Carpentry construction: roof truss price per linear metre 2026 and installation steps

The roof frame is the skeleton of your roof. It carries the covering with all its layers, plus snow and wind, and passes that weight down into the external walls and the internal load-bearing walls. A good design does more than stop the timbers from breaking, though. The roof must not sag, the rafters must not spread apart, and the whole structure has to cope with wind that does not only press down on the roof but also tries to lift it. This is called wind uplift, and people often forget about it when they choose lightweight metal roofing. Heavy tiles hold a roof down by their own weight. Light sheet metal does not.

Why do cracks sometimes appear just under the roof? Sloping rafters naturally try to push their feet outwards. Something has to hold that force back: collar ties, ceiling joists, the ceiling structure or a reinforced concrete ring beam. If someone removes the part that was doing this job during a renovation, for example when opening up a loft, the external wall under the wall plate can start to lean outwards. The usual giveaway is diagonal cracks in the corners just below the roof.

In a collar roof, each pair of rafters is joined by a collar tie to form a stiff A-shaped frame, with no posts in the middle. This type suits steeper gable roofs with smaller spans, roughly up to eight or nine metres depending on the pitch and the loads. The loft stays open, and the collar ties are often used to fix a ceiling to.

You will find purlin roofs on many traditionally built houses, especially across Central and Eastern Europe. The rafters rest on horizontal beams called purlins. These are carried by posts, and paired ties clamp the rafters and posts together into one solid unit. A purlin roof can span further, but those posts will stand in the loft, so you need to plan around them if you want rooms up there. Hip roofs add hip and valley rafters, with shorter jack rafters connected to them. Here careful measuring and cutting make all the difference: they decide whether the roof slopes end up flat or wavy.

A trussed rafter roof is built from factory-made trusses. They use slimmer timber, and the joints are held by steel nail plates with pressed-in teeth. The supplier designs and makes them to suit your project, and they arrive on site ready to fit. One rule has no exceptions: nobody may cut or drill them on site, and no diagonal may be left out. Every single member is part of the calculation. Standard trusses fill the whole roof space, but attic trusses are also available, and these leave an open space in the middle.

The structural engineer sizes the timbers for a specific strength class, for roof frames usually C24 to EN 338. Timber straight from the sawmill that nobody has graded may not be that strong. Structural timber should therefore be strength graded and marked to EN 14081. When the timber is delivered, look at the delivery note: it should show the strength class and the CE marking or the equivalent marking used in your country.

Freshly sawn spruce often holds more than 30 per cent moisture. If it only dries out once the roof is finished, it shrinks, twists and splits. Joints work loose and the roof slopes start to ripple. For roof frames, timber with a moisture content of usually no more than 20 per cent is recommended. KVH, a kiln-dried structural timber, is dried to roughly 15 per cent, planed and chamfered on the edges. It twists less, and insects are less keen on it. Glued laminated timber, known as glulam or BSH, is used where a longer span is needed or where the beams will stay visible inside the house.

In practice, timber often lies on site for several weeks straight on the ground with no cover. It belongs on bearers above ground level, with each layer separated by spacer battens, and covered so that air can still move under the sheet.

EN 335 sorts timber into use classes 1 to 5 according to how much moisture it is exposed to. A roof frame in a dry loft falls into the lower classes, a pergola standing in the rain into the higher ones. The class tells you which product to use and how to apply it. Brushing or spraying protects only a thin layer on the surface, dipping goes deeper, and pressure treatment at the factory lasts much longer. Keep in mind that every time a timber is shortened or drilled on site, untreated wood is exposed. Cut ends and tenons therefore need an extra coat of preservative.

Treatment against fungi and insects is not the same as fire protection. Fire-retardant treatment is only required for certain buildings, as set out in the fire safety design, and it is a separate product. No chemical will save timber that stays wet for long periods, however. Good detailing comes down to three things: the timber does not touch concrete or soil, water runs off it quickly, and air can circulate around it.

Snow loads are worked out to EN 1991-1-3 together with the national annex for your country. The national annex divides the country into snow load zones and also takes altitude into account. The same house may need deeper rafters in the mountains than on a lowland plain. The structural engineer also allows for the weight of the covering, the roof pitch, sheltered spots where snow drifts and builds up, and wind.

A qualified structural engineer carries out the structural assessment, not the carpenter and not the homeowner. You need one even when you change an existing roof: when you swap a light covering for concrete tiles, fit solar panels, add a dormer or want to remove posts to convert the loft. Solar panels do not only add weight. Their mounting brackets also pass wind forces into the rafters.

