Flat Roofs: Waterproofing, Falls and Drainage

Why a flat roof is never truly flat, how falls are created, the order of waterproofing and insulation layers, outlets and parapet details.

Flat roofs are widespread in contemporary villa architecture: simple volumes, the option of using the roof as a terrace and a clean facade line make them attractive. But unlike a pitched roof, a flat roof does not shed water immediately by gravity. Water stays on the surface, even if only briefly, and finds every weak spot. So a flat roof's reliability depends on three things working together: falls, waterproofing and drainage.

A "flat" roof is not actually flat

Despite the name, a flat roof is never built perfectly level. The surface is given a slight fall so water runs towards outlets or edge gutters. Without enough fall, ponds remain on the roof. Standing water speeds up membrane ageing, collects dirt and dust, loads the layers through freeze-thaw cycles in winter and, most importantly, turns a minor defect straight into a leak.

Falls are created in several ways:

• the roof slab itself is cast to a fall;
• a lightweight concrete or screed layer laid to falls is placed on the slab;
• ready-made tapered insulation boards are used.

The minimum fall is set by the requirements of the membrane system used and the applicable regulations. The design should clearly show fall lines, crickets that split the flow, and outlet positions, and these should be checked with a level on site.

The order of the layers

A typical conventional flat roof is built from the bottom up as follows: structural slab, vapour barrier, thermal insulation, a screed to falls (if the insulation does not provide the fall), waterproofing membrane and a protective layer. Each layer has a job, and removing one damages another.

The vapour barrier stops moisture rising from inside the house from reaching the insulation. Without it the insulation gets wet, stops insulating and blisters form under the membrane. Polymer-modified bitumen sheets, PVC or TPO membranes, and in some cases liquid polyurethane coatings are commonly used for waterproofing.

Another approach is the inverted roof. Here the waterproofing sits directly on the slab, and the insulation goes on top of it, weighed down with gravel or paving. The membrane is then protected from temperature swings and mechanical damage, but the insulation must be a special water-resistant board.

Drainage: where the water goes

Water leaves a flat roof through internal outlets or scuppers in the parapet. The number and size of outlets are calculated from the roof area and rainfall intensity. Important rules:

• every catchment zone should have at least one outlet, ideally with an overflow as well;
• outlets are fitted with leaf guards;
• where there is a risk of freezing in winter, heated outlets can be considered;
• internal downpipes are insulated so condensation does not form on them.

On a roof with parapets an overflow is especially important. If the main outlet is blocked by leaves or a plastic bag, water builds up inside the parapet and can rise above the height of the membrane upstand. An overflow then discharges water towards the facade at a safe level.

Details: the main source of leaks

Most flat roof problems start not in the open field of the roof but at junctions. The membrane must be turned up the parapet, walls, ventilation stacks and air-conditioning supports, with its top edge closed under a metal flashing or parapet coping. Around an outlet, the membrane is clamped into the outlet flange.

Equipment added to the roof later — air-conditioning units, solar panels, antennas — is the most common cause of damage. Waterproofed plinths for their supports should be planned at the design stage. Every bolt fixed through the membrane is a future leak.

Roofs used as terraces

If the roof will be used as a terrace or green roof, the system becomes more complex. A root-resistant layer, drainage mat, protection layer and finish — pedestal-supported paving, timber decking or a growing medium — are added above the membrane. Setting paving on adjustable pedestals is the better route: water runs freely beneath the slabs and the outlets remain accessible for inspection.

Inspection and maintenance

A flat roof is not a "build and forget" structure. It should be inspected at least once or twice a year, particularly after autumn and before heavy rains:

• are outlets and scuppers clear?
• does water still stand on the roof some time after rain?
• has the membrane come away at parapet and wall junctions?
• are there blisters, cracks or separations at the seams?
• is there visible damage around equipment supports?

Fixing a small defect in time is always easier than waiting until a leak shows as a stain on the ceiling. In its construction service Zaferoğlu İnşaat plans roof construction together with the architectural solution in the design.