The Construction of Roofs

Roofs are seen in a great variety of shapes and are made from an equally wide variety of materials. There are pitched roofs, mono-pitched roofs, flat roofs, gable roofs and hipped roofs; they can be covered with felt, asbestos, slate, clay tiles, concrete tiles and translucent plastic. Flat or low pitched roofs can be hidden behind parapets and gable or hipped roofs can have deep overhanging eaves, in fact, the whole appearance of a house can be altered by the design of the roof.

Correct detailing and careful workmanship are needed if gutters behind parapet walls are not to give trouble and constructing a hipped roof requires an understanding of roofing geometry if the double angle cuts around the hips are to be accurate.

Flat roofs which have in fact got a slight fall, and mono-pitched roofs, can overhang the building on all sides so that there is no need for fancy flashings to cover the joint between the roof and the walls.

Mono-pitched roofs are usually covered with corrugated asbestos, corrugated iron or even corrugated translucent plastic. These sheet materials are laid on to purlins which span the width of the building at right angles to the pitch or slope. The distance apart for these timbers depends very much on the type of sheeting being used. Sheets 1800 mm long generally need a purlin in the middle as well as one at the top and the bottom. Longer sheets would need two purlins spaced out along the length. Neither asbestos nor plastic sheets are safe to walk on without first placing a plank on them to span the purlins. Corrugated iron can be walked on, but it is liable to bend.

Roof covering starts at the eaves allowing enough overhang for the sheeting to reach nearly halfway into the gutter. One corrugation is generally sufficient side lap, but on exposed sites two can be given. End overlap should not be less than 150 mm.

Because corrugated iron is thin, it is simply laid on the purlins and nailed into place. Where the four corners of the sheets overlap there is no need for cutting, just hammer the nail harder so that it goes through. Thick materials like asbestos and plastic must be mitred. The sheets cannot be piled up at the corners or there will be spaces beneath them to let the rain through.

The first sheet is laid as it is, with no cutting, but all the following ones have to be mitred and this cut is the same length as the overlap and the same width as the side lap.

After the first sheet has been laid the second sheet is laid with its over-lapping edge mitred at the top corner. The next sheet is mitred in the same way and so on to the end of the row. On the next row above, the bottom corner of the sheet is mitred to match the cut on the sheet below. If there is to be a third row, the second row of sheeting will need to be mitred at the top as well as the bottom, but at the opposite corner to the bottom mitre. The top row of sheets do not need mitring at the top, of course.

Specially shaped foam filling is made to fit under the corrugations at the eaves. Alternatively, the openings can be filled with sand and cement mortar. A strip or fillet of sand and cement is applied along the wall under the sides of the sheeting to make a neat finish where the brickwork is cut at an angle under the roofing.

The purlins have metal straps screwed to them with the ends built into the brickwork about 600 mm down the inside of the building. These straps are to hold the roof against wind damage. It is never a good idea to let water drip off a roof so a gutter and some form of drainage must be provided.

Both asbestos and plastic roofing has to be drilled for the fixing nails or screws. These materials can also be fixed to angle-iron purlins by using hook bolts. These bolts must not be overtightened or the sheets will crack.

Boarded and felted roofs can be either flat or mono-pitched, but the treatment is the same. Tiles cannot be used on roofs of less than 22 degree pitch and not all types of tiles are suitable for such a low pitch, so enquiries must be made before deciding to tile a low-pitched roof.

Joists for flat roofs need to be about the same size as those for floors and a table of sizes appears in the Building Regulations. This is because of the weight of snow that may have to be supported. Boarding can be tongued and grooved or square edged. It can be either roughsawn or planed. The roof of a habitable extension would be in planed boarding or chipboard, in order to provide a good surface on which to apply the felt. A garage or shed roof could be clad with rough timber as it would not be so important to have a smooth finish.

