Copper valley flashing and roof cement failure (C) Daniel Friedman Roof Sealants, Mastics, Coatings
Installing or fixing a leaky roof with roof cements, sealants, coatings

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Advice for choosing & using sealants to repair a leaky or worn roof surface, or to repair roof flashings, valleys, and intersections with other building surfaces.

Roofing sealants have been applied for centuries using just about anything at hand: tar, pine-sap, even sugar to try to seal leaks in a roof that's damaged or at the end of its life. Eking out a few more months or years of roof life an be very tempting but a more legitimate use of sealants is one that is more constrained.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Roofing Cements, Sealants, Mastics, Coatings

Roof Patching & Sealant Product Terms, Types, Sources

Patched skylight flashing (C) Daniel FriedmanThere are two general classes of sealant or coating products used on roofs:

  1. Roofing mastics: properly, a thick paste applied by trowel, putty knife, or using a caulking gun and used to seal exposed nail or screw heads or small areas of damage, cracks, holes in roofs, as well as along edges of abutting roof materials such as along the top edge of counter-flashing let into a chimney or into a masonry wall.

    Some roofing mastics such as the traditional black asphalt roof flashing cement include reinforcing fibers. Mastics are also used as adhesive to glue-down loose shingles and mastics are also widely used as adhesives in other building applications such as flooring.
  2. Roofing sealants: properly, liquid coatings applied by roller or brush onto large areas of a roof surface to seal or extend the the life of a leaky or worn roof surface. Some sealants can be used on many different roof materials such as asphalt shingles, roll roofing, modified bitumen, even tile or slate (though I don't recommend that), while others are compatible only with specific roof coverings, so be sure to read the manufacturer's label as well as the roof sealant instructions from the manufacturer of your roof covering.

    Cut-back adhesives or sealants
     are products that have been thinned using a compatible solvent to produce a thinner coating that may be painted onto a surface and that may penetrate into it.

The skylight repaired with asphalt flashing cement (above) is discussed at SKYLIGHT LEAK DIAGNOSIS & REPAIR.

[Click to enlarge any image]

There are several types of sealant or coating products used on building roofs by composition. Both mastics and more liquid roof sealants may have similar ingredients.

  1. Acrylic mastics and sealants, acrylic polymer based products.
  2. Asphalt-based mastics and sealants use asphalt, a petroleum product, as the primary ingredient, typically fiber reinforced. Asphalt based sealants can be cleaned-up or thinned with paint thinner or mineral spirits. Asphalt based products are also referred to as bituminous or bitumens.
  3. Emulsified asphalt-based mastics and sealants also use asphalt with the addition of bentonite clay as an emulsifier that reduces flowing and cracking.
  4. Silicone Roof Sealants, sold in caulking tubes or in gallon cans, are silicone based coatings and sealants that can be applied over many types of building surfaces, providing good adhesion, good UV resistance, and unlike the asphalt and bituminous products silicone sealants do not harden nor shrink with age. Silicone roof products are sold for application by caulk gun, brush, or trowel and in a range of colors: clear, black, white, brown and others.
  5. Surfactant-treated Roof Coatings add a wetting agent to improve penetration or coating bonding to damp or wet roof surfaces.
  6. Polyurethane Roof Coatings & sealants including some spray-on products may be used as patching or repair coatings on asphalt roofs.
  7. Urethane based coatings are used in spray-on roof systems built over urethane foam

Watch out: as you'll see when shopping for roof patching, sealing or coating materials, manufacturers may mix these terms. For example Vinimex®, a widely used liquid roll-on roof sealant is labeled by Comex®, its manufacturer, as a roof mastic.

Watch out: some older roof mastics or sealants contained asbestos as a reinforcing fiber and some may have also included asbestos powder as a filler. See MASTIC, CUTBACK ADHESIVE, FLASHING CEMENT ASBESTOS for details.

Because builders, contractors, roofers, and vendors use and mix and match so many similar terms for roofing coatings, mastics and sealants we list all of them below along with some illustrations and descriptions of how these products are applied and of their intended use.

Prices for roof sealant/mastic/cement range from less than $10. for a tube of caulk-like sealant to over a thousand for a thick silicone-based roof sealant coating in a 5-gallon container. Don't buy any sealant or roof cement before reading on the product label the roof material types with which it's compatible.

