A patch on a shingle roof looks like a fix from the ladder. A gray blob of roof cement sits over a crack, the area around it looks dark and smooth, and nothing is dripping. The question a homeowner needs answered is whether that patch is keeping the water out or only covering the place where the water got in. From the ground, those two conditions look the same, and they lead to very different repair decisions.
Most homeowners find an old patch when they notice a stain, a soft spot in the attic, or a shingle that has lifted in a storm. By that point the patch may have been there for years, and nobody remembers who put it on or what was under it. That is the situation this page is written for. If you are deciding whether to pay for another patch, a spot repair, or a larger job, the old patch is the first thing to examine.
What a roof cement patch is made of
Roof cement, sometimes sold as plastic cement or asphalt mastic, is a thick tar-based compound. It is spread with a trowel over a crack in a shingle, over a nail head, or along the edge of a lifted tab. It stays soft in warm weather and stiffens as it cools. On a St. Louis roof that means it works through the summer, flexes poorly through a hard freeze, and can split along its own edge when the deck moves during a cold snap in January.
Patches are often applied in layers. A crew comes back two years later, sees the first patch splitting, and adds a second over the top. Some older roofs carry three or four generations of compound, each one thicker than the last. Each layer adds stiffness and weight to the shingle beneath it, and each layer hides a little more of what is underneath. The top layer may look fine in June while the layer beneath it has been cracking since the last freeze.
Why the patch can hide the crack
A crack in a shingle is usually not visible from above. Cracks often begin at the nail line, at the edge of an exposed course, or where a shingle has been lifted and set back down many times. Compound covers the top of the crack, but water can still run along the slot beneath the patch, reach the underlayment, and travel down the deck to the rafter before it shows anywhere inside. A stain can appear several feet from the patch for this reason, because water follows the deck and the framing before it drops through the ceiling.
The clearest sign of a hidden crack is a patch that looks intact on top but feels loose or cold at the edges, or a patch whose edges have pulled free from the shingle. Lifting the edge of a patch with a putty knife is one way to look, but it can tear the shingle, so that check should be done by a roofer who can see what they are doing. Ask for photos before and after the check, so you have a record of what was under the compound.
Water also takes the path of least resistance once a patch has failed. If the patch sits on a shingle that is already lifting at the tab, the water may come in at the lift and be carried sideways under the shingles above it. That is why the stain in the attic may not line up with the patch on the roof. The patch is one clue, and the attic is the second. Both should be checked before anyone decides where the leak starts.
The three questions to ask about an old patch
Before deciding whether to repair again, ask three questions. First, how many layers are on the spot? One layer of compound on a sound shingle can be a reasonable temporary measure. Three layers on a shingle that has lost its granules is usually a sign that the shingle needs to be replaced, not sealed. Second, does the shingle still bend? A shingle that cracks when it is lifted slightly has lost the flexibility needed to be pulled out and replaced cleanly. A roofer can test this gently on an edge that is already loose. Third, is the crack in the shingle itself, or in the seal line where the shingle meets the course above it? A crack at the seal line may point to a problem with the sealant strip rather than a torn shingle, and the repair is different.
If the answer to the first question is more than one layer, and the answer to the second is that the shingle does not bend, the patch is no longer a repair. It is a place where the roof is waiting to fail, and every new coat of compound adds a little more weight to a shingle that cannot carry it.
What a proper repair looks like
A proper repair starts with removing the old compound, not adding new compound over it. The roofer lifts the surrounding shingles carefully, scrapes away the old material, checks the underlayment and the deck below, and then decides whether the shingle can be replaced with a new one. A shingle that has been sitting under compound for years is often brittle along its nail line and will not come out clean. In that case the crew cuts it out, slides in a new shingle, and seals the nail heads with a compatible sealant. Keys installs Atlas or Malarkey shingles, and the crew can match the exposure and the color of the surrounding roof where the new shingle goes.
The roofer should also check the nail heads in the area. A nail that has backed out of the deck, or one that was driven at a slant through the shingle, lets water in on its own, whether or not a patch is present. If the patch was covering a nail, the nail needs to be reset or replaced, and the shingle over it needs its own check. A patch that was hiding a nail is not a repair of the shingle, and it should be written up that way.
Timing matters as well. Compound applied in cold weather may not cure properly, and compound applied in direct summer sun can soften enough to slide. A repair done on a warm, dry day, with the surface clean and the shingles at a workable temperature, is more likely to hold. If the work has to be done in cold weather, ask the roofer how the crew will handle sealing and whether the repaired area will be checked after the next freeze.
What to do before the roofer arrives
You can do a few things yourself before anyone climbs the ladder. Photograph the patch from the ground, and if you can do it safely, photograph it from a second angle with a coin or ruler for scale. Note the date of any stain you have seen inside the house, and take a photo of the stain itself. Write down when the patch was done, if you know, and who did it. If the patch was put on by a previous owner, the records may be in the closing file or with the prior seller.
Do not climb the roof to test the patch yourself. A wet or frosted roof is slippery, and the compound is more likely to tear under a person's weight than under a trained crew's. Keep the attic hatch clear so the roofer can look at the deck from below, and do not put boxes or insulation over the area until the inspection is done. The attic view is often where a hidden leak shows its path.
What to ask the roofer on the first visit
- How many layers of roof cement are on this spot, and will the crew remove them or work over them?
- Will the shingle be replaced, or only sealed, and why?
- What does the deck look like under the patch once the old material is cleared?
- Will you send written notes and photos of the area before and after the work?
- If the leak returns after the repair, who comes back to look at it, and on what schedule?
A roofer who answers the first question with "we will just add another coat" is describing the same repair that has already failed once. A roofer who says the old material has to come out first is describing a repair that starts from the shingle, not the stain. Both answers are worth hearing before you pay for anything.
When to stop patching
Patching is a reasonable short-term step when the roof is otherwise sound and the crack is isolated. It stops being reasonable when there are several patches across one slope, when the shingles are curling or cupping, or when the granules are gone from the exposed area. At that point the money spent on more compound is usually better spent on a plan for the slope or the whole roof. Missouri roofs in their second or third decade, with patches on more than one slope, often reach that point, and the cost of repeated patching can add up to a meaningful share of a replacement.
If you want someone to look at the old patch, check the shingles around it, and tell you in writing which path the roof is on, call the Keys office at (314) 220-2333. Ask for the written notes and photos before you agree to any work, and keep them with the rest of your roof records.