Installation of Traditional Timber Roof Trusses

The first step is measuring the building. The drawings assume ideal dimensions, but real masonry is rarely perfect, and the ring beam or wall top may not be level all the way round. An experienced carpenter checks the actual dimensions first and only then starts preparing the timber.

A traditional roof frame is made up of several kinds of members:

• Wall plates sit on the load-bearing walls
• Rafters form the sloping roof planes
• Purlins support the rafters along the length of the roof
• Posts and struts carry the load further down
• Collar ties or binders connect opposite sides of the roof

Every piece must be the right size, cut accurately and protected against moisture, mould and wood-boring insects.

The installation itself usually follows this order:

1. Check the design, the structural calculations and the real dimensions of the building
2. Prepare, cut and treat the timber
3. Set the wall plates and anchor them to the ring beam or wall top
4. Fit the posts, purlins and rafters
5. Add binders, struts and wind bracing
6. Check the joints, anchoring, alignment and readiness for the next layers

Throughout the work, the carpenters keep measuring the pitch, the ridge height and the rafter spacing. A mistake in the roof frame carries through into the battens, the roof covering, the insulation and the roof windows.

Gable roof

This is the usual choice for family houses. It has two roof planes meeting at a single ridge. That makes it straightforward to design, quick to build and easy to check.

Hipped and half-hipped roof

On a hipped roof, the roof slopes down on all sides of the house. On a half-hipped roof, the gable ends are only partly sloped. The roof looks lower and stands up well to wind, which is why people often choose it for bungalows. The frame needs more angled cuts, hip rafters and precise connections, though.

Collar roof

A collar tie is a horizontal timber that links two opposite rafters and stiffens them. This type suits small and medium-sized houses where you want a habitable loft without posts standing in the middle of the room.

Purlin roof

On wider spans and more complex roofs, the rafters rest on purlins, which are held up by posts and struts. This is a stable solution. It does call for precise work and close cooperation with the structural engineer.

Speed of construction should not decide the roof type. What matters is how heavy the roof covering is, the span of the house, whether you plan to use the loft, where the roof windows will go and how much open space you want under the roof.

How demanding the installation is, and therefore what it costs, depends on more than the roof area. A simple gable frame on a level ring beam, with good lorry access to the site, goes up much faster than a complex roof. These things usually add time:

• More hips, valleys and dormers
• Chimneys and vent pipes passing through the roof
• A wider span and a larger volume of timber
• Poor site access and no space to store long timbers
• An uneven or damaged ring beam that has to be repaired first
• On renovations, taking down the old frame and strengthening the masonry

When you compare quotes, check whether each one covers installation only, or also the timber, treatment, fixings, delivery and handling. Two quotes with a different scope cannot be compared fairly. It is also worth planning the frame together with the work that follows it: sarking boards, breathable membrane, counter-battens and battens, insulation and the roof covering.

The roof frame is a load-bearing structure, so it belongs in the building design and the structural calculations. Timber structures in Europe are designed to Eurocode 5 (EN 1995), together with the national annex of the country concerned. In practice this means one simple thing: the timber sizes and rafter spacing should come from a calculation, not from a guess.

For a new building, the roof is part of the building approval. On a renovation, if you are changing the roof shape, raising the ridge or converting the loft, ask your local building authority in advance which procedure applies to your case under the local building regulations. Do not assume a simple notification will be enough. In a block of flats, the roof is the responsibility of the building management or the owners' association, so any work has to go through them.

Building a roof frame means working at height with heavy timbers. It is a job for experienced carpenters with proper protective equipment and fall protection. DIY installation is not recommended.

• Poor initial measuring, so deviations in the masonry only show up when the roof covering goes on
• Damp or badly stored timber that twists and splits after installation
• Wall plates that are not level or are poorly anchored, which affects the whole frame
• Rafter spacing that does not suit the insulation, roof windows or roof covering
• Inaccurate angled cuts at hips and valleys
• Missing or inadequate wind bracing, which is what stops the structure moving in the wind
• On renovations, a new frame placed on a damaged ring beam without dealing with the cause of the damp or the pests

In practice, a roof frame built cheaply and in a hurry often only reveals its faults when the roof covering is laid or the loft is insulated. Putting it right then means taking apart finished layers of the roof.

Before the roofers and insulation installers start, go through these points with the carpenter:

• The wall plates are level and firmly anchored to the ring beam
• The roof pitch and ridge height match the design
• The rafter spacing is even and suits the planned insulation and roof windows
• The roof planes are flat, with no visible sagging or waviness
• The joints are tight, all fixings are in place and nothing moves
• The wind bracing is installed as designed
• The timber is treated and free of rot, splits and signs of insects
• Openings for the chimney, dormers and roof windows are in the right places
• The site is tidy and leftover timber has been removed or neatly stored