The joists must be well anchored down and are set at 400 mm centres so that a ceiling of plasterboard or other sheet material can be fixed if required. Herringbone bridging is also required for spans of over 2 m. The amount of fall required is a minimum of 1 : 60 which is 50 mm in 3 m. The fall should not be too slight, because if any settlement takes place, the fall could be reversed.

Tapered fillets called firring pieces are nailed to the top of the roofing joists to provide the fall. If the joists are at right angles to the roof fall, then battens reducing in size for each joist are used. The roofing boards are nailed through these firring pieces.

A wide board or fascia is used to cover the ends of the joists and the roof boarding projects over this fascia to almost halfway over the gutter. The first layer of felt is usually nailed in place with galvanised clout nails and the following layers are laid in either hot or cold laid bitumen. The top layer is mineral faced.

As felt cracks fairly easily it is best to remove all sharp edges of the boards and to provide an angle fillet at the junction between the roof and the adjoining walls. This includes the parapets if there are any.

It is easier to allow the roof to overhang the building on as many sides as possible, because the flashing needed at the wall junction creates a lot of extra work, and if it is not carried out properly the roof will never be thoroughly waterproof. Guttering is screwed to the fascia board and is given a slight fall to the outlet. The insulation may be placed on top of the plasterboard ceiling making the roof itself cold, or a warm roof can be created by fixing the insulation under the roof covering by using insulating slabs for the roof boarding. A vapour check is always inserted on the warm side of the insulation to prevent condensation problems within the cold roof space, or within the structure itself where the insulation is above the roof void.

The roof covering is carried over the angle fillet and up the wall to a total of 150 mm above the roof surface. A cover flashing of felt or soft metal such as lead or soft aluminium is tucked about 12 mm to 19 mm into the first convenient joint above and turned down over the roofing felt and angle fillet, but not on to the flat roof. The flashing is held into the joint by little wedges made of the same material and then the joint is pointed with cement and sand.

Felt or metal sheeting can be used to cover flat roofs and some large tiles can be used on very low pitched roofs, but small tiles need a pitch from 35 degrees upwards. Because the tiles have their heads hooked over battens and their rails resting on the course of tiles below, the actual angle of the tile is slightly less than the pitch of the rafters of the roof structure.

The actual type of roof structure is determined by the span which has to be bridged. A simple lean-to roof is suitable for spans of up to 2.4 m. A simple pitched roof can be used for spans up to 3.6 m. If the ceiling joists are used to tie the rafter feet, the span can be increased to 5.4 m. It is usually sufficient to space the roof rafters at the same centres as those of the ceiling joists.which are themselves governed by the width of the plasterboards for the ceilings. This gives 400 mm centres to meet the requirements of a 1200 mm wide board which is the size most commonly used for housing.

A mono-pitch or lean-to roof is the simplest type of pitched roof and the top of the rafters are fixed to a wooden plate attached to the rear wall. At the eaves the rafters are birdmouthed over a wallplate carried by the wall. Intermediate support may be given to the rafters by purlins carried on the end walls. This type of roof relies on the fixing at the top of the rafters and the security of the wall plate for its strength. If the fixing at the head gives way the rafters will slide slowly down the wall and either push the lower wallplate off the top of its supporting wall, or it will push the wall over.

Coupled roofs of the double pitch type derive their strength from the principle of triangulation, and all struts and hangers are positioned so that triangles are formed. When a triangle is soundly joined it will remain rigid and cannot be distorted. Loads placed on one member are transferred to the others and the bending of members is restricted by placing struts to the purlins and hangers from the ridge which again form triangles. Removal of one of these struts or hangers would weaken the roof and allow one of the triangle sides to bend and possibly break, if its sectional area were not big enough. This is a serious consideration when roofs are being altered to make attic rooms.

Roofs have to withstand possible snow loading as well as wind loads and it is therefore important that the roofs of dwelling houses are designed by competent people. Pitched roofs for sheds and gardens are not so important, but as the loads present an outward thrust at the eaves some form of restraint is required. Ceiling joists are not needed usually so collars are placed about halfway up the rafters to tie them together, and form the required triangle. It is best to have a tie on each of the rafters, but they can be reduced to one every two or three rafters.

