Roof membrane on flat roof: 7 mistakes you should avoid

Liquid roofing membrane is one of the easiest ways to get a tight roof. One coat with a roller or brush, and the roof is sealed for decades. But we see the same few mistakes repeated when people write to us afterwards. None of them are about the membrane itself. They are about what happens before and during. Here are the seven you should avoid when applying roofing membrane to a flat roof.

1. You are putting roofing membrane on a dirty roof

The most common mistake, and the most costly. Roof membrane bonds to the substrate, not to the layer of dust, algae and loose slate granules that lie on top. If the roof is not clean, the membrane sticks to the dirt, and the dirt doesn't stick to anything.

Sweep the roof thoroughly, rinse it with a garden hose or low-pressure cleaner, and let it dry completely. Remove any green deposits with an algaecide a few days before. If you can scrape some off with a fingernail, the roof is not ready.

2. You jump the cracks

Roofing membrane is elastic and seals small cracks in the surface. However, an open joint in the roofing felt, a loose edge or a hole in a penetration must be sealed first. Otherwise, the crack will continue to work its way under the membrane, and at some point the membrane will follow.

Use MaxFlex flashing for joints, risers and penetrations, and lay the roof membrane over it afterwards. This will create one continuous, waterproof layer all the way through.

3. You are laying the roof membrane too thinly

The membrane only lasts if there is enough of it. A 20 kg bucket covers up to 13.3 m², and that is not a number you should try to beat. If you stretch a bucket to 20 m², you will get a layer that is too thin to hold tightly and too thin to last for 40 years.

Measure the roof area, not the floor plan of the house. Divide by 13.3 and round up to whole buckets. If in doubt, you can calculate it on the product page, where the calculator gives you the number directly.

4. You forget about standing water

A flat roof with puddles, where water remains for days, is tough on all roof coverings. The roof membrane can handle it better than roofing felt, but the problem itself will not go away on its own. Water that cannot escape will always find the weakest joint.

Clean drains and roof drains before you start, and check that the water is actually draining away. If there is a deep puddle, level it out before you lay the membrane. This will help the roof membrane hold, and so will the rest of the roof.

5. You pick the wrong day

Roof membrane must be laid on a dry roof, at above 5 degrees, and there must be a few hours without rain afterwards. The membrane is dry to the touch in 1 to 2 hours and hardened after 24 hours. Rain in the first few hours can wash away the outer layer, and dew on the roof in the morning means that the membrane does not bond properly.

Start in the middle of the morning, when the dew has cleared, and don't apply it on a day when showers are on the way. You can read more about the seasons in our guide to when in the year you can apply roof membrane.

6. You scroll too quickly over the edges

The middle of the roof is easy. It's at the edges, the upstands and around the skylights that things go wrong. Most people roll too fast and too thinly there because it's difficult. And that's exactly where the roof tends to leak.

Use a brush at edges and penetrations, and pull the roof membrane at least 10 cm up along the edges and vertical surfaces. These are the places you need to spend time on. The rest will take care of itself.

7. You mix and dilute

Luna Roof roofing membrane is 1-component and ready-mixed. It does not need to be diluted, mixed, or primed beforehand. We regularly get questions about whether it can be thinned and made to last longer. You can't do that without compromising durability.

Stir the bucket well and apply it as is. That's the whole point of the product.

What a properly laid roof looks like

When you are finished, the roof membrane should lie as one continuous, black or white layer without visible transitions, and all edges and penetrations should be completely covered. The surface is slightly shiny for the first day and becomes matte when it has hardened. The roof is then sealed, and it will remain that way.

If you have a larger roof, or are you a craftsman and install roof membranes for others, you can buy on pallet with quantity discountAnd if you have any questions about your specific roof, write to us before you start. It's easier than fixing it afterwards.