Installation of Timber Trussed Rafter Roofs

Trusses can span the full width of the house and carry the load down to the external walls. You need fewer internal load-bearing walls, so the layout can be more open. On a simple gable or hipped roof over a regular floor plan, you also benefit fully from factory precision and fast installation.

So why are they not right for every house? A standard truss has a web of diagonal members between the bottom and top chords. These diagonals fill the roof space, so a standard trussed roof does not suit a habitable loft. There are trusses designed for room-in-roof use, but the design has to include them from the start. On very complex roofs with dormers and unusual details, manufacture and installation become much harder. A traditional cut roof built by a carpenter may then be the more sensible choice. Compare both options at the design stage, not after the walls are already built.

The roof is part of the approved building design. If you want to change the roof shape, pitch or height compared with the approved drawings, check with your local building authority first. Different changes need different procedures, and a simple notification is not always enough.

The most important work happens before the crane arrives. The manufacturer needs the floor plan, sections, roof pitch, type of roof covering, overhangs, the position of the chimney and any penetrations, and details of the insulation. From this, the manufacturer prepares the structural calculations, the production drawings and an installation plan. On site, installation usually runs like this:

1. Unload the trusses and check that every piece is there and undamaged
2. Set the first trusses as shown on the installation plan, usually the gable or end trusses
3. Fix temporary bracing so the trusses cannot lean or topple sideways
4. Place the remaining trusses one by one at the specified spacing
5. Fix every truss to the ring beam or wall plate with the specified holding-down fixings
6. Add the permanent bracing and check that the whole structure is plumb, straight and stable

Each truss has an exact position in the roof, and pieces must never be swapped by guesswork. A single truss standing on its own is very unstable sideways. The roof only becomes a rigid, stable structure once it is braced, so this stage must never be rushed. The fixings have to carry more than the weight of the roof and snow. They also resist wind uplift, which on light metal roofing can literally try to lift the roof off.

Once the structure is complete, the roofers follow with the breathable membrane, counter-battens, battens and the roof covering. Fast installation only pays off if the roofers carry on without a long gap, so the building does not stay open to the rain.

Precisely made trusses need an equally precise building to sit on. In practice, delays are often caused by an uneven ring beam or poor site access, not by the trusses themselves.

• The building measured before production: wall lengths, diagonals, heights and the level of the ring beam
• A finished, properly cured concrete ring beam or a wall plate with fixing points ready
• Steel members and beams already in place, if the design includes them
• Access for a long delivery lorry and firm ground for storing the trusses
• A solid, level spot for the crane, if one is used
• Scaffolding or working platforms, with obstacles around the house cleared

Coordinate the follow-on trades in advance as well. The chimney installer, the ventilation contractor and the solar panel installer should know where their penetrations will go and what loads the roof will carry. If the overhang, roof covering or chimney position changes after production, the trusses cannot simply be altered on site.

Installation is simpler, and therefore cheaper, when the roof is regular, the span is ordinary, the ring beam is accurate and both the lorry and the crane can get right up to the house. A crew can then often set the roof structure of a family house in a few days.

The job becomes harder, and more expensive, in these situations:

• Large spans and long trusses that are harder to handle
• A complex roof with hips, valleys and many penetrations
• Heavy roof coverings such as concrete or clay tiles, which need heavier members
• A location with heavy snow loads or strong wind exposure
• A narrow or sloping plot where the crane is hard to set up
• An uneven ring beam that needs correcting, and waiting time because the site is not ready

When you compare quotes, check exactly what each one includes: structural design, production drawings, timber strength class, delivery, unloading, crane, fixings, fasteners and bracing. A quote that leaves something out can end up costing more. If one company makes the trusses and another installs them, agree in advance who deals with inaccuracies and missing pieces.

Work at height with heavy components under a crane belongs in professional hands. A homeowner should not attempt to install trusses alone.

The biggest risk is altering trusses afterwards. A truss works as one designed unit, so a cut diagonal, a shortened chord or a hole drilled for a pipe can weaken the whole roof. Every change like this must be approved by a structural engineer or the manufacturer.

• Weak or missing temporary and permanent bracing
• Truss spacing different from the installation plan
• Too few fixings, or a different type of fixing than specified
• Trusses swapped and placed in the wrong positions
• Trusses stored directly on the ground, without bearers or protection from moisture
• No check that the roof is straight before battening and laying the covering

Prefabricated trusses with nail plates are covered by the European standard EN 14250, which sets requirements for manufacture, materials and joints. You can ask the supplier for the declaration of performance for the product.

Before the roofers start, go through these points with the installation crew:

• Every truss is in its place according to the installation plan and at the correct spacing
• The trusses are plumb, and the roof plane looks straight when you sight along the ridge and eaves
• Every truss is fixed to the ring beam or wall plate with the specified fixings and fasteners
• The permanent bracing is complete, not just the temporary battens
• The nail plates are pressed in over their full area, not lifting or bent
• No member has been cut or drilled without the structural engineer's approval
• The timber is not split or damaged from handling
• You have received the documents: structural calculations, installation plan and declaration of performance