Aluminium backed butyl tape is one of the few repairs that genuinely works first time. It is a thick, permanently tacky rubber sealant on a foil carrier, and it bonds to metal, concrete, plastic, glass, felt and most masonry. It will seal in the rain, which is usually when you need it.
What it will not do is fix a roof that has a structural problem. Knowing the difference is most of the job.
When butyl tape is the right answer
- A split or pinhole in a gutter, downpipe or gutter joint
- A hairline crack in a corrugated or metal roof sheet
- A lifted or failed flashing joint where the material itself is sound
- A seam that has opened up on a felt or rubber flat roof
- A leaking window or conservatory seam
- Sealing around a pipe or cable penetration
When it is not
- Missing or slipped tiles and slates. Tape over a gap is a patch on a symptom. Refit the tile.
- A sagging or spongy roof deck. That is water already in the structure, and sealing the top traps it.
- Rotten timber or crumbling mortar. Tape needs something sound to bond to. It cannot bond to material that is coming apart.
- You cannot find where the water is getting in. Water travels a long way along a rafter before it drips. Taping the spot where you see the drip usually seals the wrong place.
- Anywhere you would have to work at height without proper access. This is the important one. See the note at the end.
Surface preparation, which is the whole job
Almost every butyl tape failure is a preparation failure. The tape is not the variable.
- Get the loose material off. Wire brush or scrape away rust, flaking paint, moss, old sealant and lichen. You want a surface that is not going to detach and take the tape with it.
- Get the grease off. Wipe with a cloth and a solvent cleaner, then let it flash off. Butyl sticks well to clean metal and badly to oily metal.
- Dry it if you can. Butyl will bond to a damp surface, which is its great advantage, but a dry surface still gives a better bond. If it is raining, wipe the area immediately before applying and work quickly.
- Check the temperature. Our butyl tape performs from -15C to 45C in service, but it goes stiff and hard to work in the cold. If it is near freezing, keep the roll indoors until the moment you use it and warm the surface if you can.
Applying it
- Cut a length that overlaps the damage by at least 50mm in every direction. Do not just cover the crack, cover well past the end of it, because cracks travel.
- Peel back a few centimetres of the release liner rather than all of it. Butyl is aggressively tacky and will grab itself instantly if you let it fold over.
- Position one end, then lay the tape down progressively as you peel the liner, smoothing as you go. This is how you avoid trapping air.
- Press it down hard along the whole length. Use a roller, the back of a wallpaper seam roller, or a gloved thumb. Pressure is what makes butyl flow into the surface texture and seal, so this step matters more than it looks.
- Work the edges down especially firmly. A lifted edge is where water gets under and where the next failure starts.
- On a sloping surface, always lap upper pieces over lower ones, like tiles, so water runs over the joint rather than into it.
- Overlap any joins in the tape by 50mm.
Getting the most life out of the repair
Round the corners of your patch. Square corners are where peeling starts. Trimming the corners off with scissors takes ten seconds and makes a noticeable difference.
Do not stretch the tape as you lay it. Stretched butyl will slowly try to return, and that pulls at the edges.
Press it, then press it again. Go over the whole patch a second time after a few minutes.
Check it after the first hard rain, then again after the first frost and the first hot spell, because that is when thermal movement tests the bond.
Wear gloves
Cut edges on corroded metal roof sheet and old gutter are genuinely sharp, and rust makes for a nasty cut. A pair of leather work gloves is worth having on for this job, and they also keep the butyl off your hands, which is otherwise a solvent job to remove.
Know when to call a roofer
We would rather you fixed this yourself, and for a gutter split or a flashing seam you almost certainly can. But roof work is where DIY injuries happen, and a ladder on wet ground is not worth it for a patch.
Call a professional if the repair needs you on a pitched roof, if you would be working above a conservatory or glass, if the deck feels soft underfoot, if the leak has been going long enough to stain ceilings, or if you simply cannot locate the entry point. A roofer finding the actual source is cheaper than sealing the wrong spot four times.
The guidance here is general. It is not professional trade advice, and some roofing work is regulated. If you are in any doubt about a job, get a qualified tradesperson to look at it first.
The tape itself is here: butyl waterproof tape.