How To Install A Flat Roof


The decision to rebuild a flat roof is not to be taken lightly.

The construction work is straightforward enough but it’s a job which takes time and a great deal of care. It could involve boiling up bitumen for the roofing felt for example. Hauling up buckets of hot bitumen to your roof can be dangerous so you may prefer to call in a roofing contractor for this stage of the work.

Why renew a flat roof?

Before you start on a job quite as big as this make sure that it is really necessary. Many flat roofs leak even after repairs. But that may be because the real cause hasn’t been identified. So make a careful inspection of your roof to convince yourself that a complete rebuilding job is really necessary.

Your problem may be something very simple. It’s much more sensible (and a lot cheaper) to work out how and why the leak occurs before deciding to renew the whole roof. On the other hand your roofing felt may have worn out. It may need replacing — but without extensive structural work. The important thing is to find out whether repairs are what you really need or whether the roof is beyond this and needs complete renewal — joists, decking and all.

How To Install A Flat Roof

Fault finding

Start your fault finding by looking at the roof itself then check the ceiling and walls underneath. Then, if the problem still looks serious, inspect inside the roof.

Surface crazing: Unless you have leaks don’t worry too much about surface crazing. It’s normally only a cosmetic blemish. You can smarten it up by scraping the bitumen surface hack with a brick: use it like a scrubbing brush, you’ll soon get a neat surface again. General deterioration: The surface of a flat roof is normally asphalt or bitumen: it won’t last for ever. If the surface has clearly started lo break up, peel or degrade then you’ll have io take it up and relay it with new. If it is an old roof and has been weathertight until now, be chances are that this is all you will have to do. But don’t bank on it.

Cracks: Deep cracks in the roof surface indicate that if you haven’t got leaks now, you soon will have. They can occur for a number of reasons but the main one is differential expansion of parts of the roof structure or of the roof felt itself. You can find cracks running across the roof, around upstands and hidden underneath flashings —practically anywhere. Unfortunately they can sometimes be hairline thin — you may have to look very carefully.

You can patch cracks. But unless you get to the root of the problem, which is normally in the structure, it will occur again in a few seasons. Sorting out this problem may not, however, involve a major renewal.

Blisters: If your roof surface is beginning to develop bubbles or blisters you should cut them out and replace them with patches otherwise they’ll soon be a source of leaks. Damp patches on ceilings: These normally indicate that water is leaking tthrough the roof. But if you have recently insulated your roof, the problem may actually be condensation which has drenched the insulation. It then saturates the plasterboard on which it’s lying. You can normally solve this by installing a vapour barrier but that’s another problem altogether. Damp patches which are the result of leaks may be difficult to trace. They won’t necessarily occur straight under the leak in the roof. You’ll have to follow the path of the water inside the roof back along the joists and rafters to the leak.

Damp patches high on walls and around the edge of ceilings: These often indicate that water is leaking through cracks under flash ings in parapets. It can also mean that the flashings are defective.

Sagging roofs: All roofs bow a little. But if you have a sag which is noticeable or which results in rainwater lying around in big puddles you should have a look at the roof construction. You may have rot. Or the roof may have been loaded beyond its safe limit You’ll need to look at the construction of the roof. If there are doubts .as to its safety don’t climb onto it to check.

Planning the job

If your investigations show extensive sagging, any signs of dry rot, wet rot which has destroyed the strength of the timber, any breakdown of the roof decking (normally from damp) or any other extensive damage or collapse, you will probably have to renew the roof lock, stock and barrel.

Because it’s a major job it’s best to call in your local authority building inspector to confirm your opinion. It’s a job which is going to take time — so work out a realistic programme and stick to it.

The main steps are:

Make a preliminary check on the way your roof is built.

Decide on what kind of roof you are going to install.

Get approval for your proposals from your local building inspector. He may not want drawings, but check, just in case.

Strip the existing roof.

Fix a temporary roof cover.

Inspect the structure and replace any defective structural timbers.

Fix the new deck and prepare it for the waterproof membrane.

Fix the waterproof membrane and insulation.

Fix flashings and edges and gutters.

Put up the new ceiling and make good inside.

All these can take more time than you might think. So allow plenty of time for each stage in your programme.

Obviously you should plan the job for the earliest summer months.


Start off by removing any fittings which could I damaged by falling debris or by rainwater as well as furniture and carpets.

