The short answer
As a stopgap, sometimes. A sealant or repair tape can slow a small leak at a clean, dry, sound joint, such as a hairline gap in a flashing or a split in a gutter joint, and buy a few weeks. They do not cure a fault, they rarely last on moving surfaces, and when used over wet or failing material they trap water and hide the cause. A lasting repair puts the right part back.
What sealants, tapes and liquid coatings can and cannot do on a roof, and how to use one without making the repair harder.
Where a sealant does help
A sealant has a place as a short-term measure. It works best on a small, clean, dry gap in a rigid joint: a hairline crack where a flashing meets a wall, a split in the seam of a gutter, or a pinhole in a metal run. Used there, a bead of the right product can slow a drip while you arrange for someone to look properly. Self-adhesive repair tape is similar, and it can hold on a clean flat surface for a while.
The words that matter are small, clean and dry. The surface has to be free from moss, dust, old sealant and moisture, otherwise the product sits on top without bonding. The product also needs to suit the material, since a sealant that grips glass may not grip lead or felt, and one made for gutters is not made for a roof slope.
Think of the result as a plaster on a cut. It protects the area for now, but it does not heal the underlying problem, and nobody can say how long it will last. If it stops the leak, treat that as time to book a survey, not as a repair that is finished.
Where sealants trap water and hide faults
Most roof leaks do not come from a gap that a bead of sealant can reach. Water enters beneath the visible covering, through a slipped tile, a rusted nail, torn felt or a worn valley, then travels along timber to drop somewhere else. Smearing sealant on the spot where the stain appears treats a symptom.
Applied over a damp surface or a cracked tile, a sealant can seal moisture into the roof, so timber and felt stay wet and the real fault goes on getting worse out of sight. Because the leak may stop for a while, the problem looks solved. When it returns, it is often bigger, and the roofer has to clean away layers of old product before the proper repair, which adds time.
The Energy Saving Trust gives a related warning about putting spray foam over a roof that already leaks and leaving the leak unfixed: "We don't recommend doing this." The same thinking applies to any coating laid over an unsolved fault. Our page on repairing a roof in the rain shows why damp surfaces defeat adhesives.
Tapes, liquid coatings and flat roofs
Flat roofs are where people most often reach for a product. Liquid coatings, rubberised sealants and bituminous tapes are sold to patch felt, EPDM and GRP, and on a small split in sound, dry membrane they can work as a temporary seal. They struggle where the roof ponds, because standing water softens a patch, and where the deck beneath is wet or soft, because a coating cannot hold on a surface that is moving or rotten.
Felt is expected to last between 10 and 20 years and EPDM between 20 and 30, according to published guides, and a roof near the end of that range may leak in several places at once. Patching each one is a poor use of money, and a re-cover may be the better buy. Our page on emergency flat roof repair goes through the options by membrane type.
Gutters and cracked chimney caps are another common target. A bead of sealant can calm a drip, but if the gutter joint has opened because the run has dropped, or the cap is crumbling, the part needs resetting or replacing, not coating.
What to do instead, and what to ask
If you have a leak and no roofer yet, the best move is a make-safe, not a tube from the hardware shop. A roofer can cover the area, find the cause and tell you whether it is a quick job or a larger one. Our page on whether a slipped tile is an emergency shows how to judge urgency.
If a sealant has been used already, whether by you or by a previous contractor, tell the roofer and say roughly when. They can assess whether it is still stuck fast, whether it is hiding a crack and whether the area needs to be cleaned back before the repair. Photographs of the roof as it was, if you have them, help.
When you ask for a price, ask what is being repaired and what is being covered. A good answer names the failed part, such as a tile, a lead soaker or a section of felt, and says what replaces it. A vague answer that mentions only a coating, a seal or a treatment should make you ask more. Our survey ends with pictures of the fault and a single written total, so you can see what the money pays for. The emergency and general roof repairs page explains how that works for urgent jobs.
More on this in Emergency & General Roof Repairs in Crawley
Two more sealant questions
Is roof sealant as good as a proper repair?
No. A proper repair removes the failed part, such as a broken tile, torn felt or worn flashing, and puts a sound one in its place. Sealant covers the fault and relies on the bond holding. It is a temporary measure for a clean, dry joint, and it is not a substitute once a roofer has found the real cause.
Can a roofer repair over old sealant?
Usually the old product has to come off first, because new material will not bond reliably to it, and a sealant layer can hide cracks beneath. That adds a little time to the job. Telling the roofer where and roughly when sealant was used lets them allow for it in the total they quote.