Fitting A Suspended Timber Ceiling

Installing a suspended timber ceiling is a task you may decide to undertake if you have an older house and feel a ceiling height of 3m (or 10ft) or more is excessive. Alternatively your home could be part of a large house which has been divided up and the relation of room height to room width might seem disproportionate, making small rooms look even smaller. Partitions sometimes cut across a decorative plaster cornice and you may wish to conceal this or you might have a ceiling with pipes and cables across it which you want to hide.

Whatever the reason for this type of modification, you can successfully construct a suspended timber ceiling yourself, provided you take proper care. Before you begin, remember if the room is a ‘habitable’ one under the Building Regulations — a living room, kitchen or bedroom — the new ceiling must not be less than 2.3m from the floor for at least half its area.

Fitting A Suspended Timber Ceiling

Preparing the installation

You will first have to decide which way to span the joists; normally this will be across the shortest width of the room. However, if this dimension is 3m (or 10ft) or more, you would need joists of about 125 x 38nun (5 x llin) timber and you may find it more economical to fix a beam or binder across the shorter span joined to smaller intermediate support joists across the longer span. You should also determine which direction will involve the least cutting of plasterboard sheets, if using these for finishing, and examine each wall to see if there are any fixing problems likely to arise.

Find out if the internal walls are constructed with a timber stud framework or from brickwork, or if any of them are very irregular so extra cutting and fitting of ceiling timbers will be required.

Check if there is a possibility of support for a binder from the existing floor above; the existing joists should be at least 150 x 50nun (or 6 x 2in) to provide this support. Another point to consider is whether you will need an access trap to get at the space above the new ceiling, for example to check plumbing or gain access to an electrical conduit. Decide the location of the trap and how large it should be — it ought to be at least 450 x 450mm (or 18 x 18in) to enable you to get your shoulders easily through the opening. 1 Using long spirit level as guide, mark position of underside of new ceiling on walls right round room 2a To support binder, fix timber hangers to ceiling joists above 2b Use wood offcut with housing cut to size of binder end to provide support at wall 3a With lime-mortared brick wall, you can use cut nails to fix wall plate 3b Alternatively drill wall and insert hardwood plugs to take fixing screws 3c With timber stud wall. Screw wall plate through plasterboard into studs 3d Where there is old wall lining, use timber packing to provide firm surface for fixing Choosing size of timbers When buying timber for the ceiling, ask for SS or MSS grade timber. The table below indicates the sizes of timber required to carry a ceiling over various spans, if the joists are spaced 400mm (or 16in) apart. This figure is a multiple of the standard sizes of plasterboard sheets. If you are going to use a material which is heavier than plasterboard, the timber sizes will need to be increased. If in doubt, ask your supplier.

Size of joist (mm) maximum span (m)

38 x 75 1.90

38 x 100 2.53

38 x 125 3.15

38 x 150 3.77

Beams (binders) supporting ceiling joists size of beam spacing for max span of (m) beam (mm) 1.20 1.50 1.80 2.10 2.40 38x 125 2.14 1.92 1.75 1.62 1.52

38x 150 2.56 2.30 2.10 1.95 1.82

38 x 175 2.98 2.68 2.45 2.27 2.12

38 x 200 3.40 3.05 2.79 2.59 2.42

38 x 225 3.82 3.43 3.14 2.91 2.72

At the wall, joists should be fixed to a length of timber known as a wall plate and you should allow for this when buying the wood. The wall plate timber should be of the same width and depth as that required for the joists. If you are going to use a binder, this will be supported by timber hangers fixed to the joists above; again, you should allow for timber for the hangers.

Warning

Often the timber available will have a moisture content of 17 to 20 percent. If you use this timber in a room which has central heating, you will eventually get cracks in your new ceiling as the timber dries out. Ask for drier wood or store the timber for at least one month in warm, dry and well ventilated conditions.

