Structure

Flat Roof Structures, Joists and Decking

A flat roof is a structure with a covering on it, and most of what decides how it performs is finished before any waterproofing arrives. We build that part too: the joists and firrings, the deck, the falls, the kerbs around whatever comes through the roof, and the junction where new work meets the building that was already there.

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The parts

What a flat roof structure is actually made of

Four things, each of them decided rather than left to habit. Get any of them wrong and the covering spends its life compensating for somebody else's shortcut.

The joists. The timbers that carry everything, sized and spaced by what the roof has to hold and how far it has to span. That is a design question, not a habit, and where it needs a designer we say so.
The firrings. Tapered timbers fixed along or across the joists to create the slope. They are how a flat roof gets its fall without the ceiling below going out of level, and they are the part most often left out of a cheap job.
The deck. The boarding the covering is bonded to. Its material, thickness and how its joints are arranged all change what can go on top, so it is specified alongside the waterproofing rather than chosen on the day.
The kerbs. The raised frames around rooflights, hatches and anything mechanical, built square, built high enough for the covering to be dressed up them properly, and built before the roof is closed. Where the roof is also going to be stood on, the same discipline governs building a roof terrace.

Falls

Why the fall is designed rather than discovered

Because no flat roof is truly level, and the moment one is treated as though it were, water starts deciding for itself where to sit. The slope, the direction it runs and the point it runs to are worked out before anything is cut, together with where the water leaves and what happens if that route blocks.

Skipping it produces the commonest fault we survey. Standing water is not fatal to a modern covering, but it goes to work on every seam it can reach, and correcting it later is expensive because the correction sits under the waterproofing. What standing water actually does to a roof is worth reading first.

Junctions

Where new structure meets an existing building

This is the part that separates a roof that lasts from one that is watertight for a season. New timber meeting old brickwork, an extension tying into a house, a dormer cheek meeting a pitched slope: each is a junction between two things that move differently and were built decades apart.

The structural side of that is settled alongside the waterproofing rather than before it, because each keeps changing the other. How the new work is supported and fixed, what it bears on, how much height there is at the wall for the covering to turn up properly, and where existing structure has to be opened up and made good.

Re-roofing

Replacing a deck that has failed

A deck that has been wet for years does not recover, and no covering bonds to boarding that gives under a foot. Where a strip and renew opens up soft or rotted boarding, replacing it is structural work we carry out ourselves rather than an extra line on a roofing quotation.

How much comes out is genuinely unknowable until the covering is off, which is why a quotation for a re-roof separates the deck we can see from the deck we cannot. Where joists underneath have gone as well, usually at the wall or around an old outlet, that is found at the same moment and priced the same way: shown to you, written down, decided before anybody carries on.

Working together

Working alongside a builder

On an extension or a loft conversion the carpentry often belongs to somebody else, and that works perfectly well provided two things are agreed rather than assumed.

The first is who sets the slope. A builder boarding to our specification is an ordinary arrangement; a builder boarding level because nobody said otherwise is a roof that will pond from the day it is finished. The second is when we are needed on site: after the structure and boarding are complete, before any other trade starts walking on it, and before internal finishes fix the heights we have to work to. The wider programme question, including who supplies what on a new roof for an extension, is set out there rather than repeated here.

Before pricing

What has to be settled before we can price it

A structure cannot be quoted honestly while the things that determine its depth are still open. Four answers unlock a real figure: what the roof has to carry, where the water is meant to go, what is coming through the roof, and what finished height the building can accept at the wall and at any door.

Until those exist, any figure is a guess dressed as a price. Where they exist only as a sketch, the written report from a survey turns them into something to build from and to price against.

Limits

Where a structural engineer takes over

Some of this is carpentry and some of it is design, and the honest position is that we do not blur the two. Anything that changes how load travels through a building, forms a new opening of consequence, or asks an existing structure to carry more than it was built for needs a designer, and we will tell you that rather than sizing it by eye on the roof.

What we then do is build to what the design requires. The approved details for the project decide, the specification records them, and where a kerb, an opening or a rooflight is involved, the structural drawing and the waterproofing detail are read together before anything is cut. Guessing at that on somebody's behalf is how roofs end up rebuilt twice.

Questions

Frequently asked questions

Do you build the structure as well as the covering?

Yes. Joists and firrings, the deck itself, the kerbs around openings and the tie-in to the existing building are all work we carry out, not something we ask somebody else to leave ready for us. On plenty of jobs a builder does the carpentry and we specify what we need; on plenty of others we do both, and the quotation says which.

Do I need a structural engineer?

Sometimes, and you will hear it from us rather than find out later. Anything that changes how load reaches the walls, opens a new hole in a roof of any size, or alters what an existing structure carries belongs to a designer. We then build to what that design requires, and the specification records the approved details rather than our opinion of them.

Can falls be corrected without rebuilding the whole roof?

Often, yes. Where the joists themselves are sound, the slope is usually reset in the layer above them rather than by touching the structure, which is why so many ponding problems turn out to be cheaper than feared. Whether that applies to your roof is a survey question, because it depends on what is under the covering and how much height is available.

Who is responsible if the builder has already boarded the roof?

Whoever laid the deck is responsible for it, which is why we look at it before agreeing to waterproof over it. Where boarding has gone down at the wrong falls, in the wrong material or with the joints in the wrong places, we say so in writing and price the correction separately. Covering a deck we have doubts about would move the problem rather than solve it.

Building a roof, not just covering one?

Tell us what is planned and what stage it is at. If a designer is needed we will say so early, which is the cheapest time to hear it.

Coming out and pricing the work is free; a written condition survey carries a fee, agreed before the visit and taken off the price of the work if you go ahead.