When the brickwork has been completed, roofing starts by bedding a timber wallplate around the top of the wall. It can be on either the inner or outer leaf of cavity walls, although it is more usual to place it on the inner leaf. The rafters are cut to form a birdsmouth over this plate and are cut at the top to fit against the ridge board. The angles for these cuts are easy to obtain by drawing the roof triangle to a large scale on a board.

An adjustable bevel can then be set to the required angles. The birdsmouth cut must not be taken more than halfway into the rafter. The amount of rafter beyond the cut depends on the amount of overhang that is required at the eaves of the building. This too can be ascertained from the scale drawing.

To erect the roof, four rafters are made up into trusses by laying them on the ground and nailing a board or gusset across the top, keeping the rafter heads apart just sufficiently to take the ridge board and so that the ridge board will rest on the gusset. A ceiling joist is nailed across the foot of the rafters flush with the top of the birdsmouth. The inside or vertical cuts of the birdsmouth must be exactly the same distance apart as the width of the building between the outside edges of the wallplate. These two sets of rafters, or more if it is a long roof, are then ready for lifting into place.

The wallplate is set out with all the rafter positions carefully marked. Then the two rafter trusses are lifted up and nailed into place. This will be two positions clear of the ends, if it is a gable roof or in line with the top of the hip if it is a hipped roof. They are held plumb using temporary battens.

The ridge board, which is also marked with the rafter positions, using the wallplates as a template so that they are in exactly the same places, is lifted into the slots provided at the top of the rafter trusses and secured. The rest of the rafters are then fixed in pairs at each side of the roof, leaving the two end rafters out to make room for the bricklayers to build the gable walls.

Roofs for domestic property are built-up using internal loadbearing partitions to take the thrust of the purlin struts. Ceiling joists act as ties and if they are in two lengths they are lapped or cleated together over the supporting walls. Plates are also positioned on top of the ceiling joists over the supporting walls so that the struts can be birdsmouthed over them. Binders are positioned at the centre of the ceiling span and hangers are nailed to them and to the ceiling joists to support the ceiling and reduce the size of timber that would otherwise be required. In this way the roof and ceilings are locked into a rigid framework of triangles.

The ties, struts and hangers are all equally spaced at about 1.5 m to 2 m centres. If the spacing is increased it will put more load on the purlins and from them on to the struts, hangers and binders, their sizes being related to the spacings.

Rafter feet are cut to receive the fascia and the soffit using the same bevels as those used for the birdsmouth. The fascia is fixed about 38 mm higher than the rafters to allow for the tilt of the tiles. A special tilting fillet should be fixed to the top edge of the rafters to hold the tiles up, but the fascia is often used and is backed by a piece of angle fillet. It makes for easier repairs at a later date of fascia boards if the weight of the tiles is taken by a separate tilting fillet.

The soffit or level board beneath the eaves is fixed to battens which are nailed to the rafters at one end and to a length of timber fixed to the brickwork at the other end. A wide overhanging eaves not only gives character to the property, but shelters the top of the wall and the upper window frames from a lot of bad weather.

Weather proofing around chimney stacks and other projections through the roof is done by a system of flashings. Trimming around the stack follows the same principle as that for floors. First, a horizontal trimmer is fixed between the rafters that run on either side of the brickwork and then the short rafters are cut and fixed up to the trimmers. At the back of the stack a flat gutter is formed and this is covered with a flashing material such as lead, zinc, felt or other sheet material. The gutter is 150 mm wide and a further 125 mm of flashing is taken up the roof under the tiles, and over the top a length of tilting fillet that is required for the tiles immediately over the gutter. Another 125 mm is turned up the face of the brickwork. This back gutter apron has to be fitted before the roof covering is fixed.

