What a coating actually does
A roof coating is a liquid, usually brushed or rolled on, that dries into a flexible film over the felt. Its job is to cover the surface so sun, frost and rain wear on the coating instead of the felt beneath. Some are reflective, some seal hairline cracks, and some are thick enough to bridge small gaps.
That is all it does. The coating is a skin on a skin. It adds no strength to the roof and takes nothing out of the structure. Where the felt is firm and watertight, a coating can slow the decay. Where the felt is cracked, split or lifting, the coating can look tidy for a while and then lift with it.
The condition of the surface on the day matters more than the product. Dirt, moss and loose grit have to be cleaned off before anything goes on, or the coating bonds to the dirt and not to the roof. Brushing a coating over a damp surface can seal in the very moisture it was meant to keep out.
Where felt life stands
Age tells you whether a coating is a sensible spend. HomeBuilding gives felt (torch-on) a typical life of 10 to 15 years, and Less puts it at 10 to 20 years. The two guides disagree, so both are named, and both are ranges, not promises. A roof built to the shorter end can need attention sooner than one built to the longer.
| Guide | Typical felt life |
|---|---|
| HomeBuilding | 10 to 15 years |
| Less | 10 to 20 years |
If your roof sits well inside those figures and shows only surface wear, a coating is a reasonable thing to ask about. If the roof is at or beyond them, coating it is likely to delay a decision, not avoid it. The real age of a roof and the age on paper often differ, especially where earlier repairs were patched over, so a survey is the better guide than a date on a receipt. Ask what the felt feels like underfoot, whether the laps are tight and whether the ceiling below has stained, before anything is bought.
Faults no coating reaches
Three faults are beyond any coating.
- A wet deck. If the boards beneath are damp or soft, the new film will not dry them, and it can seal the dampness in.
- Failed laps. Where the sheets overlap, a lap that has opened lets water in at the join. A coating across the join may hide the gap without closing it.
- Blisters and splits. A bubble or a crack is a break in the covering, and it needs cutting out and patching, not painting over. Our page on blisters on a felt roof explains the difference.
There is also the matter of what happens later. Once a roof has been coated, any future repair has to deal with the coating as well as the felt, which can complicate a patch and make the surface harder to bond to. If the roof is likely to be replaced within a few years, a coating may be money that is spent and then stripped off with the felt. That is worth weighing against the cost of simply replacing the covering now. Ask any roofer who suggests a coating what they expect it to add in years, and what they will check first.
Coat, repair, or start again
The choice is easier than it looks once you know the state of the deck and the laps. Sound felt with years in hand can be coated, and the coating is then a sensible way to slow the wear. Felt with a few local faults can be repaired and left. Felt that is cracked, blistered and soft underneath is past both, and a new covering is the straight answer. Deciding between the three is easier with photos of the laps, the edges and the ceiling below.
Laying a new layer over the old felt is another route, but it has its own limits; overlaying an old flat roof explains when it helps and when it does not. For the bigger decision, repair or replace a flat roof goes through each case in turn.
Send close photos of the surface, the laps and any stains inside. We look at the roof before we price it, and you get a single written total once the roof has been looked at. A coating is only ever offered when the roof can carry one. More on this work is under flat roofing.