Measuring and cutting

Measure up from the floor at least 2.3m at various points round the room and, with a long spirit level as a guide, mark this distance on the walls right round the room. This line indicates where the underside of the ceiling will reach, rather than the position of the joists. It will also show up any obstacles such as pipes, pilasters, and light fittings, which must be moved or cut round. If the line does not join up, check your spirit level for accuracy.

Trim the joists to size with a panel saw (if necessary) and cut sloping or raking notches in the wall plate at 400mm (or 16in) intervals, taking care not to cut through more than one-third of the thickness of the timber or you will weaken it. Use a sliding bevel to mark the joist ends with the correct angle to match the wall plate notches and cut the joist ends to shape.

Installing the basic framework

To fix a binder, cut holes in the old ceiling so you can screw timber hangers, spaced about 900mm (or 3ft) apart, to the sides of the joists above. Use two or three coach bolts to fix the binder horizontally to the bottom of each hanger. Use screws to fix a 38mm thick wood offcut, with a housing cut to the size of the binder end, to the wall. Position the binder end in the housing; if it is loose, drive timber wedges into the housing to ensure the binder end is securely held.

Fixing wall plate

If the house was built more than -60 years ago, the walls are likely to have lime mortar between the bricks. In this case you can nail through the wall plate into the bricks using 75 or 100mm (3 or 4in) cut nails at about 300mm (or 12in) intervals. Don’t use other types of nails, since they may not grip — some may bend over or even split the bricks.

A slower but safer method is to plug the wall. Drill holes through the centre line of the wall plate, place the wall plate in position and push a long nail through each hole to make pilot holes. Remove the wall plate and use a masonry drill to make 6 or 10mm holes at least 50mm (2in) deep. Drive hardwood plugs into the holes and fix the plate with 75 or 90mm No 10 screws.

If the wall is constructed with a timber stud framework, find out where the uprights or studs are by tapping; where the tap sounds dull rather than hollow, make a mark. The marks should be at about 350-450mm (or 14-18in) intervals and you should nail or screw through at these points to fix the wall plate.

In late Georgian or early Victorian houses, particularly if the wall has only 229mm (or 9in) thick brickwork, there may be a wall lining of rough battens at about 400mm (or 16in) intervals covered either with lath and plaster or with tightly stretched hessian which has been papered. It is usually not safe to fix the wall plate directly to the lining; cut away rectangular sections of it at about 600mm (or 2ft) intervals and, using cut nails or screws and plugs, fix blocks of wood to finish flush with the wall surface and fix the wall plate to these.

Warning

Treat all timber to be fixed directly to brickwork with wood preservative. When fixing, watch out for buried pipes and electric cables; avoid nailing, for example, directly above an electric switch.

Fixing joists

Joists should literally drop into the notches in the wall plate; if you have cut too much off a joist end, fill the gap between it and the surface of the wall plate notch with a thin piece of scrap plywood. Skew-nail with at least two cut or round wire nails through the joist and into the wall plate. As you work, from time to time use your spirit level to check the undersides, or soffits, of the joists are all in one plane. If you are working on your own, you will find it worth fixing a length of batten under the wall plate opposite the end of the joist you are fixing to support the other end of the joist.

When fixing joists to a binder, skew-nail with at least two cut or round wire nails up through the joist into the binder. Alternatively a more secure method of fixing is to use timber offcuts as hangers so the fixing nails are at right-angles to the weight they are supporting.

Allowing for access trap

Cut the trimmers (timbers which run at right-angles to the joists) and the trimmed joists which meet the trimmers at the proposed access trap. To fix the trimmers and the trimmed joists to each other — and the trimmers to the joists (trimming joists) which run down the ends of the access trap — you can use shouldered tenon joints. Keep the tenon to the middle third of the depth of the joist and fix it in position with at least two nails. Alternatively you can use metal joist hangers so the strength of the joist is not reduced by mortising. If you want your access trap to be square rather than oblong in shape, fix a length of timber between the trimmers towards one end of the trap at the right distance along the trimmers.

