Asphalt roof repair means one of two completely different jobs, and the first thing to settle is which. On a sloped roof, asphalt means shingles, and repair means replacing units and reworking flashings. On a flat or low slope deck, asphalt means a built up or modified bitumen membrane, and repair means cutting out failed material and rebuilding the layers. Buying the second repair for the first roof, or the other way round, wastes the money and usually leaves the leak running.
That matters because most commercial buildings with asphalt on them have been repaired by whoever was cheapest and nearest, and asphalt is forgiving enough to hide a bad repair for a season or two. By the time the patch fails, the record of what was done has gone, and the next contractor prices from guesswork.
Two roofs, one word
A built up roof is layers of reinforcing felt bonded together with asphalt, then surfaced with gravel or a granulated cap sheet. Modified bitumen is its modern form: the same asphalt with polymer added for flexibility, supplied in rolls and bonded by torch, hot asphalt, cold adhesive or a self adhesive backing. Both are multi layer membranes, and both are repaired by rebuilding layers.
Asphalt shingles are a different animal. Each one is nailed near its upper edge and bonded to the course below by a strip of factory adhesive. They shed water by overlap rather than by sealing, so a repair is a matter of replacing units and getting the flashings right. Shingle repair on a commercial building covers that roof in full. This page is about the flat one.
Where a built up or bitumen roof actually fails
It fails at the laps and the details long before the field of the roof gives out. The field is several layers thick; a lap is a line where two sheets meet, and a detail is where the membrane has been asked to turn a corner or go around something.
| What you see | What is happening | What it needs |
|---|---|---|
| Open or lifted lap | The bond failed or was never made | Cut back, rebond, new cap strip |
| Blister | Air or moisture trapped between plies | Cut out and rebuild plies |
| Ridging in lines | Movement in a joint under the roof | Rebuild at the line, relieve the stress |
| Alligatoring | Asphalt hardened by sunlight | Surfacing or coating if no splits yet |
| Split | Brittle membrane plus deck movement | New plies across the split, lapped wide |
| Bare patches | Gravel or granules lost | Resurface before the asphalt oxidizes |
The pattern worth noticing is that only two of those are repairs to the membrane itself. The rest are consequences of something else: a bond that failed, a joint that moves, sunlight reaching asphalt that should have been covered. Repairing the symptom without naming the cause is how a roof gets the same patch three times.
Why the leak is almost never where the stain is
Water that gets through a lap travels sideways on top of the insulation, through the joints between insulation boards, and comes down wherever it finds a deck seam or a fastener. On a large deck with a shallow fall, that can be a long way from the entry point. Chasing the stain upward on the roof surface is how crews end up patching sound membrane.
Finding the entry point means working from the roof side, following laps and details uphill from the stain, and in most cases confirming the wet area with a moisture survey rather than a torch and an opinion. The practical cost of guessing is high: every unnecessary cut is a new lap, and every new lap is a future leak. Roof leak detection methods sets out what each test can and cannot tell you.
The details leak first, and they are cheap to get wrong
Every place the membrane stops is a detail: a parapet, a curb under a rooftop unit, a drain bowl, a pipe, a scupper. On an asphalt roof those are built by turning the plies up a vertical surface, usually with a cant strip at the angle so the material does not have to fold at ninety degrees, then capping the top edge with metal or a counterflashing so water cannot get behind it.
Three faults account for most detail leaks. The plies were turned up too low, so wind driven rain goes over the top. The top edge was sealed with compound instead of being capped, and the compound has shrunk. Or somebody drilled a fixing through the upturn to hold something down. All three are visible from the roof, all three are repairable in an afternoon, and none of them needs the field of the roof touched. Flashing repair goes through each detail in turn.
What a repair should actually say it will do
A repair quote on an asphalt roof should read like a specification, not a sentence. The things that need to be on it:
- Where. A marked plan or dimensioned photographs, not a compass direction.
- How far back. The cut out extends to sound, dry material, and the quote says who decides where that is.
- Dried or replaced. Wet insulation under the cut is removed, not covered. Saying so now avoids the argument later.
- Ply count. Matching the existing build up, stated as a number.
- Laps. Each ply lapped wider than the one under it, edges staggered.
- Surfacing. Gravel reset or a cap sheet applied, so the new asphalt is not left bare.
- Flashings in the area. Inspected while access is paid for, and replaced if they are near the end.
That list is also the inspection you do afterwards. A finished repair should be a clean rectangle of new material, wider at every ply, surfaced to match. If it looks like a smear of black compound, it is a smear of black compound.
Wet insulation decides whether repair is still the right word
Asphalt systems are usually thick and heavy, and the insulation under them holds water quietly for years. Wet insulation does three things: it loses most of its thermal value, it keeps the underside of the membrane permanently damp, and it adds weight the deck was not designed to carry. None of them show on the surface.
