Short answer: the black tar-like coating on an older foundation wall is asphalt damp-proofing, and it belongs on the suspect list rather than the confirmed one. Asbestos was used as a filler in a range of asphalt and bitumen building products, and no one can identify it by sight. It matters most not when the wall is dug up but when it gets scraped or ground back for re-coating. A single sample runs about $100 to $150.
This guide covers what the coating is, why it is a suspect material rather than a known one, and the specific moment in a drainage job when it stops being harmless.
What is the black coating on the outside of a foundation wall?
Damp-proofing, in almost every case. It is a brush or spray applied asphalt coating put on the exterior of a below-grade wall before backfill, and on concrete block walls it usually goes over a parge coat that smooths the blocks first. Its job is to slow moisture moving through the wall, and on Fraser Valley homes built from the 1950s through the 1980s it is close to universal.
Damp-proofing is not waterproofing, and the difference explains a lot of wet basements. Damp-proofing resists moisture. It does not bridge cracks and it does not hold back standing water under hydrostatic pressure. A modern waterproofing membrane does both. That is why a 1970s house with intact damp-proofing and a collapsed perimeter drain still gets water in the basement: the coating was never designed to be the last line of defence.
Most homeowners never see it. It goes on before backfill and stays under a metre or more of soil for the life of the house. The exceptions are the moments when something digs the wall out again, which in practice means a perimeter drain replacement, a foundation repair or an addition.
Is that coating an asbestos material?
It is a suspect material. That distinction is the whole point of this post, and overstating it in either direction does homeowners a disservice.
What is established: asbestos was used as a filler and reinforcement in asphalt and bitumen based building products. That is why Ontario’s guide to its asbestos regulation, whose sample list of suspect asbestos-containing building materials is widely used across Canada as a starting point for survey scoping, includes entries like construction mastics, adhesives, roofing felt and base flashing alongside the better known drywall and flooring products. Asphalt-based coatings sit in that same product family.
What is not established, and what nobody should assert about your particular house: that the coating on your foundation contains asbestos. It might not. Formulations varied by manufacturer, by product line and by decade.
The reason this cannot be settled visually is the same reason it cannot be settled for any other material. HealthLink BC puts it plainly: “You can’t always tell just by looking whether a material contains asbestos. It is always best to hire a professional to collect and test a sample.” Black tar looks like black tar whether it contains chrysotile or nothing at all.
Scale is worth keeping in mind too. WorkSafeBC’s homeowner information sheet notes that asbestos fibres “can be found in over 3,000 household materials.” The well-known ones, popcorn ceilings, vermiculite, pipe wrap, are a small fraction of that list, and the exterior of a house is not exempt from it. The same sheet says a survey report should describe the materials used in each room of the home “and on the exterior (siding, roofing, window putty, etc.).” Exterior materials are explicitly in scope for a survey.
Why does scraping matter more than digging?
Because excavation moves the material and preparation destroys it.
An excavator pulling soil away from a foundation wall is not, by itself, the high-risk step for a coating that is bonded to concrete. The high-risk step comes a day or two later, when the wall gets prepped for whatever goes on next. Prep means wire-brushing, scraping, grinding, or pressure washing. Each of those attacks the coating directly and turns a bonded film into airborne particles and slurry.
That sequence is the practical reason to sort this out before the trench is open rather than after. Once a crew is standing in a trench on a dry August day with a membrane scheduled for tomorrow, nobody wants to hear about a one to three day lab turnaround.
The regulatory framing is the same as for interior work. Section 20.112 of WorkSafeBC’s OHS Regulation Part 20 requires that before demolition, salvage or renovation work begins, all employers responsible for the work and the owner must ensure a qualified person inspects the building and the work site to identify hazardous materials, including asbestos-containing material. The wall is part of the building. Our guide on asbestos checks before a pre-1990 foundation excavation covers the wider sampling scope around a trench, including the drain pipe itself.
How do you tell damp-proofing from a modern membrane?