Alphabetical List of Types of Roof Mastics, Sealants, Coatings

Acrylux roof sealant mastic - at

Black-Knight(R) roofing mastic installation - a Tarland Products asphaltic roof mastic at

Roofers Choice brand Plastic Roof Cement at

Comex Vinimex roof sealant - at

Product description: Impermeabilizante acrílico de alto poder cubriente que forma una capa homogénea capaz de impermeabilizar todo tipo de superficie, aún en superficies verticales. Cuenta con garantía de 3 ó 5 años. Ideal como reflejante en techos y azoteas, lo cual colabora en el ahorro de energía y evita el calentamiento de las viviendas.

Leaky built up roof (C) Daniel Friedman

Tar is occasionally mis-applied as an adhesive as we illustrate below.

Asphalt roof single tab sealant bleed-out staining on a Maryland roof © InspectApedia Bob Sissons

See ASPHALT SHINGLE TARRY BLEED for details about the messy roof shown above.

Pro-Gard Liquid Rubber EPDM Roof Coating at

Black Jack Ultra Roof elastomeric reflective roof coating from Black Jack, at

Henry Tropi-Cool silicone roof sealant, at

Watch out: Be sure to read the application instructions and roof material compatability remarks provided by the manufacturer of any roof patching compound or sealant you are considering buying. Applying the wrong sealant can actually damage a roof covering if its solvents are not compatible with the roofing material. In particular, some roof coatings or flashing cements are not compatible with asphalt shingles, asphalt roll roofing, EPDM, rubber, or modified bitumen roof coverings.

Life Expectancy of Roof Mastics & Sealants

Despite standards and claims, in our opinion there is no single "right answer" to "how long will this coating, patch or repair last?" I've inspected 20-year old roof coatings in remarkable condition and I've seen coatings and repairs fail from the day after installation. But certainly some materials are more resistant to the wear-forces at work (heat, UV, thermal movement) than others, as we elaborated in the list of coatings and mastics above.

Below we illustrate use of a black asphalt-based flashing cement on a roof surface. Observe how it has aged, dried, shrunk, cracked.

Roof cement failure (C) Daniel Friedman

Watch out: as we illustrate in seveal photos throughout this article, applying most sealants across large or long gaps, cracks, or openings without adequate reinforcement by a mesh or fabric is likely to be a short-lived repair.

Heat causes the loss of volatiles from roofing mastics and sealants, causing most products to become more brittle with age. UV radiation also weakens the material and probably oxidizes some of its constituents. Thermal movement caused by changing temperature then pulls-apart and cracks the sealant, as we illustrate above.

How to Use a Reinforcing Fabric When Patching with Roof Cement

Roof cement gaps and leaks (C) Daniel FriedmanIf I were using a roofing mastic to repair a valley I'd pick up a reinforcing mesh sold alongside the mastic in any building supply house; those mesh reinforcers may be fiberglass or synthetic fabric. The mesh gives tensile strength to the repair by being placed down onto the surface and smeared over with mastic sealant. That adds some life to the job.

Here we illustrate a typical application of roofing cement along the intersection of a roof flashing and wall where leaks had occurred. Over time the combination of drying out of the sealant, thermal changes, and movement cause the sealant to crack and leak again.

Use a patching fabric over projections, larger cracks, gaps, and holes to obtain a longer-lived more durable repair. Using a patching fabric is simple:

  1. apply a layer of the mastic or patching compound over the problem area
  2. cut and place a section or strip of patching fabric over the area
  3. spread an additional layer of roof cement or patching compound over the area
  4. let the patch cure

I've also re-lined valleys and built-in eaves-trough gutters or yankee gutters with self-adhering ice and water shield products.

Roof Flashing Cement & Sealant Research & Standards

Using Roof Sealant to Repair a Leay Roof Valley

Reader Question: repairing a roof valley with roofing sealant vs. other approaches

2016/06/10 Rebecca said:

My recently purchased 1960's house with a two-layer asphalt roof has a leaking valley. The rest of the roof seems sound (excepting need for gutters, etc.). I want to repair this myself (and a roof tear-off is simply not a financial option right now). What challenges might I face beyond replacing the roof roll and the puzzle-like replacement of two layers of shingles?

This question appeared originally at DRIP EDGE FLASHING for ROOFS

Shown below: hanging plastic bags in the attic (or putting buckets under leaks) makes sense as an emergency stop-gap way to face some roof leaks but it's hardly a reasonable nor effective repair.

