Roof drainage in practice: fall, wedges, and the fight against pooling

If you ask an experienced roofer what wears most on a flat roof, the answer is rarely the sun or frost alone – it is standing water. Water that stays in depressions on the roof surface, the so-called hollows, keeps the membrane wet and cold for long periods, collects dirt and algae, freezes to ice in winter, and eventually finds its way through the smallest weakness in a joint. Therefore, drainage is not a detail in the roof project – it is the foundation itself.

§How much slope should there be?

The industry's recommendation as established in the former TOR guidelines from the Roofing Paper Industry Information Council and today continued in manufacturers' instructions is a fall of 1:40 on new roofs — that is 25 millimeters fall per meter. Falls down to around 1:100 have been considered absolute minimum but are discouraged by the industry: With so little fall, execution must be perfect because even small deflections in the deck or inaccuracies in the insulation create voids. With 1:40 there is margin for reality.

§Wedge-cut insulation: slope built into the insulation

On many flat roofs, the concrete deck or wooden deck itself is horizontal, and the slope is created instead with wedge-cut insulation: insulation boards that are cut with a slope so the roof surface after insulation slopes toward the drains. The supplier develops a wedge plan — a drawing of how the wedges, direction changes, ridges, and gutters should lie — and the roofer's task is to lay the plan precisely in reality. If two boards are swapped or a wedge is reversed, a pocket forms, which is only discovered when it rains.

• 01Follow the wedge plan slavishly — each pane has its place and direction.
• 02Check deck flatness before placing wedges; major deviations must be reported to site management.
• 03Put the knife down with the blade pointing inward/downward when you leave the station.
• 04Check with straight level or laser that slope actually goes toward drains before membrane seals everything

§Roof drains, downpipes, and emergency drains

Water must not just go away from the surface — it must go into the drainage system. Roof drains are placed at the low points of the roof surface, and the membrane is connected to the drain's flange tightly and according to the manufacturer's instructions; it is one of the details where most errors occur. Gratings and leaf guards must be mounted so the drain does not clog with leaves and slate granules. In addition, many roofs are designed with emergency drains or overflow — e.g. spouts through the wall crown — which come into operation if the main drains clog during heavy rain, so water does not build up above the roof edge height and seep into the building.

§Inspection upon delivery

Drainage must be inspected before the roof is handed over. In practice, this is done by inspecting the roof surface after rain — or by controlled watering — and seeing if permanent water accumulations remain when the surface has had time to drain. Minor wet areas that dry quickly are normal; actual puddles with permanent water are a deviation that must be recorded and handled. Document the inspection with photos and record it in quality assurance — it protects both the customer and you.

“Water is the world's most patient craftsman. It finds the fault, no matter how small it is — so find it before the water does.”

— Mouth luck among roofers