By age, texture and edges. This is not a definitive test, and it does not replace a sample, but it tells you which conversation you are having.
| Feature | Older asphalt damp-proofing | Modern waterproofing membrane |
|---|---|---|
| Application | Brushed or sprayed on, follows every bump in the wall | Rolled sheet, sprayed elastomeric, or dimpled drainage board |
| Thickness | Thin film, often patchy, thin at corners | Consistent thickness, usually visibly thicker |
| Texture | Matte to slightly glossy black, can be brittle and flaky when old | Rubbery and flexible, or a visible dimpled plastic sheet |
| Edges | Ragged, hand-applied, stops unevenly near grade | Clean terminations, often with a termination bar or trim |
| Typical era on Fraser Valley homes | 1950s to 1980s | Roughly 1990s onward, or after any retrofit |
| Suspect material? | Yes, sample before disturbing | No, these post-date the asbestos era |
If what is on the wall is a dimpled plastic drainage board or a flexible rubbery sheet, the coating question is usually already answered: someone has been down there since, and the original coating was either removed or covered. If it is a thin, brittle, hand-brushed black film that flakes when you scratch it, treat it as suspect.
The other thing worth settling at the same time is what the wall is going to get instead, because that decision changes whether the old coating has to come off at all. Where a new system can be installed over the existing coating, the disturbance never happens. That is a drainage and waterproofing question rather than a testing one, and foundation waterproofing is a separate trade from ours worth pricing on its own terms before the sampling scope is set.
How do you handle it without stalling the excavation?
Sequence it against the quoting stage, not the digging stage.
- Ask, while collecting drainage quotes, whether the scope includes removing the existing coating. If the answer is yes, or “we will see when we open it up,” you have a sampling question.
- Get the coating sampled during the quote period. Results run one to three business days, which fits inside the time it takes to compare bids. Rush analysis adds roughly $75 to $150 where a deadline is genuinely tight.
- Sample the other suspects in the same visit. A surveyor already on site for the foundation can cover the drain pipe, any parging and the interior side of the same wall for far less than three separate call-outs.
- Put the result in the drainage contractor’s hands before they mobilize. A negative result frees the prep work immediately. A positive one changes who does the removal, since abatement contractors have had to be licensed in B.C. since January 1, 2024, and their workers certified.
- Keep the original lab report. WorkSafeBC’s guidance is that the report should include the original laboratory results, not just a summary, and that anything reported as “less than 0.5%” or “trace” must be treated as asbestos-containing unless further lab work pinned down the percentage.
The short version
Black foundation damp-proofing is an asphalt product from a family where asbestos was used as a filler, which makes it worth sampling and not worth panicking about. The risk is not the coating sitting on the wall. It is the wire brush and the grinder that come out once the trench is open and the crew is prepping for a new membrane. One sample, one to three days, roughly $100 to $150, booked while you are still comparing drainage quotes.
Planning a perimeter drain or foundation job on a pre-1990 home in Abbotsford, Chilliwack, Mission or Langley? Call (604) 800-7217 or book independent asbestos testing. We sample and report only, with no removal work to sell you, and most results come back within one to three business days.
Frequently Asked Questions
Is foundation damp-proofing always asbestos?
No, and nobody should tell you it is. It is an asphalt-based coating, and asbestos was used as a filler and reinforcement across a range of asphalt and bitumen building products, which is why mastics, adhesives, roofing felt and base flashing all appear on published lists of suspect materials. Whether the specific coating on your wall contains it is a lab question. HealthLink BC's position applies directly: you cannot always tell just by looking.
The coating is outside and buried. Why would it matter?
Because it does not stay buried once a drainage crew opens the trench. The moment that matters is not the excavation, it is what happens next: scraping, wire-brushing, grinding or pressure-washing the wall to prep it for a new coating or membrane. That is the step that turns a stable buried material into airborne dust, and it usually happens within a day or two of the wall being exposed.
Can I just leave the old coating on and put the new membrane over it?
Sometimes, and it is worth asking your drainage contractor whether the new system can be installed over the existing coating rather than down to bare concrete. Where that works it avoids the disturbance entirely, which is the cleanest outcome. Where the old coating is failing and has to come off, sample it first so the removal is planned rather than improvised in the trench.
Does a hazardous materials survey even cover the outside of the house?
It should. WorkSafeBC's homeowner guidance says an asbestos survey report should describe the building materials used in each room of the home and on the exterior, naming siding, roofing and window putty as examples. Exterior materials are explicitly in scope. If a survey report for a foundation project has nothing exterior in it, ask why.
How much does it cost to sample the coating?
Roughly $100 to $150 for a single material test, with results typically back in one to three business days. Against a perimeter drain replacement at $15,000 to $30,000, or a foundation waterproofing membrane adding $3,000 to $8,000, the sample is a rounding error. The expensive version is discovering the question after the wall has already been ground back.
Published August 20, 2026 · Last updated August 20, 2026 · Fraser Valley Asbestos