Converting a loft: what holds the roof up, and what replaces it

What is up there now

Open the hatch of an older house with a cut roof and this is roughly what you find. The rafters are the two slopes, from the ridge down to the outer walls. Part way down each slope, a strut carries it down onto an internal wall. Across the bottom run the ceiling joists. They hold up the ceiling of the rooms below and tie the feet of the rafters together, and they were sized for plasterboard, not people.

Two kinds of roof

This model has a cut roof, built piece by piece on site. Newer houses usually have trussed rafters: triangles made in a factory, with diagonals inside them. Trussed rafters became common later in the 1945 to 1979 period and are typical from 1980; older houses usually have cut roofs. Typical for the period; we confirm it from photos.

In a trussed roof the diagonals are what hold the roof up, and none of them can be cut out. That roof needs new structure, designed as a whole, before anything is removed. The rest of this sequence is a cut roof, like the model.

The struts are in the way, and they are working

The struts stand exactly where a room wants to be. They are also doing a job: they carry the roof slopes down onto the walls below, and from there to the foundations and the ground. So they cannot go until something else does that job.

Steel goes in first

One answer is a steel beam along the ridge and a steel purlin under each slope, each drawn end on as a steel I because it runs away from you. They bear on padstones in the party wall, our half, and on the gable, out of this view. It is one way of several.

Held at the top, the rafters no longer push the walls out at the eaves. Until now the ceiling joists, and any collars, stopped that as ties.

Now one strut can go

Only now does the front strut come out, shown dashed. The rear one waits for the dormer. The order is the whole point: the new support goes in before the old one comes out, so the roof is never left with nothing holding it.

A floor for people

The new floor is a new set of joists at close centres, laid beside the old ceiling joists and clear of them, so the ceiling below carries nothing new. On this section they span front to back, between walls that run into the page: both outer walls and both internal walls. Where a span is too long for timber, a steel beam picks the joists up part way.

The model changes

The photograph is now the converted model: the struts gone, the floor in, and a full-width dormer on the right-hand slope. Everything drawn over it is still the engineering.

The dormer has its own structure

It runs from the party wall to the gable. Its frame is drawn here on its own. Rafters are cut to make the hole. Each side of it, a doubled trimming rafter runs down the slope. A steel purlin across the top carries the cut rafters above it and one end of the dormer’s flat roof joists. Those run out to the dormer face, which stands on the rear wall at the eaves. The cheeks, the side walls, are framed on the trimming rafters or the new floor.

Where the new load goes

The roof now comes down the rafters to the steels and the outer walls. The new floor hands its load to all four walls. Each wall takes it down to its foundation, and the foundation spreads it into the ground.

The steels carry their share into the page, to the party wall and the gable. None of it comes down through the room.

The stair, and next door

The new stair comes up through the new floor, so the floor needs an opening, trimmed like the dormer’s hole.

On a semi or terrace, each steel bears in a party wall, in our half, never through it. That is work to a party structure, notifiable under the Party Wall etc. Act 1996 at least two months before the work starts.

Source: Party Wall etc. Act 1996, section 3.