A tube of roof cement costs less than a sandwich, and it is responsible for more expensive roof damage in this region than almost any other single product. Not because the material is defective, but because of what it is used for.
A roof does not keep water out by sealing. It keeps water out by geometry — overlapping materials arranged so that gravity carries water from each surface onto the one below it, always outward, never into a joint. Shingles lap. Flashing laps. Underlayment laps. Nothing in a correctly built roof depends on adhesion to stay watertight.
That is the whole design, and it is why properly detailed roofs last decades with no maintenance to the joints.
When roof cement is applied to a leak, it is almost always because the geometry is wrong — a lap is missing, reversed, or too short, or a flashing was never installed. The cement is now holding back water by sticking to two surfaces, on the exterior of a building, through everything the exterior throws at it.
In Seattle that means constant UV in summer, thermal cycling every clear day as the roof heats and cools, dimensional movement of the materials it bridges, and eight months a year of sustained wetting. Asphalt-based cement becomes brittle as its volatiles escape. It shrinks. It develops hairline cracks that are invisible from a ladder and entirely sufficient for water. Two to four seasons is a realistic life, and the failure is gradual rather than obvious.
So the leak returns, and by then it looks like a new problem rather than the same one.
This is the real cost. A generous smear of black cement across a chimney base or a vent penetration conceals the actual condition underneath. The next person to look — a home inspector, another contractor, you — cannot see whether there is step flashing under there, whether a cricket is missing, whether the counter flashing was ever let into the mortar.
Diagnosis now requires removing the cement, which is unpleasant work that bonds aggressively to shingles and frequently damages them on removal. So the defect stays hidden, and the pattern that follows is familiar: repair, fail, repair, fail, and eventually a homeowner who has spent more on three unsuccessful repairs than the correct fix would have cost, with an additional season of water in the assembly each time.
When we inspect a roof and find cement smeared across chimneys, valleys, and penetrations, we read it as a map of every place someone was unable to find or unwilling to fix the real problem.
It is a real product with real uses, and being absolutist about it would be wrong.
Sealing exposed fastener heads. Standard practice where fasteners must be face-driven.
As secondary protection behind correct geometry. Bedding a flashing lap that would shed water perfectly well without it. Belt and braces, with the belt doing the work.
As genuinely temporary emergency mitigation. Stopping active water intrusion in a storm until a proper repair can be scheduled in weeks, not seasons. This is a legitimate and useful application, and the word that matters is temporary.
Manufacturer-specified applications. Some assemblies call for a sealant at defined points, and following that specification is correct.
The distinction throughout: sealant supporting correct geometry is fine. Sealant substituting for absent geometry is a countdown.
Ask any contractor proposing a repair one question: what is the geometry fix, and where does sealant fit into it?
A good answer describes a physical correction — new step flashing woven into the courses, a kickout fitted at the wall termination, counter flashing let into a mortar reglet, a cricket built behind the chimney, a boot replaced — with sealant, if mentioned at all, as secondary.
An answer that begins and ends with sealing it up is proposing a temporary measure at permanent-repair prices. That is worth knowing before you agree to it, and it is worth asking what the permanent version would cost, so you can make the choice deliberately rather than discovering it in two winters.
Rejecting roof cement as a substitute for geometry does not mean rejecting sealants. It means using the right one in the right place.
Asphalt roof cement. Compatible with asphalt shingles and underlayment, which is its genuine advantage. Appropriate for bedding a flashing lap as secondary protection, sealing exposed fastener heads, and true emergency mitigation. It becomes brittle with age and UV, so it is never the primary defence.
Polyurethane sealant. Retains flexibility far longer than asphalt cement, adheres well to metal and masonry, and is paintable. A better choice for sealing counter flashing into a mortar reglet, where the joint must accommodate movement between dissimilar materials for years.
Butyl. Excellent adhesion and permanent flexibility, commonly used in metal roofing at seams and fastener points. Does not cure hard, which is precisely why it survives thermal cycling.
Silicone. Durable and weather-resistant, but it adheres poorly to asphalt and, critically, nothing else adheres to it afterwards — including a future proper repair. Silicone on an asphalt roof creates a surface that resists any subsequent bonding, which turns a bad repair into an obstacle.
The general principle: match the sealant to the materials and the movement, and never ask any of them to hold back water on their own.
When a contractor proposes sealing something, this is what the geometry-based alternative would be at each of the common failure points.
Roof-to-wall junction. Not a bead along the joint. Individual step flashing pieces, one per shingle course, woven so each laps over the one below and under the shingle above, with a kickout at the base and either siding or counter flashing lapping the upper legs.
Chimney. Not cement smeared around the base. Apron flashing on the downslope face, step flashing up the sides, counter flashing let into a groove cut in the mortar and mechanically secured, and a cricket on the upslope side where the width warrants it.
Vent penetration. Not a ring of cement around the pipe. A new boot with its base integrated into the shingle courses, or at minimum a retrofit collar seated properly on a sound base.
Valley. Not sealant along the shingle cut lines. Membrane beneath, correct metal, proper cut lines, and a termination that laps over the drip edge into the gutter.
Skylight. Not caulk around the frame. The manufacturer's flashing kit — sill, step, and head components integrated into the courses.
In every case the proper repair costs more than a tube of cement and less than doing the cement repair three times before doing it correctly.
If you have inherited a roof covered in black smears, removal is unpleasant and worth understanding before someone attempts it aggressively.
Aged asphalt cement bonds tenaciously to shingle granules. Scraping it off typically takes granules with it, which is why over-enthusiastic removal can damage otherwise serviceable shingles. Cold weather makes it brittle and easier to fracture off; warm weather makes it gummy and more likely to smear.
The realistic approach is to remove what is necessary to expose and correct the underlying defect, and leave the rest alone rather than chasing cosmetic cleanliness at the cost of granules. Where cement covers shingles that will be replaced during the repair anyway, that is the easy case.
And there is a diagnostic use for it before removal. Cement marks the locations where somebody previously believed water was entering. Even if they were wrong, that is a record of the leak history of this roof, and it is worth photographing and noting before anything is disturbed.
This post is part of our in-depth coverage of roof repair & leak detection in Seattle.
How Roof Leaks Actually Travel → Roof Repair & Leak Detection →
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