[Click to enlarge any image]


Sorry I don't fully understand the situation;

Tar or asphalt patched roof valley on a clay tile roof (C) Daniel FriedmanWatch out: From what I think is the case, you face these risks, in order of importance:

1. falling off the roof and being injured or killed
2. causing more damage to the roof when you walk on it, increasing the size of the repair job
3. finding it difficult to cut nails and lift shingles on the older roof sufficiently to place down a new layer of valley liner.

If the roof is in the last 1/4 of its life I'd prefer to stay off of it as much as possible: brittle shingles easily damaged by foot traffic; I might try working with care on padded surfaces to patch or seal the valley - a "band-aid" temporary repair that you can make more durable if you combine a fabric with your roof mastic to seal the valley.

Our photo shows black asphalt roof cement patching coating in a leaky roof valley on a clay tile roof in New York. That roof is discussed at CLAY TILE ROOF FLASHING LEAKS

Reader follow-up: what tools do I need?

Rebecca said:

Point number 3 is more along the lines I meant--sorry to leave you a vague question. We had a cricket added to the chimney, due to leaking there caused mostly by tar repairs causing dams, and general neglect.

It appears tar dams are the primary culprit for leaking along this valley, as well. I plan to inspect the roof deck from inside the attic to determine the extent of the roof deck damage.
I do plan to use rope/safety gear when on the roof for this extended period of time.

What tools should I have ready to remove the shingles and nails?

Moderator reply:

I generally do not recommend roof repairs by someone with no experience, no training, and no one with experience to help them, because of the risks involved.

A problem is that you don't know what you don't know - you don't even know that you're missing something; the result is possibly an unnecessarily costly and ineffective or short lived repair. While small repairs such as sealing a little hole or leak in a roof are within the grasp of many homeowners who are otherwise able to tackle the dangers of fooling around on a roof, even then I'd at least read some roofing instruction guides or get help from someone with experience and training.


Asphalt shingle roofs use normal carpentry & roofing tools including a few flat bars; when pulling up a lot of shingles roofers use a stripping tool but typically one can get by with a short-handled flat-bladed shovel; the added width of the blade can reduce un-wanted shingle damage. Occasionally I'll use a slater's nail cutting tool - that's a long flat bar with notches - the tool can be driven up under shingles and then hammered down to cut nails. You shouldn't need that for repairing a valley.

For applying roof mastic I use a small mason's trowel and of course disposable clean-up solvent & towels. Often having a pair of trowels or a trowel and a putty knife allow me to scrape mastic off of one blade onto another for more careful and detailed work. Be careful how you dispose of that trash: leaving it in a hot trash can can start a fire.

Keep in mind however that if you only seal the open surface of the valley water may back up under the shingles that abut the valley and might still leak, particularly if the valley doesn't drain rapidly off of the roof.

All of this to me sounds as if patching valleys by coating is the cheapest, quickest job that's also short-lived and a bit ugly.

But doing a proper valley-liner replacement requires lifting shingles along the valley; on an old roof you'll find the shingles fragile.

Reader follow-up: what's roofing mastic?

Roof sealant failure (C) Daniel FriedmanRebecca said:
And would you please expand your roof mastic comment--my research so far on repairing the valley has not included roof mastic. Thank you so much!

Photo: roof sealant used around a dryer vent installed through a roof surface.

[Click to enlarge any image] Take a closer loo9k to see how this sealant has dried and cracked.

Moderator reply:


Roofing mastic aka roof flashing cement is a thick tarry sealant, typically sold in gallon or larger sized containers or more expensively in caulk/sealant tubes for very small jobs. I'd consider coating with a high quality rubber roof sealant unless the damage in the valley is severe.

The roof cement is traditionally an asphalt-based product with reinforcing fibers. There are other substitute roof leak sealing coatings including thick paints. All roofing mastics will in my experience ultimately fail by drying out and cracking, though there are some rubber roof coating products that claim they're immune from that problem.

Copper valley flashing and roof cement failure (C) Daniel Friedman

The copper roof valley shown above cracked and leaked from thermal movement; you can see that repairs were attempted using a black asphalt-based flashing cement. Unfortunately not only did the flashing cement give only short term relief, its chemistry attached the copper below, thinning and corroding the copper. Below is another example of roof flashing cement failure on the same roof.

Roof valley flashing cement patch failure (C) Daniel Friedman


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