Turning an Existing Flat Roof Into a Terrace

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

It is careful to say that being walked on does not tell you where the insulation sits, because a terrace can be built over a warm deck or over an inverted deck. What the terrace owns is everything above the waterproofing: the wear surface, its supports, the void between, the upstand heights a finished level forces, the threshold conflict and the fact that the outlets are now under something.

The record asks whether the structure has been assessed for the weight of the surfacing, its supports and the intended use of the terrace. That is three things rather than one: what is laid, what carries it, and what will be on it and standing on it once it is in use.

Where the roof is inverted there is more. The inverted record asks whether the deck has been assessed for the weight the arrangement relies on, including any surfacing intended later, because in that build-up the weight is doing mechanical work rather than sitting on top of the roof. Both records send the question to the engineer responsible for the deck rather than answering it.

The guarding record describes the assembly as a load path in which the weakest link is normally the substrate behind the fixing, and states that guarding fixed to a finish, a coping, an outer leaf or a surface build-up has been fixed to something that was never designed to take it. On a roof terrace the structure able to receive that load lies below insulation and a water-resisting layer.

That produces the conflict the record names directly: every base fixing is doing a structural job and a weathering job at once, and the two pull against each other. A rigid fixing concentrates movement at the closure around it, while a fixing allowed to move slightly protects the closure and softens the chain, and neither is resolved by sealant alone. The trafficked record adds that guarding posts are concentrated at the perimeter, which is the same zone where the waterproofing is already turning up an upstand.

The trafficked record calls the door the single most demanding junction in the assembly. 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.

The threshold record describes the trade in a chain: each increment of level access removes upstand, the removed upstand has to be replaced by interception, and interception is only worth anything if it discharges somewhere that will not surcharge back into it. It also notes that where the external surface is a roof, the consequences of an unresolved junction move from an inconvenience at a front door to a leak into a room below.

Once a surface is laid, the outlets serve the concealed level and are reached through whatever is above them. The record states that whether they can be inspected and cleared without dismantling the terrace is decided when the paving layout is set out, not when the roof needs maintenance, and that a terrace laid as a continuous bonded field over its outlets has been designed to be opened up destructively.

The void is also where everything washed through the joints ends up. The pedestal record notes that debris falls through the joints and collects where nobody looks, and that the first sign of a blocked outlet is often water appearing somewhere unrelated to the blockage. How the void is kept clear, and who is expected to do it, is a question the record puts at design stage.

This is the difference that most changes what a terrace means. Where the insulation sits above the waterproofing, the surfacing is also acting as the ballast holding the stack down, so lifting paving for maintenance removes restraint as well as access. That is not true of a terrace over a warm deck, and the record draws the distinction explicitly.

The inverted record explains why the restraint matters: the weight above is resisting wind from one direction and flotation from the other, because water standing on the waterproofing surrounds loose-laid boards and boards surrounded by water want to rise. It cautions that removing ballast or paving, even temporarily and in a small area, removes wind restraint from the layers beneath and should not be done without advice.

The records treat the terrace as something that has to be maintainable and alterable, and both of those are decided by the layout rather than discovered later. Which areas can be opened, how surfacing is returned afterwards, how debris is removed and where guarding posts pass through the layers are all described as provisions decided at design stage.

There is also a warning about ordinary later work. Fixing anything into a completed terrace, including a planter, a screen or a light, may penetrate a concealed water-resisting layer, and the record sends that to the roof designer rather than treating it as a domestic job. The pedestal record adds that standing a heavy fixed object on a loose-laid terrace converts part of the assembly into something that must be broken to reach the deck.

The record is unambiguous that this is not a design preference. Whether a roof may be occupied, and what guarding and escape provision that brings, is determined by the relevant authority and the designer, and access requirements at the threshold come from the project's access strategy and the relevant authority rather than from any general reference.

That makes the first move an enquiry rather than a specification. Establishing who is making those determinations, and on what basis, comes before choosing paving, because the answer can change the loading, the guarding, the threshold and the route out.