Down the sides of the chimney stack soakers are fitted between each course if plain tiling or slating is being used as the roof covering, but the over flashing method is used where contoured tiles or sheet materials are used as roof covering.

Soakers are strips of flashing material which are turned up the brickwork 75 mm and under the tiles 100 mm. The length of the soaker is determined by the length of plain tile or slate being used. It equals the gauge of the tile or slate, that is the amount that is exposed on the surface of the roof, plus the lap, which is the amount that one tile or slate overlaps the course next but one below it.

Therefore, if the tile is 266 mm X 165 mm and is laid to a gauge of 95 mm the length of soaker required would be 95 mm + 75 mm = 170 mm. An extra 25 mm can be added so that the top of the soaker can be turned down the edge of the tile batten and nailed to hold the soaker in place.

The cover flashing which cloaks the upturned edge of the soakers is stepped to suit the courses of bricks. It overlaps the soakers by 50 mm to 65 mm. The flashing is placed against the wall and the courses are marked on it so that the steps can be cut in it. Then the mortar joints are raked out about 12 mm to receive the flashing which is held into the joints by wedges made of the flashing material in the same manner as the flashing for flat roofs. Where soakers are not used, the cover flashing is made wide enough for it to be dressed down over the roofing material a distance of about 150 mm to 175 mm.

At the front of the chimney stack an apron flashing is necessary and this extends beyond the brickwork by the amount of the side apron or soakers. It is turned up the brickwork about 150 mm to the most suitable bed joint, which is raked out to receive it as with the other flashings. It is dressed over the roof covering by 150 mm to 175 mm. The extended ends of the front apron fit under the side flashings, of course.

At the gables, the end or verge of the roof overhangs the brickwork and the tiles are bedded in mortar over the wall so that they project a few inches; the underside then has a sand and cement fillet applied to cover any uneven brickwork and make a neat finish. Where there is a deep overhang at the eaves, it is usual to carry the roof over the verge the same amount and to do this the wallplates and ridge are projected beyond the gable and an extra rafter is fitted outside the wall. This is done by joining the last two rafters together with short lengths of timber like a ladder. The two rafters are then fixed in place with the outer rafter gaining support from the short timbers over the gable wall.

The sloping soffit is lined with the same kind of boarding that is used for the soffit at the eaves. It is nailed or screwed to the horizontal timbers. Wide barge-boards are then cut to the same angles at the top and the bottom as were used for the rafter head and foot cut. These boards are nailed into place and the tiling is allowed to overhang them a few inches. A batten is then scribed to fit under the tiles to make a neat finish.

Dormer window treatment follows the principles applied to trimming for a chimney stack. The rafter at each side of the dormer is doubled to take the extra weight. Rafters are trimmed at the top and bottom of the window opening. The dormer roof is often rested on the top of the window frame, but it is better if a separate timber can be arranged, as window and door frames should not be made load bearing. Side cheeks of the dormer can be either glazed or solid. Glazed side cheeks are frames made up on the bench and fitted into place as a single unit. Solid cheeks have the frame work for the cladding built-up on the site. Flashing is by means of soakers which are fitted under the roof tiles and up behind the cladding of the dormer cheek.

It is never a good policy to let the collected rainwater simply drip off the edge of a roof, so the dormer should either be fitted with a gutter and downpipe, or it should be given a fall to the rear. In either case the roof treatment is similar to the back gutter of a chimney stack. It has a flat area for the collection of water and the covering material is carried up the roof under the tiles and over the tilting fillet.

This will mean that a great deal of water will pour down the roof at the side of the dormer so that flashing between the side cheeks and the main roof will have to be carefully installed.

Roofs for conservatories are either translucent plastic sheeting or glass. The latter is generally the Georgian wired type. Glazing bars which are 75 mm deep and 38 mm thick are double rebated to take the glass on either side and are supported by purlins. Where the glass has to be in more than one piece the bottom piece is fitted first and then the next one is bedded into place allowing it to overlap the first piece by about 50 mm. A tack is driven into the glazing bar just below the bottom edge of the glass to prevent it slipping down the roof. A fillet of putty is bevelled on the top of the glazing bar in the same way as glazing windows.

