Leaky roof? How to find the hole and seal it

The stain in the ceiling is almost never right below the hole. Water runs along the rafters and insulation before it shows up inside, which is why so many people spend an entire weekend looking in the wrong place. Here's the order a roofer goes, and what you can do yourself.

The five places a flat roof most often leaks

The surface itself is rarely the problem. It's the transitions:

• Collections and deliberations in the roofing felt where the glue has come off
• Upward edges against masonry, fire ridge and neighboring building
• Skylights and light boxeswhere the covering has loosened
• Penetrations for ventilation, exhaust, antenna and solar cells
• Drains and roof drainswhere leaves collect and water stands

If you follow these five places, you will have found the cause in most cases.

How to find the hole

1. Aim from the inside, but lead from the outside

Mark where the stain is inside and measure the distance to the nearest exterior wall. Water travels, so move in a radius of a few meters upwards from that point, never just straight across.

2. Walk the roof on a dry day

Look for cracks in the surface, raised edges, bubbles, and places where the stone coating on the roofing felt has worn away. A bubble often means moisture trapped in the structure, and this needs to be factored into the calculation.

3. The water test

If you're still in doubt, grab a garden hose and water one area at a time for ten minutes while someone else watches inside. Start at the bottom of the roof and work your way up. If you water the entire roof at once, you're still not knowing anything.

4. Check for rain, not during

If puddles of water remain a day after rain, the rain has fallen too badly in that area. This is not a leak in itself, but standing water wears down the roofing felt faster and is often the reason why the same spot leaks again.

Patch or seal the entire surface?

A patch on one hole will do if the rest of the roof is healthy. On the other hand, if the roofing felt is dry, cracked or worn over most of the surface, there will just be a new hole somewhere else next winter. Then it is cheaper to seal the entire surface at once.

A liquid roof membrane applies as one continuous layer over joints, ridges and penetrations, so that there are no edges left to penetrate. It is applied cold with a roller or brush, and the roof is dry again after an hour or two.

When a membrane is not enough

The membrane seals the surface. It cannot fix what is underneath. Get a professional to do it if:

• the insulation is wet and has been for a long time
• there is soft or dark wood in the sub-roof or rafters
• the roof has settled, so there is a visible gap
• there is mold inside along the outer wall

In these cases, the structure must be dried or replaced first. If you seal over wet insulation, you lock in the moisture.

Do it on time

A hole the size of a coin costs nothing to seal. The same hole a year later has typically taken the insulation, and then it's a completely different calculation. If the roof is dry inside first, sealing is the cheap way to go.

If you need to calculate your own roof, you can enter the square meters into the calculator at product page, then it puts the right number of buckets in the basket. If you are in doubt whether the roof can handle a membrane, send us a few pictures and we will take a look.