Fixing blocks

Plan where you will want to fix items such as light fittings to the ceiling. If these positions do not coincide with the joists, you should install fixing blocks by nailing blocks of wood between the joists.

Finishing the ceiling

Having completed the structure, you can now fix the cladding. Plasterboard sheets and tongued and grooved timber boarding are two of the most suitable kinds of material you can use for this. Using plasterboard sheets These come in standard 2400 x 1200mm (8 x 4ft) sizes, which may be too unwieldy — and you will need someone else to help hold them up while you fix them. Alternatively, you can use gypsum plank — plasterboard which is available in sizes of 600 x 2350-3000mm (2 x 71— 10ft); place the lengths across the joists. You may consider using foil-backed plasterboard, which is fixed with the foil uppermost to keep the heat in. When storing the plasterboard, take care to stack it neatly so the sheets are nearly vertical against a wall or the material may be damaged.

Using No 12 cadmium-plated nails to guard against corrosion, fix the plasterboard at not more than 150mm (or 6in) intervals and not closer than about 16mm to the edge. Start nailing at the centre of the fixing edge and work outwards to the ends. Drive nail heads flush with the surface. Don’t use a panel saw for cutting: with a straight-edge as a guide, use a very sharp knife to cut through the paper covering on both sides and break the board over a length of wood. Fix any rounded edges of the plasterboard with a small gap between them and tightly butt cut edges against each other. Fill the joints flush with the surface with gypsum (board finish) plaster or a proprietary filler to ensure a smooth finish.

For a really neat finish, use tapered edge or recessed edge plasterboard and stick down scrim tape along all the joints with wallpaper paste. Fill the joints flush to the surface with gypsum plaster, smoothing it down with a small float or flexible filling knife. If there are any irregularities in the filling, brush them away with a wet distemper brush. Line the whole surface with lining paper before decorating.

Tongued and grooved timber boarding

One advantage of using this material as cladding is that individual lengths are easily handled and all but very large rooms can be tackled single-handed. Order the boards slightly oversize so they can be trimmed to length when fixing. After you have bought them, store them in the room where they will be used so they become conditioned to the room temperature and humidity; if you do this, the risk of shrinkage when the cladding is in place will be greatly reduced. Before fixing, lightly rub the face which will be visible with fine glasspaper — or use an orbital sander fitted with a fine abrasive sheet — to clean off any dirt or raised grain.

The boarding should run at right-angles to the joists of the suspended framework. Measure between the walls at the new ceiling level and trim the first board to length, cutting for a tight fit; any extra trimming required can be carried out with a block plane working in from each edge. Place the groove of the first board hard against the wall and drive panel pins through the face of the board into the joists above, keeping the pins as close to the grooved edge as possible without breaking away the grain. Then drive panel pins obliquely through the tongue into the joists above for a secure fixing.

Cut the second board and place it in position, sliding the groove over the tongue of the first board. Use a hammer to tap the board into place along its length, protecting the tongue with an offctie from a tongued and grooved plank. Drive pins through the tongue of the second board. Continue this procedure until the final length is ready for fixing; this may have to be trimmed along the tongue edge before it can be eased into position. Then drive pins through the face of the board close to the wall at the joist positions.

To finish the job, fix mitre-cut lengths of scotia moulding in matching timber as cove. To bring out the colour of the grain and provide a protective coating, apply teak oil, matt polyurethane lacquer or another suitable sealer to the cladding. Apply two coats with a lint-free rag, allowing the first coat to dry before applying the second.

Line the access trap by nailing timber battens round the inside edges of the trap. Then fix strips of timber to these for the trap door to rest on. Fix architrave moulding to the ceiling surface with oval nails to overlap the timber battens. Measure up carefully so the inside edge of the architrave is flush with the inside face of the batten and mitre all corners for a neat finish. Cut a piece of ply-faced chipboard to size for the trap door and fix it with backflap or butt hinges, if the clearance above the ceiling permits hingeing.

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