So the question that separates a repair from the beginning of a replacement is not how the membrane looks. It is how much of the assembly underneath is wet. Under roughly a quarter wet, cut out the wet areas during repairs and keep going. Approaching a third and spreading, you are paying repair money into a roof that is already a capital item, and the honest answer is a remaining life figure to plan against. Repair or replace works through the comparison, and roof repair cost covers what moves the number on the repair side.
Where to go next
If the roof has a specific wet area and nobody knows how big it is, a moisture survey before any repair is the cheapest money on the job, because it decides the size of the cut rather than discovering it. If the asphalt is sound but bare and crazing, roof coatings is the page to read, with the caveat that a coating buys time on a sound roof and hides a failing one. And if the split you are looking at runs in a straight line across the roof, something underneath is moving, and that is a question for a survey rather than a patch.
Common questions
Is an asphalt roof the same as a shingle roof?
Not necessarily. Asphalt shingles are one kind of asphalt roof, laid in overlapping courses on a slope. The other kind is a built up or modified bitumen membrane, rolled and bonded in layers over a flat deck. Both are asphalt, and almost nothing about repairing them carries across.
- Shingles are individual units nailed in courses on a slope
- A built up roof is felt plies bonded in hot asphalt
- Modified bitumen is the rolled successor to built up roofing
- Shingle repair replaces units, membrane repair rebuilds plies
- Establish which one you have before accepting any quote
The confusion costs money because the wrong trade arrives. A crew that repairs shingles well may never have cut and rebuilt a bitumen membrane, and a flat roof crew makes a poor job of weaving step flashings into a slope. Plenty of commercial buildings carry both, with a membrane on the main deck and shingles over an entrance or a mansard. Say which roof you mean when you call, and ask who on the crew does that work.
What is the right way to repair a blister on an asphalt roof?
Cut it out and rebuild the plies. A blister is trapped air or moisture between layers that never bonded, so the delaminated area is removed back to sound material, dried, and replaced with new plies lapped over the sound surface. Coating over a blister traps the problem underneath.
- The blister is opened and the loose plies cut away
- The area underneath has to be dry before anything goes back
- New plies are set in with each lap staggered outward
- The patch finishes wider than the hole in every direction
- A cap sheet or surfacing goes back over the repair
Not every blister needs cutting. A small, firm blister in the field of the roof that is not growing and sits away from traffic can be recorded and watched. A blister underfoot or in a walkway route will break, and once it breaks it is a hole. The decision rule is movement and location rather than size alone, which is why a repair visit should produce a marked plan of what was cut and what was left.
Why does asphalt crack and split as it ages?
Asphalt hardens. Sunlight and heat drive off the lighter oils that keep it flexible, so the membrane that once moved with the building becomes brittle. The deck underneath still expands and contracts every day, and the stress has to go somewhere. It goes into cracks and splits.
- Ultraviolet light and heat oxidize the asphalt over years
- A brittle membrane cannot absorb normal deck movement
- Fine surface crazing is called alligatoring
- Long straight splits usually follow a joint underneath
- Surfacing exists to shield the asphalt from sunlight
This is why gravel and granules matter more than they look. They are there to keep sunlight off the asphalt, and a roof that has lost its surfacing in patches is aging much faster in those patches than anywhere else. Bare asphalt showing across a wide area is a planning signal rather than a repair one. Fine crazing with no splits can often be surfaced or coated; splits mean the movement has already won and the plies need rebuilding at that line.
Is roof cement an acceptable asphalt roof repair?
As a repair on its own, no. Roof cement is a bedding and sealing compound, not a membrane. It shrinks, cracks and pulls away from what it is stuck to within a few seasons, and because it is black and thick it hides the defect it was spread over while the leak continues underneath.
- Cement has no reinforcement, so it cannot bridge movement
- It is correct as bedding under a new ply or a patch
- Spread alone over a split it cracks along the same line
- A thick layer holds water against the asphalt underneath
- It makes the next proper repair harder and dirtier
There is a legitimate version of this repair: cement plus reinforcing fabric, lapped and then surfaced, which is a membrane in miniature. The illegitimate version is a bucket and a trowel. You can tell them apart from the ground after the fact, because the proper repair looks like a rectangle of new material and the other one looks like a smear. Ask which is being quoted, and ask for the fabric to be named.
How many plies does an asphalt repair have to rebuild?
As many as the roof has. If the roof is a four ply built up system, a repair that puts back two plies has left the patch weaker than the roof around it, and that patch becomes the next failure. The repair should match or exceed the original build up.
- Count the plies at a cut edge or in an existing core
- The patch matches the original ply count or beats it
- Each ply is lapped wider than the one beneath it
- Laps are staggered so no two edges stack up
- The scope should state the ply count in writing
This is the single most useful thing to ask for in writing, because it is checkable and it is where cost gets cut quietly. A contractor who states the ply count has told you what you are buying. One who writes repair leak at northwest corner has told you nothing, and nothing is what tends to get installed. If nobody knows what the original build up is, a small test cut answers it in an hour and informs everything that follows.