Build temporary wooden boxes around baths and handbasins and water closets which could be racked and anything else which could be damaged and can’t easily be moved. Drape polyethylene sheeting over anything which could be damaged by rain — and that includes plastered and tiled walls.

Most importantly, disconnect any electric power and lighting circuits — even if they don’t run through the ceiling. They’ll be exposed to the weather at some time and will be a real risk unless you isolate them. Occasionally you will find pipes for water or gas running through the ceiling structure. Make sure that these are disconnected as well before starting work.

Because you’re working overhead and because there will be a lot of dust you should wear protective goggles and a hard hat, as well as overalls, working gloves and tough shoes or boots.

When you are certain that it is safe to start stripping the roof, begin inside with the plasterboard ceiling.


Remove the ceiling lining starting at the Inspection holes you made in your preliminary check. Try to do as little damage to the tops of the walls as you can to minimize patching later.

When you are removing the ceiling lining, work along the line of the joists. One of the most useful tools for this job is an old narrow bladed garden spade with a long handle which can be slid between the lining and the joists. It will cut off the fixing nails as it goes.

You will be working overhead so make sure that your working platform is stable and really secure. And before you rip large sections of the ceiling down take a look to see what else is likely to come down with it.

Make random criss-cross cuts across the ceiling with a sharp knife. That will encourage the lining to break off in sections rather than in big sheets. And clear away the rubble as you go using plastic bags. Otherwise you’ll find yourself tracking plaster all over the house.

While the roof is still reasonably watertight take a final careful look at the structure you have exposed. Make a note of any differences in the pattern of the joists you worked out from your earlier inspection. Measure the depth and thickness of the joists for any replacements and check around the roof to see if any of them are different.

If any of the joists are in a very bad state, prop them up before you go on the roof. Use a pair of 100mm X 50mm pieces of wood for each of the props with a shorter length of 100mm X 50mm underneath the defective joist. Use another at the bottom to protect the floor surface from damage.

Now is the time to check that your proposals for the new roof will actually fit in with the existing structure. It’s best to leave ordering the new timber and decking until this stage. Whatever you do, don’t remove the old decking and felt until you are absolutely ready to commence work. It’s better to have a mess in the room below than no roof and the risk of flooding when it rains.


With the ceiling cleared away start on the roof. You will need several ladders so that you can get up and down from the roof from different positions.

You will also need a supply of long crawl boards for when the decking has been stripped (you can use sheets of the new decking if it has been delivered), a big jemmy to prise the decking off and a shovel to strip off the bituminous roof felt. Start around the edges of the roof. Remove any battens and guttering tying down its edges at eaves and carefull bend up any flashing where the roof butts up against the wall of your house or a parapet. Use the spade as a quick way of loosening the felt from the deck. You should be able to pull it back by hand in many places. Make sure you’re not near the edge when you do this. Roofing felt tears easily and unexpectedly.

With most of the roofing felt off, starting iy ing the decking. If it is made from of ply or chipboard, use a jemmy to prise it away from the joists. Tongued and grooved boarding decks are tacked in the same way although you will need to take out one of the middle boards n hi make it easy to get the others up.

Work systematically stripping from the middle of the roof to the edge where you can get down safely. As soon as you have removed a section of the deck, lay crawl boards across the open joists: there’s nothing heroic about falling through a roof.

Break up the old decking and dispose of it. And then arrange your crawl boards for the next stage.

1. Remove the guttering carefully and then unscrew the brackets holding it in place before tackling the fascia board itself

2. You may be able to remove some of the (locking with the felt in place: use a bolster to loosen the overhanging edges from the joists

3. As soon as any holes are made in the roof itself lay crawler boards for your own safety. You can use planks or old, sound decking


If you are not absolutely clear why a roof leaks or if it is clearly beyond repair you will need to find out how it is constructed. Unless there are gaping holes in the top of the roof it’s easiest to work from inside. Start inside by cutting out several 1000mm X 500mm sections of the ceiling lining. Make sure that the cutout spans the bottom of several joists. You’ll need to do this in several places just in case the construction is different in different parts of the roof. The first obvious place to start cutting is a damp patch.

Clear out any loose debris or

insulation and take a look inside. Use a torch to see more clearly. Obviously it’s best to make your inspection while it’s raining outside.

The things you will need to take note of are:

The depth of the joists (which you’ll need to know if any have to be replaced).

