Trafficked Roof Deck and Terrace Assembly

A trafficked roof deck has water at two levels at once. There is the surface people stand on, which sheds rain into its joints, and there is the waterproofing below, which is the level that actually has to reach an outlet. The assembly exists to move water that has passed the surface across to a drain that nobody can see, without the traffic above damaging the layer doing the work.

The boundary against the warm deck flat roof is not a layer order at all. A terrace can be built over a warm deck or over an inverted deck, so being walked on does not tell you where the insulation sits. This entry owns everything above the waterproofing: the wear surface, its supports, the void between, the upstand heights a finished level forces, the door threshold conflict and the fact that the outlets are now under something.

Two demands pull against each other throughout. The surface should be level and comfortable, while the roof beneath it must fall towards its outlets, and something has to absorb the difference between the two planes. That absorbing layer is where the assembly's void comes from, and the void is also the space debris collects in, drainage runs through and anything mounted on the roof has to be founded in.

Access is the last part of the concept rather than an operational afterthought. Because outlets, upstands and the waterproofing all sit under the finished surface, the layout of that surface decides whether the roof can ever be maintained. A terrace laid as a continuous bonded field over its outlets has been designed to be opened up destructively.

The level surface and the falling roof beneath it only coexist because the support layer varies in height across the roof. That variation makes the void deepest where the roof is lowest, which is exactly where the outlets are, so the deepest, least visible part of the assembly is also the part that has to stay clear for the roof to drain.

Water divides at the wear surface and is never reunited. Some runs off the paving to its edges, and some passes through the joints into the void, where it meets a plane with its own falls. If the surfacing sheds towards a perimeter while the waterproofing falls towards a central outlet, the roof has two drainage layouts working against each other rather than one working in layers.

The threshold is where the assembly argues with the building. A doorway that has to be level for use and an upstand that has to be measured up from the concealed waterproofing are competing for the same vertical distance, and neither the surfacing nor the waterproofing can concede it alone. The usual resolution is drainage placed in front of the door and a change in the build-up around it, which is a design decision rather than an adjustment made on site.

Anything anchored through the surfacing lands somewhere on the waterproofing. Guarding posts, screens and planters all concentrate load and require fixings at the perimeter, which is the same zone where the waterproofing is already turning up an upstand. That coincidence is why guarding support is agreed with the roof design rather than added to a finished terrace.

Wind acts differently on loose-laid surfacing at the edges and corners of the roof than in the middle of it. Units near the perimeter, at corners and around any upstand face conditions the field does not, which is why the edge of a terrace is often arranged differently from its centre even though it looks continuous.