The wooden bars are birdsmouthed over the bottom plate in the same way as roof rafters, but the glass rebate should not be allowed to come in line with the bottom plate; a space between the bottom plate and the glass is necessary to let the inevitable condensation escape.

Patent metal glazing bars are obtainable and the manufacturer’s fixing instructions should be followed. This usually means fixing the bars to the bottom wallplate and to another timber plugged and screwed to the wall at the top of the roof. The number of intermediate supports necessary will depend on the size of the bars and the span of the roof. There is no putty required as the glass rests on a soft strip fixed to the metal and the top seal is generally by means of lead also fixed to the metal bars. This is simply tapped down over the glass using a wooden mallet.

Plastic sheeting is fixed with springhead nails and clearance holes must be bored as the sheets are easily cracked. Mitring at the joints of the sheets has to be carried out in the same way as for asbestos sheets. One corrugation overlap is usually sufficient for the side joint. End overlaps are about 150 mm. Soft foam infill strips are made to fit the corrugations and stop the draughts at the eaves. Condensation is frequently a problem with these roofs.

It is important to remember in the interests of safety, that glass, plastic and asbestos roofs are unsafe to walk on and will give way without warning. A plank which spans from purlin to purlin must be placed on the roof to walk on, both during construction and afterwards when maintenance is being undertaken. Snow guards are also necessary at the eaves of the main roof above conservatories, to prevent a sudden rush of snow sliding down on to the lower roof and collapsing it under the weight.

Access to windows and gutters above the conservatory roof has to be provided by screwing certain of the sheets in place so that they can be removed reasonably easily on those occasions when painting or other maintenance has to be carried out. With glass roofs the putty will have to be scraped out and the glass removed as required.

Rainwater disposal has to be arranged and in some areas this has to be taken to soakaways and in others, the water can be taken to the nearest drain.

The guttering and downpipes can be made from plastic, asbestos or metal. Asbestos is relatively cheap and is made half-round pattern. Putty or mastic is used for the joints which are held by small, galvanished 6 mm gutter bolts. The half-round gutter is supported on galvanised brackets screwed to the fascia board. Down pipes are screwed to plugs set in holes bored in the brickwork.

Metal gutters can be in half-round or ogee pattern; the joints are also made with putty or mastic and held with gutter bolts. Metal brackets screwed to the fascia are used to support these gutters and the downpipes are fixed to the walls with plugs and screws.

Both asbestos and metal rainwater systems can be painted to suit the colours of the house. Asbestos gutters need to be painted on the inside to prevent water penetrating to the back of the paint and pushing it off. The material needs sealing with emulsion paint on the outside before the gloss paint system is applied. If this is not done the alkali in the asbestos will combine with the paint to make a crude soapy mess that will not dry.

Metal gutters also benefit from a bitumen treatment on the inside, but it is not essential. The appropriate primer should be used on the exterior before the undercoat and gloss are applied.

Both asbestos and metal rainwater goods can be cut with a hacksaw and the holes can be bored with metal cutting drills and countersinks. No special tools are needed.

Plastic rainwater systems are easy to install. The joints are usually sealed with some form of patented Neoprene gasket and the systems simply snap together. The brackets are in plastic and are screwed to the fascia board. In some makes the brackets form the joint between the lengths of guttering. Whatever the material used for the gutters, there should always be a support at the end joints and at least one intermediate support. If the lengths of gutter are more than 2 m then two intermediate supports will be required. The various systems all make provision for the expansion and contraction of the plastic which would otherwise cause the gutters to buckle in the summer.

Plastic systems can be in the half-round pattern or they can be of the modern version of the ogee pattern which is square at the back and double curved at the front. There are special fittings made so that the plastic system can be jointed to existing metal gutters of half-round or ogee shape.