Whether there is any insulation in the roof space and what state it’s in — wet? Dry? Flattened? And has an air gap been left between the top of the insulation and the underside of the decking?

What is the roof decking made

from (it could be chipboard, ply, woodwool, or tongued and grooved boarding)?

The general state of the joists and decking. Is there any sign (or smell) of rot? Have the joists sagged noticeably? It’s often easier to see this with your eye lined along the joist than it is outside. Has the decking (which supports the roof felt) deterior ated in any way and are there water stains on the timbers? Now you will have to do some detective work to trace where the water is coming from. And that’s when you’ll have to wait for a rainy day.


When you are replacing joists it is essential that the new ones are of exactly the same depth. Otherwise you’ll find it impossible to put the ceiling back.

If the ends of the joists are supported in steel stirrups attached to the wall, replacing them is relatively simple. You lift them out of the stirrups and drop in the new ones. In practice you will probably have to prise them out from below using a jemmy. Work one end up a short distance and go to the other end and do the same until the joist is free. In some cases the joists are secured with nails driven through the side cheeks of the stirrups. Use a cold chisel to break off these nail heads.

If you are replacing more than a few joists, check that the supporting walls are adequately braced: apart from supporting the roof, joists tie the tops of walls together and add structural stability.

Removing a joist will normally involve taking off one or two struts fixed at right angles between joists. If they show signs of rot throw them away — but first measure them as you’ll need to replace them. They are needed to keep the joists stable and as supports for the edges of roof decking sheets.

In older roofs the joists will be built into the tops of the walls. Taking them out calls for care if you are to avoid damaging the wall. It’s best to cut them off about 500mm from their ends with a handsaw and to scrape out the mortar around their ends.

Work them carefully out of the loosened socket. Resist the temptation to wrench them out, you’ll only have to repair the brickwork. If the wall is a party wall even more care is called for.

If the walls are timber framed, the joists will sit on the timber top plate of the wall with blocking pieces between the ends to keep the joists square and in place. Keep these end pieces — or if they are in poor condition measure them for replacements. Remove the end pieces on either side of the defective joist and prise it off with a jemmy.

Older houses built during the austerity period of the late 1940s will almost certainly be made from some odd sizes of timber. And as you remove each joist check quickly that it matches its neighbours and measure up two or three of them to get an idea of the replacement sizes you will need. If you can’t get exact replacements then you will need to go a size up — in depth, if not thickness. You can always cut down a joist that’s too deep, but it is very inadvisable to pack out below a joist that is too small simply to bring it up to the same level as the other joists alongside.


Working on roofs — even flat ones — is dangerous. The roof is at its most dangerous with the ‘lacking off but you must be just as much on guard when you’ve replaced it.

Plan the arrangement of your boards over the open joists so that you can get around the roof safely and so that you can rearrange them easily. Crawl hoards are needed during your check on the state of the structure but use the decking boards thereafter. They’ll also provide a support for the temporary roof.

Shift ladders around’ so that you always have immediate access to ground level from the crawl boards.

Put up at least one ladder, preferably two, so that you can get down quickly but safely. Have one for access to the ground outside and one for the inside.

Make sure that ladders are tied securely at top and bottom and are 1m out for every 4m up.

Work systematically. Reposition crawl boards if you can’t easily reach your work. And carry tools and fixings in a shoulder bag.

Wear soft-soled non-slip shoes and never work with your back to an open space.


You’ll need some kind of simple waterproof cover to protect the rooms below — and later the decking — before the waterproofing goes on top of them.

Use a big heavyweight polyethylene tarpaulin, preferably in a single sheet. They’re available in large sizes but if you can’t get one big enough, use two side by side. \\—timber batten Overlap their edges by at least a metre and seal the joint with polyethylene adhesive tape for good measure.

The tarpaulin should be big enough to cover the roof and drop down about a metre around the eaves. Turn the edges over several times to form a kind of hem and staple (or tack) through them to long lengths of 100mm X 50mm timber. They will act as weights helping to stop the material blowing off in winds. And you can use them as rollers to wind up the polyethylene neatly when you start work for the day.

If one side of the roof is attached to your house or has a parapet, tuck one of the timberreinforced sides of the tarpaulin under the flashing which you bent out when you were taking the roof off. Bend the flashing down again over the edge. You can fix this edge more securely for a few days with masonry nails.

