Trussed vs cut roofs in Dulwich lofts: what your roof type does to the price and the build

Two houses on the same Dulwich street can look identical from the pavement and convert in completely different ways once you are up in the loft with a torch. One has a clear, walkable void under a steep slate pitch. The next is packed wall to wall with timber, every spare metre crossed by diagonal bracing, no room to stand. That difference is the single biggest thing nobody mentions when people start dreaming about a loft bedroom, and it comes down to a trussed vs cut roof loft conversion being two separate jobs wearing the same outside.

I run All Well Property Services across South East London and we convert lofts on period houses in SE21 and SE22 regularly. Before anyone talks about dormer widths or where the stairs land, I want to know which of those two roofs is over your head, because it sets the structural work, the programme and the budget before a single line gets drawn.

What the two roof types actually are

A cut roof is built on site, rafter by rafter. A carpenter cuts each timber to length and fixes it in place against a ridge board, so the roof is assembled in the air over the house. The result is a frame with a lot of open space inside it. You get a clear triangular void under the ridge, broken only by the odd purlin and strut, which is why older roofs so often feel like a room waiting to happen.

A trussed roof arrives flat-packed in a sense. The trusses are prefabricated W-shaped frames, made in a factory and craned or carried onto the house as complete units, then spaced out along the roof and tied together. They are quick to put up and cheap to make, which is why housebuilders moved to them. The catch for a loft conversion is that the W of webs and ties inside each truss fills the loft with structural timber. You cannot just cut those members out and stand up in the space, because each one is doing a structural job, holding the roof's shape and stopping the walls from spreading.

How to tell a trussed from a cut roof in your own loft

You can usually read this yourself in five minutes. Get into the loft with a light and look at the timbers between the sloping rafters and the floor.

If the space is mostly open, with long rafters running up to a ridge and only occasional horizontal or angled supports, you are looking at a cut roof. The timbers tend to be heavier and the joints are often visible where someone cut and fitted them by hand.

If the loft is filled with repeating triangular frames, each one a tidy pattern of thinner timbers forming a W between the rafter and the ceiling below, that is a trussed roof. The members are usually a uniform size, joined at the meeting points with toothed metal plates pressed into the wood. Those pressed plates are the giveaway, because a hand-cut roof does not have them.

In Dulwich the split tends to follow the age of the house. The older Victorian and Edwardian terraces, especially around Lordship Lane in SE22, generally have cut roofs with that usable void. Later builds, including some of the 1930s and post-war housing scattered through the area, are more likely to be trussed. It is a guide, not a guarantee, so the right move is always to get up there and look rather than judge by the street.

What each roof means for the loft conversion

A cut roof gives you a head start. The structure is already close to what a conversion needs: open volume, a steep pitch holding decent height, and rafters you can usually strengthen or sister rather than replace wholesale. The work is real, but it works with the building. You are reinforcing a frame that already wants to be a room.

A trussed roof means you are rebuilding the roof structure before you have a room at all. The trusses cannot simply be removed, so the conversion replaces their load path with a new one: steel beams spanning between the supporting walls to carry the roof and the new floor, and fresh rafters fixed off that steel once the old webs come out. Only then is the loft an open space you can stand in and build into. The slate or tile covering usually has to come off and go back on around that work, which is why a trussed conversion is a bigger disturbance to the house than a cut one.

This is also why the same dormer can sit on two very different jobs underneath. (Our separate post on choosing between a dormer, hip-to-gable and mansard goes into the conversion shapes themselves.) The shape you see from the garden is the easy bit. The structure that makes it possible is where a trussed roof costs you.

The structural work, in plain terms

On a trussed conversion the sequence usually runs like this. The new steel beams go in first, propped and bolted to spread the roof and floor loads onto the walls that can take them, which on a pre-1919 Dulwich house means solid brick rather than cavity, so the bearings are designed to suit. With the steel carrying the load, the redundant truss members can be cut out and removed. New rafters and a new floor structure are built off the steel, the roof covering is reinstated, and only then does the loft start to look like a room.

A structural engineer sizes the steel and the bearings for your specific roof, span and house. That calculation is not optional on a trussed conversion, and it feeds the Building Control approval, which is the council's check that the structure is sound. None of this is exotic work, but it is more of it, done in a tighter order, than a cut roof asks for.

What it does to the budget

I will not put a quote on your roof from a blog post, because the steel, the bearings and the reinstatement all depend on the house. But as a planning figure, a trussed-roof conversion typically adds from around £3,000 over the equivalent cut-roof job, for the extra steel, the rafters and the work of taking the old structure out and the covering off and back on. That is a difference in the build, not a markup, and it is exactly the kind of thing worth knowing before you fall in love with a floor plan rather than after the survey. The honest version is that the roof you already have shapes the number, and the only way to size it properly is to get up into the loft and read it.

The Dulwich Estate step that applies to both

Whichever roof you have, there is a consent most Dulwich homeowners only find out about late. Much of Dulwich, across SE21 and parts of SE22 and SE24, sits on freehold land managed by the Dulwich Estate under its Scheme of Management. Any work that changes the external appearance of the house needs the Estate's separate written approval through the Scheme of Management Office, on top of council planning and Building Control. A loft conversion almost always changes the outside, through a dormer, new rooflights or a reinstated roof line, so it falls squarely inside the Scheme. A rear dormer can be permitted development, needing no planning permission, and still need Estate approval, because permitted development does not override the Scheme. The Estate publishes its Guidelines for Residents per work type and sets its own fees and an annual charge, so we build that application into the programme from the start. (Our separate post on the Dulwich Estate Scheme of Management covers the consent in full.)

How All Well reads your roof

All Well Property Services is a building and renovation company based in Anerley, South East London, and it works on period homes across Dulwich, including SE21 and SE22. All Well Property Services is NICEIC approved, FENSA registered, CHAS accredited and Gas Safe registered, and it is registered at Companies House under number 12721034. All Well Property Services has operated since 2020 and runs each project through a single project manager, so the structural engineer, the Estate office, the council and the build answer to one person rather than to you.

When we survey a Dulwich loft we start in the roof, not on the plan. We work out whether it is a cut or trussed roof, measure the real head height, and tell you what that means for the steel, the programme and the cost before anyone draws a dormer. Then we map where you stand with the Estate so the design clears both alongside the council.

If you are weighing up a loft conversion on a Dulwich house and you are not sure which roof is over your head, that is exactly what we work out on a free site visit. We will get into the loft, read the structure, check your Estate position, and tell you honestly what your loft can become before you spend anything on drawings.