Converting a Trussed Roof

Telling the two apart in thirty seconds

Put your head through the hatch and look at the timbers. This is the single measurement that most affects what a conversion will cost, and it takes no expertise.

A trussed roof looks like a series of identical triangular frames, closely spaced, made from relatively thin timbers — often around 35 to 50mm thick. Inside each triangle are diagonal members forming a W or M shape, and the joints are made with toothed galvanised steel plates pressed into both faces. The space is criss-crossed and you cannot walk through it. These are prefabricated in a factory and craned on. They are standard in houses built from roughly the mid-1960s onwards.

A cut roof is built on site from heavier timber. You see rafters running up each slope, one or two horizontal purlins partway up supporting them, and ceiling joists across the bottom. There are no diagonal webs through the middle, so the space is open and walkable down the centre. The joints are nailed or bolted, with no pressed plates. Typical of houses up to the 1960s.

An attic truss is the exception worth knowing about. Some newer houses were built with trusses specifically designed to leave a clear rectangular space in the middle, with the webs pushed out to the sides. If your loft has a clear open area bounded by vertical timbers at each side, the house may have been built conversion-ready, and the structural work required is minimal.

The galvanised plates are the giveaway. If you can see pressed steel plates at the joints, it is a truss, whatever the shape looks like.

Why you cannot just cut one out

This matters enough to state plainly, because people do it.

A truss is not a rafter with some extra bits. It is a single engineered component in which every member is doing a job, and the diagonal webs are carrying load in tension and compression between the rafter and the ceiling tie. Remove one web and the frame is no longer the thing that was calculated; the rafter above it loses its support and can deflect or fail.

Trusses also work as a set, braced to each other laterally. Cutting one transfers its load to its neighbours, which were not designed to take it.

The failures this causes are not always immediate, which is what makes it dangerous. A roof with a cut truss can stand for years and then deflect under snow load, or spread at the eaves and push the walls out. By then the damage is structural and extensive.

The same applies to the ceiling tie across the bottom. It looks like a floor joist and it is not — it is holding the two rafter feet together, stopping the roof spreading. Cutting it to make a floor is one of the more serious things that gets done in lofts.

So the sequence is always: install the new structure first, transfer the load onto it, confirm it is bearing, and only then remove the truss members.

What the restructure involves

1. A structural engineer designs the replacement. This is not optional and the calculations go to Building Control.
2. New steel beams at floor level, spanning between the supporting walls, to carry the new floor and take the load the ceiling ties were resisting.
3. New floor joists spanning between or hung from those beams, forming a floor that can carry a room.
4. A new support structure for the roof slopes — typically a steel ridge beam, or new purlins carried on steels, so the rafters have support partway up their span.
5. New rafters, or the existing truss rafters retained and strengthened, depending on the design.
6. Temporary propping throughout, holding the roof while the transfer happens.
7. Only then, removal of the truss webs and ceiling ties.
8. Lateral restraint straps tying the new structure back into the walls.

It is well-understood work and any competent loft conversion contractor does it regularly. The point is that it is a real piece of structural engineering happening before the conversion proper begins, and it needs design, steel, propping and crane or manual handling of beams up through the house — all of which is why it costs what it does.

What it adds, and what else to check

Head height is the second issue with many trussed-roof houses and it compounds the first. Houses from the 1970s and 80s frequently have shallower roof pitches than earlier ones, so even after restructuring there may not be the height for a comfortable room. Measure the ridge height before paying for a structural survey.

Before you commit

• Measure floor to ridge. Under about 2.2m and a rooflight conversion will not work regardless of the roof type, and a trussed roof with low height is the least promising combination.
• Check the pitch. A shallow pitch gives little usable width even at a good ridge height.
• Look for an attic truss. A clear rectangular void in the middle means the house may be conversion-ready.
• Check what the trusses sit on. They bear on the external walls, and the new steels need to bear somewhere adequate too — usually the same walls, sometimes needing padstones.
• Consider the alternative. Where the restructure cost is high and the head height marginal, an extension or a garage conversion may deliver the same space for less.
• Get the engineer in early. A structural assessment before a detailed design tells you whether the scheme is viable and roughly what the steel will be.
• Do not take a builder's word that the trusses can simply be cut. That is the one answer that should end the conversation.

What it costs

These are the loft conversion figures published across this site:

Those figures assume a cut roof. A trussed roof needing a full restructure sits above them, because the steel, the design and the programme all increase before any of the conversion work begins. It is the single biggest variable in a loft quote, which is why establishing the roof type is the first question worth answering.

Frequently asked questions