During the time after you’ve stripped the decking off, support the tarpaulin with spare timber or boarding laid loose across the open joists. Use trestles or temporary stacks of wood to give the tarpaulin a good fall so that it drains water off and over the edges.

If there is any chance of even the most moderate of winds, weigh the edges down paying particular attention to the corners and tie the edge timber securely back to the wall to improve the weatherproofing.

Your biggest problem, of course, is organization: you shouldn’t assume that a temporary roof will be completely weatherproof, so try to organize your work in such a way that you need to rely on it for only two or three days at most. Even so, you’ll have to take your chance with the weather.

The simplest answer is not to remove the old roof covering until you’re ready to fit the new one, and to organize things so you can do the job in a day or two.


It’s not impossible to lay your own roof felt. Hut it’s a very tricky and messy business and calls for experience. It is far better to call in a roofing contractor for this part of the job.

When you do, make sure that he follows good practice — your building inspector may insist on it anyway.

The main points are that there should be three layers of felt. The top and second layers should be Type 3B glass fibre base felt bonded in bitumen. The bottom layer should be Type 3G glass fibre base pert rated felt partially bonded with bitumen to the deck over most of its area but fully handed in a 500mm band around the edge of I lie roof and around any skylights or pipes.

The surface should consist of l0mm stone chippings laid in a bitumen based adhesive compound. This layer should be thick enough to hide the roofing felt completely. The chippings should be the lightest colour you can get for maximum heat reflection.

When you have chosen your roofers make ,arrangements with them about what you need to have ready for them, how much space they will need and, if the only access to the roof is through the house, what sort of protection your carpets and walls will need.

On the day don’t be afraid to watch what they are doing and if you are unhappy about anything — for example, not fully bonding the bottom layer at the edges of the roof and around upstands, or bridging the movement gap between upstands and walls — don’t be afraid to question what’s going on. It will be your only chance and you are going to have to live with the roof — not the workmen.

Roof finishes

An alternative to stone chippings as a roof cover is lightweight paving. You may be able to do this yourself, but even the lightest paving stones demand a strong, tough roof construction. The only real justification for laying paving is to use the roof as a balcony and this is a conversion which could possibly be attempted in the future. You should consult the contractor at the planning stage and be clear about what you’re doing so that you don’t have to change plans halfway.


Flat roofs are constructed in a series of layers: ceiling, decking, waterproofing (which itself is a series of layers), vapour barrier, insulation, surfacing and the grid of the supporting structure. The difficult bit is deciding on what order to put them together.

Probably the most effective roof is known as the ‘inverted’ or ‘upsidedown’ roof. It has the insulation on top of the waterproof layer. That means that the membrane is protected from the sun and that the whole structure is kept at the same range of temperatures.

If the roof structure is concrete instead of timber the insulation has the effect of turning the roof slab into a heat store. But concrete

roofs are rare in houses. And a timber roof structure has to be stronger than usual because the upsidedown roof needs a layer of heavy gravel, paving slabs or brick pavers to keep the insulation in place. And it calls for about 20 per cent more insulation than other types.

An upsidedown roof is worth considering if your building inspector wants you to renew all the existing joists. And it could be worth considering if your roof has a parapet wall all the way around and an internal drain. That will save you having to build special details. There are socalled ‘warm’ roofs which also have the insulation on top of the deck but with the waterproof layer on top of that.

Then there is the ‘cold’ roof which is probably what your present roof is. Providing your building inspector doesn’t want any major structural changes it is probably the most practical one to go for because it will involve fewer changes and less work. Working from the bottom, the cold roof sandwich is constructed like this:

Plasterboard ceiling.

Vapour barrier to stop the very considerable amount of water which develops in ordinary rooms from condensing in the insulation.

Roof structure made up of joists, bracing and nogging pieces.

Layer of insulation with an air space between it and the underside of the deck. (Air should be able to circulate freely above the insulation and be vented to the outside).

Roof deck, normally pre-felted plywood or chipboard to which three layers of roofing felt are bonded with bitumen. The first layer is normally perforated so that the second layer is partially bonded to the deck.

Layer of stone chippings bedded in a bitumen-based adhesive. This serves as a protection for the felt from the effects of the sun.

An alternative is to use an aluminium reflective paint.


