Roof terraces
Roof Terraces: Structure, Waterproofing and the Surface You Walk On
A roof terrace is three pieces of work stacked on each other, and we carry out all three: the structure that holds it, the waterproofing layer everything else sits on, and the surface you actually walk on. Where a terrace already exists and is letting water into the room underneath, that is balcony and terrace repair rather than this page.
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The stack
What a terrace is, from the joists upward
Three layers, built in order, each one committed before the next arrives.
- The structure. Joists, boarding and the slope built into them, which is structural and decking work in its own right and is where a terrace is won or lost.
- The waterproofing. The layer that actually keeps water out, taken up every upstand and closed into every edge. On these decks we use the HYDRONYLON cold liquid system, which runs continuously round obstructions instead of being cut and lapped around them; whether a given surface suits it is settled on the deck itself.
- The surface. Paving on adjustable pedestals, or timber and composite decking, laid over the waterproofing rather than through it.
The difficulty
Why a terrace is the hardest thing on a flat roof
Because the part that keeps water out ends up buried under the part people stand on. On an ordinary roof a defect is visible, reachable and repaired in an afternoon. On a terrace it sits beneath a finished surface, so finding it costs the surface as well as the fault, and a small area of failed detail can mean lifting far more of what sits above it than the fault ever justified.
Everything therefore gets decided before the surface goes down. Where the water leaves, where the balustrade fixings land, what sits at the door, how anybody reaches the membrane again in ten years: those are design questions, and answering them afterwards means undoing work somebody has already paid for.
Boundaries
What we do not do
A terrace always raises three things that are not ours, and it is cheaper to know that now than to find out on the day.
- Balustrades and railings. A separate trade. We design the waterproofing around their fixings, which is why we need their positions early.
- Drainage runs and outlets under the surface. Installed by others. We build falls that deliver water to them and detail the membrane into them.
- The door threshold detail. The doors and their thresholds belong to whoever supplies them. We waterproof up to that junction and tell you what height we need to do it properly.
We work alongside whoever handles those, and on site we say which parts are ours as we reach them. Nobody should discover a boundary halfway through a job.
Water
Where water goes once a surface sits above the membrane
Down through the joints and along the waterproofing, which is the part that surprises people. Paving on pedestals is not a watertight surface and is not meant to be: rain passes between the slabs, lands on the membrane beneath and runs to the outlet across the falls built into the structure. Decking behaves the same way.
Which means the falls under a terrace matter more than on an ordinary roof, not less, and they have to work while carrying something on top of them. It also means the void under the surface has to stay clear enough for water to move and for leaves and grit to be got out again.
The surface
Pedestal paving or decking?
Paving on adjustable pedestals
Slabs sit on screw-adjustable supports that take up the fall, so the walking surface finishes level over a sloping roof. Slabs lift individually for access to the membrane and go back where they were. It is heavier than decking and takes more height, and it asks more of the structure underneath, which is why what the roof can carry is established before the surface is chosen.
Timber and composite decking
Lighter, warmer underfoot, and often the answer where height or weight is tight. It is fixed to a frame rather than resting loose, so reaching the waterproofing later means taking up more than one board. Timber needs maintaining and composite does not, and either way the frame sits on the membrane rather than piercing it.
The constraint
Does the door threshold allow a terrace at all?
Often that is the whole question, and it gets measured before anybody commits to anything. Everything above the structure raises the level somebody steps out onto, and the door has to keep enough drop below it for water to be shed away from the building rather than run back in.
The same arithmetic governs warm roof conversion, where added depth has to go somewhere, and it applies here with a surface on top as well. Sometimes it rules a terrace out. Hearing that at survey stage costs nothing; hearing it once the structure is up costs a great deal.
Occupied buildings
Building over rooms people are using
A terrace is almost always over somebody's ceiling, which changes how the work is sequenced rather than how it is built. Nothing is opened up that cannot be closed the same day, the room underneath is protected before anything starts, and the occupier is given the order of work in advance instead of finding it out.
On larger terraces and podium decks the same discipline scales up and the access question grows with it: residents crossing the deck, deliveries and lifting, and a surface that has to be usable again at the end of each stage. That is work we are set up for, and the arrangements are agreed in writing with whoever manages the building before anybody arrives.
Questions
Frequently asked questions
Can any flat roof take a terrace?
No, and it is the first thing to establish rather than the last. What the structure beneath can carry, whether the falls can be made to work under a raised surface, and what clearance the door leaves all decide it, and any one of them can rule a terrace out. Where the answer needs a structural designer we say so before anything else is discussed.
Do you supply and fit the balustrade?
No. Balustrades and railings are a separate trade and we work alongside whoever provides yours. What we do is make sure the waterproofing is designed around their fixings rather than pierced by them afterwards, which means the balustrade positions are needed before the membrane goes down, not after.
Paving or decking: which is less trouble later?
Paving on adjustable pedestals, in most cases. Slabs lift by hand for access to what is underneath and go back down in the same positions, so an inspection or a repair does not consume the surface. Timber and composite decking is usually fixed to a frame, which means more of it has to come up to reach anything.
Can you work on a terrace over an occupied room?
Yes, and most terraces are exactly that. The room below is protected and monitored, the roof is never left open overnight, and the sequence is planned so any part opened up in a day is closed the same day. The occupier is told the order of work in advance rather than discovering it.
Who is responsible for the drainage under the paving?
Whoever installs the drainage runs and outlets, which is not us. We form the falls that deliver water to them and detail the waterproofing into them, and we say plainly on site which parts of that boundary are ours. Where an outlet position is wrong for the falls, it is better to hear it before the surface goes down.
Thinking about a terrace up there?
Tell us what the space is, what is underneath it and what stage the plans are at. The height at the door is usually the first thing we will want to know.
We need a little more before we can send that: a name, the postcode of the roof, what is happening with it, and either a phone number or an email address.
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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.