Do Roof Leak Sealants Actually Work?

What sealants are and what they are for

The products sold for roof leaks fall into a few groups. Bitumen mastic is a thick black paste that is trowelled over a crack or a joint. Self-adhesive flashing tape is a roll of bitumen and foil that sticks to a clean, dry surface. Liquid rubber or acrylic coatings are brushed or rolled on to cover a flat roof or a small area. Silicone and roofing sealant in a cartridge is gunned into gaps and seams.

Each one is intended for a specific job: bridging a hairline crack, resealing a small gap around a pipe, or patching a seam on a flat roof. They are made to fill and cover, not to rebuild.

Where they can hold

A sealant has the best chance on a sound surface that has a small, local defect. Flat roof felt that has split along a short line, an EPDM seam that has lifted slightly, a gap around a vent pipe, or the edge of a lead flashing that has opened up are all candidates. On these, a patch can keep water out for some time, particularly if the surface is clean, dry and primed as the maker advises. Many of these jobs are done in the dry months, because tapes and mastics do not stick to wet surfaces.

Where they fail

The weak spots are the places where the roof moves or sheds water in volume.

• Slipped or broken tiles. The tile needs replacing or re-fixing. A blob of sealant does not hold a slate or tile in place.
• Valleys and gutters. Water runs hard through these, and sealant tends to wash out or crack.
• Chimney flashings. The joint between brick and lead moves with the temperature and splits a sealant bead. Our guide to finding and fixing a roof leak explains how these faults are traced.
• Worn coverings. If the felt is crazed or the tiles are porous, a patch only moves the leak elsewhere.
• Damp or dusty surfaces. The product does not bond, so it peels within weeks.

The hidden cost of a quick fix

A sealant can stop the drip inside without stopping the water getting in. The water may travel along a rafter and appear somewhere else, or sit in the insulation and timber until rot begins. It is also hard for a roofer to find the original fault under layers of mastic, which can slow down and add to the cost of a proper repair. If you are making an insurance claim, a DIY sealant can also muddy the evidence, so photograph the fault first. Our guide to insurer evidence after roof damage explains why.

When a sealant is a sensible stopgap

If water is getting in tonight and nobody can come before morning, a tape or tarpaulin from the ground is worth considering, but only if you can reach the area safely without climbing onto the roof. Falls from ladders and roofs are a serious cause of injury, so do not go up in wet, windy or dark conditions. For a safe temporary measure that can be done by a roofer, see temporary roof repairs and tarpaulins.

How to apply one properly if you do

The surface is everything. Clean it of dirt, moss and loose material with a stiff brush, let it dry fully, and apply the product in dry weather above the minimum temperature on the pack. Press tapes down firmly with a roller so there are no air pockets, and overlap the edges well onto sound material on every side. Keep to the thickness the maker recommends, because a thin coat cracks and a thick one slumps. Keep photographs of the area before and after, and write down the date and product. This gives a roofer, or an insurer, a clear record of what was done.

Above all, work from a stable, safe position at ground level or from a proper platform, never from a ladder leant on a gutter or from the roof itself in poor weather.

What we do instead

We find the actual fault, photograph it, and repair it with the right material: a replacement tile, a new section of lead, a new flat roof patch or a re-bedded ridge. If a sealant is the right answer for a small seam, we use a professional product and tell you how long to expect it to last. If the roof still has years left in it, we say so and repair it. The fixed written quote comes after the survey, while the emergency and general roof repairs hub lists the types of work.