Surfacing a Terrace Over Occupied Space

Outlets, upstands and the waterproofing all sit under the finished surface, so the layout of that surface decides whether the roof can ever be maintained. The published entry states the consequence in one line: a terrace laid as a continuous bonded field over its outlets has been designed to be opened up destructively.

That is the question, and the four answers differ on it more than they differ on appearance, cost or feel underfoot. It is worth asking before the surfacing is chosen rather than when something below needs attention.

Adjustable supports of differing extension stand on the deck and take up its falls, units rest on those supports, and the joints between them are left open. Water arriving on the surface passes straight through into the space beneath and runs down the deck falls to the outlets, so the trafficked layer is a wearing surface and a shading layer rather than the layer resisting water.

The test can be applied while standing on the terrace: can a single unit be lifted by hand? The entry also names what removes the advantage. Bonding units, filling joints, bedding a course at the edge or standing a heavy fixed object on the terrace all convert part of the assembly into something that must be broken to reach the deck.

A trafficked liquid-applied system removes the covering entirely, so every requirement lands on the same build-up: it has to stay continuous, resist abrasion, hold a textured surface, tolerate sunlight and be renewable at some point in its life. Those requirements are not naturally compatible, and the layers are an attempt to distribute them.

It is bonded, which means it has no independent life from what it is stuck to. It follows the substrate's movement exactly, it reveals the substrate's cracks unless the reinforcement carries across them, and it removes the void and the two levels along with it. Where a platform has little depth to give away, that is often exactly why it is chosen.

Over a structural deck the same units may be mortared down or laid loose on supports, and the two give opposite answers on access and drainage. In bound paving the unit is bonded to a base, so lifting a unit means breaking it, and there is no open void beneath to drain, ventilate or inspect.

The build-up also provides no drainage path downward, so all water is expected to leave at the surface. Over occupied space that changes what the shaping of the surface is doing, because any water that does get in through a crack or a failed joint has no way out and works on the bond from underneath.

A timber deck spans: joists carry the surface between supports the deck itself provides, so the framing is the entity being described. Pedestal paving does not span at all, because loose units rest on adjustable supports over a structural deck that already exists.

Its surface is deliberately open, so rain passes between the boards rather than running off them, and the frame below is repeatedly wetted and has to be able to dry again between wettings. Where a framed platform sits over an existing surface, the falls of that surface and the freedom of its outlets decide whether the void drains or holds standing water against the timber.

Load does not spread through loose units the way it does through a bedded surface. A point load on a slab corner arrives at a pedestal head and then at a small patch of whatever lies below, so the protection layer, the insulation where present and the water-resisting layer are all loaded through a concentrated area.

The roof build-up beneath changes what that means. On an inverted roof the insulation sits above the layer resisting water, so supports bear on insulation and its protection rather than on the membrane, and the surfacing is also acting as the ballast holding the stack down. Lifting paving for maintenance then removes restraint as well as access.

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 void is deepest where the roof is lowest, which is exactly where the outlets are, so the deepest and least visible part of the assembly is also the part that has to stay clear. Open joints are commonly assumed to keep the void clean, when in practice they are the route by which debris gets into it.

Three things run through every one of these entries. Which areas can be lifted for access, and how the surfacing is returned afterwards. Where guarding is anchored, and whether that fixing passes through the water-resisting layer. And how the threshold at the door is resolved between a level entrance and the upstand the roof needs.

All three are decided when the surfacing layout is set out rather than when the roof needs maintenance, and all three involve more than one trade. What is handed over about lifting units and clearing the void is what decides whether the advantage of a reachable assembly is real.