Installation of Plasterboard Sloped Surfaces and Ceilings in Attics

The process is similar to an ordinary plasterboard ceiling. The difference is the base: a timber roof structure that moves with temperature and humidity throughout its life. So the boards are not fixed rigidly to the timber. They go onto hangers and metal profiles that absorb small movements.

1. Check the roof timbers: dry wood, sound rafters, no leaks and no signs of rot or insect damage.
2. Fit thermal insulation, usually mineral wool, between the rafters so that it fills the whole space without gaps.
3. Fix hangers to the rafters and clip the ceiling profiles into them, with perimeter track around the edges. Profile spacing depends on the board type and the system manufacturer's instructions. It is usually roughly 400 to 500 mm.
4. Lay a second layer of insulation below the rafters, between the profiles. This covers the timber and reduces cold bridges.
5. Install the vapour control layer on the warm side, facing the room. Tape all joins and seal it airtight to the gable walls, windows and any penetrations.
6. Screw the boards to the profiles, stagger the joints and chamfer any cut edges so they can be filled properly.
7. Fill the joints with reinforcing tape, sand them and prepare the surface for painting.

Will there be lighting or other wiring in the ceiling? Then a service void works well. This is a gap of a few centimetres between the vapour control layer and the boards, where cables and light boxes can run. The membrane then does not have to be pierced for every downlight.

Habitable attics normally use plasterboard roughly 12.5 mm thick. Where humidity is higher, for example above a bathroom, a moisture-resistant board is used. If the design or fire regulations call for higher fire resistance, fire-rated boards are used, sometimes in two layers. A double layer of boards also cuts down the noise of rain and wind.

• Sloped surfaces run along the rafters and end at the wall plate or at a low knee wall
• A flat ceiling is hung below the collar ties or ceiling joists and forms the horizontal part of the ceiling
• A combination of both is what you find in most attics. The line where the slope meets the flat ceiling must be straight, and the joint there needs proper detailing

Profiles, hangers, screws and jointing compound should all come from one system. Mixing parts from different manufacturers looks like a small thing. In practice it is often the reason for later cracks or sagging.

How demanding a sloped ceiling is varies a lot from one attic to the next. The same floor area can be finished quickly or take far longer.

• A straight, regular roof structure, a simple roof shape, good access and insulation already in place all make the work easier
• Crooked or unevenly spaced rafters have to be packed out or levelled first
• Every roof window, dormer, chimney or penetration means extra cuts, corners and details
• Many openings for lights, speakers or ventilation add time to both fitting and jointing
• A narrow staircase, or access only through a loft hatch, makes carrying the boards up difficult
• A higher finish level, for example under gloss paint or in strong side light, needs more filling and sanding

If the attic still needs insulation added before it is lined, expect the job to be larger than the lining alone.

In attics, mistakes often do not show straight away. The ceiling looks fine, and after the first winter cracks or damp patches appear.

• Fixing directly to the rafters without hangers: the timber's movement passes into the boards and the joints crack
• Too few hangers or profiles spaced too far apart: the ceiling sags or vibrates when someone walks in the loft above
• Holes and untaped joins in the vapour control layer: moist air from the room gets into the insulation, condenses and can lead to mould
• A rigid filled joint between the slope and the gable wall: a separating tape or flexible sealant is almost always the better choice here
• Rushing: fitting boards in a damp space straight after plastering or screeding, or painting over filler that has not dried

It often happens that the electrician drills holes after the boards are up and pierces the membrane. So agree the positions of lights and cables before the boarding starts.

If you are only replacing the old lining in an attic that is already lived in, this usually counts as ordinary maintenance. It is different when an unused loft becomes a new habitable room. Loft conversions usually have to meet building regulations on insulation, fire safety and escape routes, and some also need planning permission. Ask your local building authority before work begins.

In a block of flats, the roof space usually belongs to the common parts of the building. Discuss your plans with the building management or owners' association first. Wiring for ceiling lights must be done by a qualified electrician. The ceiling is fitted from a scaffold tower or stable platforms, not from a ladder leaning against a rafter.

When you accept the work, look at the ceiling in both daylight and artificial light. A lamp held so that it shines across the surface at a low angle works well. It shows up unevenness and poorly filled joints.

• Surfaces are flat, with no visible waves or sagging. A two-metre straightedge shows no deviations beyond the agreed finish quality
• The line between the slope and the flat ceiling is straight along its whole length
• Board joints are filled, with no cracks and no screw heads standing proud
• Junctions with gable walls and partitions have separating tape or flexible sealant
• Roof window surrounds are clean, the reveals are straight and the vapour control layer is sealed to the window
• Openings for lights are in the agreed places and have clean edges
• The installer can describe the build-up used: type of insulation, vapour control layer and boards
• No leftover waste or torn membrane remains in the loft space or the insulation