Once your local building inspector has approved your proposals, work out a programme. Your aim is to get the job finished as soon as possible to avoid unnecessary damage to the rooms which have been exposed to the elements. It rains even in the summer months, high winds can cause damage and your temporary roofing cannot be considered as having a long enough life for you not to get on with the job.

You should also get an idea of whether local roofing firms are likely to be able to fit you in around the date you expect to be ready for them to start work.

Your first investigation of the roof structure will give you an idea of what materials you will need to buy. But don’t order anything until your building inspector gives you the goahead. Be prepared to top up your order after the final inspection of the roof joists.


Make a final check on the reliability of all the joists and their fixings. It’s silly to build anything doubtful into your new roof.

Condition of joists: Check the joists by giving

them a good shake (they shouldn’t move appreciably) and tap them with a hammer. If one joist gives a duller thud than the others check it out along its length. It may be perfectly all right but a dull sound can indicate that the joist is defective because of rot or deterioration.

Go round the joists probing the timber with a screwdriver. Pay particular attention to the tops, any areas which are darker than elsewhere and where joists pass over the tops of internal walls.

If you can easily dig bits of timber out or if it is spongy you’ve got rot and will have to replace the joist.

Fixings: Now check the ends of the joists to make sure they are securely seated over the outside walls. They will be either built into the wall or fixed into joist hangers. Mark any rusty joist hangers for replacement and take one out as a sample to match when you are buying new ones.

Falls: All flat roofs have a slope — or they should have. The recommended minimum is around 1 in 40 which should offset occasional dips in the roof surface. But if it is clear that your roof didn’t have a sufficient fall — or even had a reverse fall — then you are faced with having to add firring pieces, tapering pieces of timber fixed to the top of each joist to give the new roof the required slope.

Ordering: Once you are satisfied that the remaining roof structure is sound, order any additional timber for the joists and the intermediate struts — and, if you are unlucky, for firring pieces.

Unless you are specific your timber merchant may deliver odd lengths so make it clear that you want timber for new joists to be of a minimum length.

When you’re ordering joist hangers make sure that they are of the same size and design as the old ones. If an exact match is not available make sure that you can incorporate the substitutes into your wall.

Stacking: When the timber arrives stack it so that you can get at it easily and make sure that it is protected from rain with a plastic tarpaulin. One of the reasons for flat roofs leaking is excessive moisture trapped in timbers and decking during building.


Start by fixing any replacement joist hangers. They vary in the way they’re designed to fix to the wall itself but invariably they have a shoe which sticks out from the wall acting as a stirrup or socket for the end of the joist.

If your new joist hangers are of a slightly different size the important thing when you’re building them in is to make sure that their bottoms line up with the bottoms of the rest of the hangers.

This can be difficult if you are working on an old brick wall with courses out of alignment. In this situation it’s best to fix the hanger so that its bottom is higher than the rest — you can then cut a notch out of the joist to ensure that it lines up. If the hangers are lower you’ll have difficulties in putting the ceiling back neatly.

Now measure up your joist timbers and cut them to length. Measure from the inner faces of the joist hanger shoes, not between the walls. The chances are that there will be variations in the wall so measure each joist individually. Using a spare joist hanger check that the ends of the joist will pushfit comfortably into the shoe. If the shoe is too big, you can pack either side of the joist with wedges. If it’s too narrow, cut out a triangular qtction from the cheek of the joist at either mid. Before you make any further adjustments drop the joist into its shoes and tap ouch end in firmly. Lay a straightedge over ,several adjacent joists to check that the top of the new joist lines up with all the rest. The hances are that you will have to take the joist out and trim (or pack) each end. You inay also find that the original builder trimmed a rebate out of the bottom of each end to make the bottom of the joist hanger flush with the joist.

Do this with each joist. Don’t forget to apply several coats of wood preservative to the end grain and any cutouts. When you have finished, make a final check that the joists form a level base for the decking.

Now start on fixing struts, blocking pieces and noggings between the joists. Replace blocking pieces either side of the ends of the new joists. Their purpose is to brace the ends of the joists. They are cut from joist timber and should be a tight fit. Skew nail them into place between the joists.

If the joists have crossed pieces of wood bracing them further along their lengths you must replace these as well. They are quite fiddly to renew in timber so consider buying metal herringbone struts (as they’re known).

