WATstop interior waterproofing
Water coming through the basement wall?
Seal it from the inside. WATstop is a three-part epoxy-cement that rolls onto bare concrete and holds back 139.6 psi of water pressure. Two coats now, paint over it in two days.
Why interior waterproofing is different
The water is on the wrong side of the wall
The textbook fix is on the outside. You dig down to the footing, coat the wall, and the backfill presses tight up against the membrane. On an existing house that means excavating, and plenty of foundations can't be reached that way: a driveway or deck is in the way, the neighbour is too close, or the dig, backfill and yard repair runs $10,000 to $25,000 on a typical detached house. Working from inside skips all of that, but it flips the physics. The water still comes from outside, so now it's pushing the coating off the wall instead of onto it. That's called negative-side pressure, or counter-thrust, and it's the rating that decides whether an interior coating holds.
WATstop is rated to 9.5 atmospheres of counter-thrust, which is 139.6 psi. For perspective, even with the water table right up at grade, an 8-foot basement wall sees about 3.5 psi at the footing. Its bond to concrete tests at 2.5 N/mm², and that bond is what keeps an interior coating on the wall. In material it runs about $850 to $1,500 for one problem wall, $5,000 to $6,500 for a full basement perimeter, applied yourself with the yard untouched; the calculator below gives you your number.
A weekend-sized job
How it goes on
Strip it to bare concrete
Take off any paint, sealer, efflorescence and loose material until you're down to sound concrete or block. Fill cracks and voids with repair mortar before you coat. WATstop seals the surface, but it won't bridge a structural crack.
Mix in the bucket
Mix the cement part (C) into the water with a drill and paddle until it's a smooth paste, then add the resin (A), then the catalyst (B). Everything comes pre-dosed in the bucket, and you have about 2 hours to use each mix.
Roll on two coats
Work the first coat well into the surface with a short-hair roller or trowel, then put the second on within 24 hours. At the wall-to-floor joint and in the corners, press Safety Joint Roll into the fresh first coat and let the second coat cover it completely. Keep the room above 5°C and under 70% humidity while it goes on; if it's musty down there, run a dehumidifier.
Make it a wall again
Paint, plaster or tile go straight over WATstop 24 to 48 hours after the last coat, so plan the finish before you start: 48 hours is the limit. The problem disappears and the room goes back to being a comfortable space.
Coverage calculator
How much do you need?
Based on 2.0 kg per m² in two coats for full waterproofing, or 1.0 kg per m² for a vapour barrier. Two coats give the best result either way; on smooth, sound concrete the vapour barrier can go on in one. The vapour-barrier rate is only for walls that test dry, under 5% moisture; if you're not sure, use full waterproofing. Rough or very porous surfaces take a little more, so round up if you're between sizes. Joints take one metre of Safety Joint Roll per metre treated; each roll carries 20 m (about 65 ft).
Not sure what you're looking at?
Send us a photo of the wall. We'll tell you if WATstop is the fix, what prep comes first and how much to order. You'll hear back the same business day.
Two sizes, one product
Pick your bucket
WATstop 5 kg
$206.99
- Full waterproofing: about 27 sq ft
- Dry-wall vapour barrier: about 54 sq ft
- The right size for one problem wall
WATstop 10 kg
$319.99
- Full waterproofing: about 54 sq ft
- Dry-wall vapour barrier: about 107 sq ft
- The better value per square foot
Safety Joint Roll
$96.40
- Elastic non-woven polyester tape for the wall-to-floor joint and corners
- 20 m roll, 15 cm wide: about 65 linear ft
- Pressed into the first coat, covered by the second
Water actively running in? Call us first.
A damp or seeping wall is exactly what WATstop is made for. A crack that's moving, or water streaming in while it rains, earns a conversation first because sometimes WATstop is the whole fix and sometimes it's the last product that goes on. Five minutes on the phone saves you a return. 1-866-326-8321
Not that urgent, or not a phone person? Email us a photo of the wall and we'll size it up from there.
Do your homework
