Flat roof in Auderghem

What does flat roof work involve

A flat roof (or one with a very slight slope) has to be watertight across its entire surface, without the benefit of gravity to drain the water quickly. Waterproofing work consists of laying a continuous covering, without joints or breaks, that stops water reaching the structure.

Several systems exist depending on the age and type of roof:

• EPDM: a synthetic rubber membrane, laid as a single sheet, heat-welded or glued. Durable, flexible, suited to complex shapes. It's the system we recommend most often for new installations and renewals.
• Bituminous membrane (roofing felt): several layers of reinforced bitumen, torch-welded. Very widespread on existing Belgian roofs. Can last a long time if laid carefully on a well-prepared surface.
• Traditional roofing: the common term in Belgium for bituminous coverings. Often what you find on extensions and garages from the 70s-90s.

When should a flat roof's waterproofing be redone

The most obvious signs: water on the ceiling below the roof, saltpetre traces on the walls, damp coming back despite replastering. On a flat roof, the leak isn't always directly above the interior stain: water can travel under the membrane before coming down.

Other less urgent but important signs: blisters on the membrane (water trapped underneath), visible cracks in the covering, upstand edges coming unstuck. It's not a leak yet, but it's a leak in the making.

A well-laid flat roof lasts 20 to 30 years depending on the system and maintenance. If yours is approaching that limit and hasn't been inspected, better to look before the interior damage starts.

Why a flat roof lets water in

The main cause is the natural ageing of the membrane. Bitumen or EPDM weaken with the heat-cold cycles, UV rays, and the movements of the structure. The upstands (the raised edges against walls and parapets) are the most vulnerable points: that's where the membrane is under tension and where the detachment starts.

Blocked outlets make everything worse. A flat roof with a blocked drain means water standing for hours or days on the membrane. In time, even a good-quality system won't withstand it.

Installation errors also play a part: membrane insufficiently glued, upstands too short, or a structure too rigid that moves without the membrane being able to follow.

What makes the price of a flat roof renewal vary

The surface is the first factor, but not the only one. The state of the substrate determines a good part of the budget: if the insulation under the membrane is waterlogged, it has to be removed and replaced — a line item that can be a significant share of the quote. Laying a new membrane on wet insulation is pointless.

The chosen system matters too. EPDM costs a bit more than bitumen to install, but needs less maintenance in the long run. For a small roof with a simple shape, both work well.

Access and the height of the building affect the time on site. A ground-floor garage is simple. A flat roof on the 3rd floor with difficult access takes longer.

Finally, if we combine with insulation on top of the roof (inverted insulation), the intervention costs are shared and the end result is much better. Roof insulation can qualify for Renolution grants.

Repair the flat roof or redo it entirely

For a localised leak on a membrane still in good condition, a targeted repair is often enough: we re-weld or re-glue the damaged area, and rework the affected upstand. That's settled in a few hours.

But if the membrane is old, blistered in several places, or the insulation underneath is damp, a complete renewal is preferable. Patching a tired membrane means buying time now for a more expensive job in two or three years. We tell you what each situation deserves.