In addition you will need to fit nogging pieces to support two of the edges of the boards which form the decking. The other two edges will rest on joists. Plan the way the sheets are to be laid out and work out where to fit them. You may find that they can double up as intermediate braces.

You may consider fixing noggings for plasterboard at this stage as well. Don’t forget to give all these cross pieces a liberal coat of wood preservative.

If you have to fit firring pieces to provide an adequate slope to the roof, nail them to the joists before you add any cross struts or nogging pieces.

Now check over all the cross members to see if any airtight compartments are going to be formed by them. It is very important if you are to avoid condensation in the roof itself that air can circulate above the level of the insulation.


Start by loose laying the 18mm prefelted chipboard or ply over the joists and mark the boards up for cutting.

Each board should butt up firmly to the next. All four edges of the board should be supported by a joist or nogging. There musn’t be any unsupported joins between boards.

Nail the boards down using 45mm nails at 120mm spacing around the edges and down the run of each supporting joist. Don’t miss out nailing to any of the joists.

Seal all the joints and exposed edges with a cold bonding mastic and tape (your decking supplier should have this in stock). Don’t leave the decking unprotected: it’s not waterproof. The pre felting is there merely to provide a bond for the layers of roofing felt.

Now go round the roof and fix timber upstands and fillets against walls or parapets or around any pipes or skylights. It’s essential that you fix these upstands to the roof deck and not to the wall.

There should be a 12mm gap between them and any vertical surface.

This allows the roof to move independently without rupturing the felt. If you have difficulty fixing the timber upstands it’s possible to buy freest4tnding metal kerbs to support them.

The ideal height for upstands is around 200mm. That is much higher than has been the customary practice, but you may have to ompromise: the dimensions of the wall or skylight or parapet may mean that you have Iii make do with less. There may simply not he enough room to build in new flashing.

Fix curbings around the exposed edges of the roof and fix the fascia boards at this stage as well. Nail a 50mm X 50mm batten to the top of the fascia board to form a drip rail. This helps throw water clear of the wall below.

Now check over the roof and ask yourself whether it’s ready for felting. Are there any hollows where water could collect? Are there any sections — for example around skylights or pipes — where you have inadvertently formed a trap for water? Have you fixed fillets to stop roof felt having to take a sharp eight angle bend? Has all the timber been treated with wood preservative? And is it completely dry?


With the waterproof layer of felt on it’s time to add flashings to anything protruding above the roof level.

If you have roof lights or pipes poking through the roof you will have bent or slid their existing flashing out of the way early on in the job. Check now that they haven’t been damaged during building work and replace them if they show any signs of deterioration. You should be able to buy special pipe collars `off the peg’ and replace roof lights without any problems.

Walls and parapets: Where your flat roof butts up against a wall or parapet you will have built an upstand on the deck which will by now have been felted. The flashing is tucked well into the mortar course above this and bent down to overlap the top of the upstand by at least 50mm.

Start by raking out the mortar course to a depth of at least 30mm.

Clean it out with a brush and water. Unroll the metal flashing, smooth it out flat and measure it for length. Allow at least 200mm extra each side of the roof. Buy a sufficient length in the first place to run the flashing in one length. If you have to make a join, overlap the ends of the two pieces by at least 200mm — but try to avoid joins if possible.

Traditional flashing materials were lead or zinc; these had to be bent into shape and bedded in hot bitumen. It’s simpler, however, to use a ‘modern, self-adhesive flashing which is just as tough but easier to apply.

Prepare a 1:3 mortar mix (one part cement to three parts sand) and add a little PVA adhesive. Use the minimum amount of water necessary. Dampen the open course and lay a thin bedding layer of this mortar.

With a helper tuck the edge of the flashing into the course for its full length. You may have to do some more raking out to get it to lie in evenly. Wedge it firmly in place at intervals with small pieces of brick or stone. Don’t bother to take them out: they’ll provide a mechhanical fixing while the mortar sets. Now point the metal flashing into the brickwork. I king a pointing trowel force `worms’ of the mortar into the open course above the flash! ig.

Smooth the surface off with the handle of the trowel and clean mortar droppings off the face of the flashing. Support the outer edge of the flashing on scraps of wood to stop it from moving while the mortar sets.

Next day, or when the pointing has set, neatly fold the flashing down over the felt upstand. The overlap should be at least 50mm.

At the sides of the roof, carefully dress the flashing over the edge with a wooden mallet so that it covers the edge and then lies flat against the wall.