On a new building, the wall plate is usually anchored to a reinforced concrete ring beam with threaded rods or anchors chosen by the structural engineer. A strip of damp-proof course goes between the concrete and the timber so that the wood does not draw moisture out of the concrete. The rafters sit on the wall plate in a notch, called a birdsmouth, and are fixed with steel angle brackets or rafter ties. These hold the rafter down even when the wind tries to lift it.

Traditional carpentry joints such as mortise and tenon, halving joints and dovetails are still found on older roof frames, and when they are well made they last a very long time. On new buildings, steel connectors are becoming more and more common. What matters is that the carpenter uses exactly the types and numbers of screws or nails that the structural engineer allowed for. Ordinary building screws are no substitute for certified fixings.

A pitched roof only works if its layers go on in the right order. A breathable roofing membrane lies on the rafters, counter-battens run along the rafters on top of it, and only then come the battens that carry the covering. The counter-battens create a ventilated gap between the membrane and the covering, and moisture escapes through it from the eaves up to the ridge. If someone nails the battens straight onto the membrane, any water that gets under the covering is stopped by the batten and soaks into the structure. Also, do not leave the membrane uncovered for longer than the manufacturer allows, because sunlight breaks it down.

Solid timber boarding is needed under standing seam metal roofing, under bitumen shingles and on some low pitches. If you want to live in the loft, insulation goes between the rafters and a vapour control layer goes on the room side. Its joints and the junctions with the walls must be taped airtight. Why does that matter so much? Water vapour from the house that slips through a gap condenses in the insulation and on the timber in winter.

On an older house, it pays to inspect the roof frame piece by piece before any new covering goes on. Oval exit holes, fine wood dust and timber turning brown below the surface can point to house longhorn beetle or rot. To check whether the wood is sound, push a sharp tool into it: soft timber gives way easily. Damaged sections can be repaired by splicing, which means cutting out the bad part and joining in a new piece. The whole frame is only replaced when the damage is widespread.

Old roofs are often covered with asbestos cement sheets or slates. Only a licensed asbestos removal contractor may take them off, and the waste has to be handled as hazardous waste under local rules. Your local authority or environmental health office can tell you what applies where you live. If you break, cut or throw the sheets off the roof yourself, asbestos fibres spread into the surroundings, including your own garden and your neighbours'.

With free-standing timber structures, most mistakes happen where the wood meets the ground. Do not concrete the posts in or bury them directly in the soil. They belong in steel post bases anchored into concrete footings, so that the bottom of the timber sits above ground level and water can drip off. Bevel or cap the tops of the posts, because end grain soaks up water much faster than the sides of a timber.

Once a pergola has a solid roof, for example polycarbonate, it carries snow and catches the wind. It therefore needs properly sized timbers and bracing that keep it standing straight. If you attach it to an insulated facade, the fixings have to pass through the insulation without damaging it or creating a cold bridge. Whether a structure needs notification or planning permission depends on your local building regulations. For a pergola, a carport or any change to the shape of the roof, check with your local building authority in advance what you need to apply for. If you live in a block of flats, any work on the roof has to be agreed with the building management or the owners' association.

You can renew the wood stain on a pergola or garden building without any carpentry experience. Do not alter any load-bearing parts, though, and only work from a ladder at a safe height with someone nearby.

1. Choose a dry day without direct sunshine, with a temperature within the range given by the finish manufacturer, and at least one day after rain.
2. Check the timber by pushing in a screwdriver, especially at the post bases and the tops of the posts, and have any soft or rotten spots looked at by a carpenter.
3. Brush the surface clean of dust, cobwebs, moss and flaking finish.
4. Sand greyed and rough areas with abrasive paper along the grain and sweep away the dust.
5. Cover paving, plants and the neighbouring wall with plastic sheeting.
6. Brush on a thin coat of the same type of finish that was used originally, working along the grain.
7. Give extra attention to end grain, joints and the areas around metal fittings where water tends to sit.
8. Apply the next coat only once the first has dried as the manufacturer instructs, and check that water drains freely away from the post bases.

Inspect the roof frame before the membrane and covering go on, while everything is still in view. Check that:

• Cross-sections, spacing and positions of the timbers match the drawings and the structural design
• The timber has the declared strength class and is not visibly wet, twisted or attacked by insects
• The wall plate is level, anchored as designed and separated from the concrete by a damp-proof course
• The rafters lie in one plane and the ridge looks straight when you sight along the roof
• Angle brackets, rafter ties and anchors have their full number of fixings
• Cut ends and tenons have been given an extra coat of preservative
• No truss member has been cut, drilled or left out
• The timber keeps the fire clearance from the chimney required by the design and the chimney manufacturer
• You have the timber delivery note, proof of treatment and photos of the anchoring

Write every defect into the handover report before the roof frame is covered up. Once the covering is on, fixing the anchoring or replacing a timber means partly taking the covering off again.