Edges: The raw edges of builtup roofing are rarely very attractive so consider fitting a metal capping around the edge to tidy it up. They come in a variety of designs but are basically a single aluminium extrusion which is fixed to the fascia board and clips over the edge of the roof.

Fix edge tappings with screws at 300mm centres to the fascia board below the edge of the roofing felt. On no account should you screw them to the felt. This sort of capping can’t be used where there is a gutter, as it would interrupt the flow of water.


1. Rake out a 30mm deep groove for the flashing two courses of bricks above the roof and along its entire length, using a bolster

2. Offer up the flashing, raking out the mortar as you do so to achieve a perfect fit, then secure the flashing with wedges

3. Bend the flashing to approximately its finished shape before cementing it in place with a 1:3 mortar mixed with PVA glue

4. Allow the mortar at least 24 hours to set, then use a timber batten and gentle mallet blows to fit the flashing to the roof shape


Flat roofs are never very pretty but that’s no reason not to tidy up at the end of the job.

Clean off mortar droppings from flashings and the roof surface.

Carefully straighten out any kinks in flashings or edge tappings.

This is also the time to renew any damaged copings to parapet walls and to repoint them carefully as well. When you are satisfied, start work inside.

It’s best to wait until there has been a serious shower of rain before fixing the insulation, vapour barrier and ceiling boards.

That way you can immediately track down the source of any leaks.

Checking ventilation

While you are waiting, make a final check that the roof is vented to the outside. This may involve drilling neat rows of 25mm diameter holes through the upper part of the fascia board through the blocking pieces between the ends of the joists. If you are worried about birds, nail or staple fine wire mesh across the holes from inside. If you haven’t done so already, fix nogging piece for the plasterboard sheets and any batten that will be needed to support light fittings.

Fitting insulation

Now cut the insulation to size so that it will fit between the joists. There should be a minimum thickness of 100mm. You can of course put in more but make sure that you leave an air gap of at least 50mm between the top of the insulation and the underside of the deck to get rid of water vapour. In practice it’s best to use a rigid closed cell insulation board such as polystyrene — it doesn’t compress if it gets wet and if it is cut carefully you can wedge it into position from below or hold it in place with nails tapped into the sides of the joists.

With all the insulation in place, staple the vapour barrier to the underside of the joists. It is a big sheet of polyethylene which stops most of the water vapour in a room from getting into the roof space. Allow a certain amount of slack in places where you propose to run electric light cables. Some forms of attack the plastic in cables so it’s to run cables below the polyethylene as I,u as you can.

With your roof watertight, ventilated and ventilated you’re ready to start fitting a new ceiling and repairing and redecorating your rooms.

Surface finishes

Fitting a flat roof isn’t simply a matter of it ting the flashing and guttering; depending on what you want to do with the roof there are a wide variety of different finishes available. The most common use for a flat (apart from the obvious one of forming a cheap extension) is the roof garden or ‘upstairs patio’. The conversion of a flat roof to this sort of use is a thought for a future project, but there are a few simple things you can do to a standard, simple roof.

Given that you won’t want to get up on the very often, your main concern is to protect the felt; this is most easily done by embedding a thick layer of stone chippings in a bed of bitumen-based adhesive on the top of the felt. An alternative to this is to apply a thick coat of aluminium paint, both treatments offering good protection against the effects of ultraviolet light. The stone chippings look smarter, however, especially from an overlooking window, so ask your roofing contractors to finish the job for you in this way. Specify light-coloured chippings with an average particle size of around 10mm, laid 25mm deep to cover the felt completely.

If your roof is strong enough (and to check this, consult your local building authority inspector) you can lay paving stones, quarry tiles, bricks, or even earth. The former must be laid on a bed of hot or cold bitumen or a concrete screed — again, you must consult an expert, not least because the extra weight will demand a strong roof construction.

A covering of earth allows you to grow a lawn, which is even more visually appealing, while earth is an excellent insulator. But consult your local authority planning department. You also face the problem of access: lawns need mowing and weeding, and it is never easy to get lawn mowers up stairs and out of windows, quite apart from the mess they inevitably create.

If you do intend to lay a covering of this type, consider whether or not it’s worthwhile carrying out a complete conversion, with doors onto the roof and a parapet or railings all round; if this is your plan, then you really need to plan all the work carefully.

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