Blocked Weep Holes Turn a Rainscreen Into a Bathtub

There is a repair that gets made on Metro Vancouver low-rise buildings every year, costs almost nothing, looks tidy when it is finished, and quietly starts destroying the wall behind it.

Someone runs a bead of sealant along the bottom edge of the cladding.

The reasoning is understandable. There is a gap there. Gaps let water in. Sealing the gap should keep water out. On a face-sealed wall that logic holds. On a rainscreen wall it is exactly backwards, and the gap that got sealed was the drain.

A rainscreen wall is designed to leak

This is the part that surprises most owners and more than a few trades. A rainscreen assembly does not try to stop water at the outer face of the cladding. It assumes water will get past it, because wind-driven rain on a Pacific Northwest building always does.

Behind the cladding sits a cavity. Behind the cavity sits the weather-resistive barrier, which is the actual waterproofing layer. Water that penetrates the cladding runs down the back face or down the cavity, hits a flashing, and exits through openings at the bottom.

That is the design working correctly. The full logic of this approach is set out in the guide on how a rainscreen works, and the short version is that the wall stays dry because it drains, not because it is sealed.

Take away the drainage and every assumption behind the design collapses.

Where the openings are

On a properly built rainscreen, drainage openings appear at predictable places: along the base of the wall assembly, above the head flashing at every window and door, above any horizontal flashing that interrupts the cavity, and at the base of each storey on taller assemblies.

They also serve a second function. Air moving through the cavity dries whatever moisture remains after the bulk water has drained away. A cavity that drains but does not ventilate stays damp longer than it should, which is why the openings appear at both the top and bottom of a cavity run on well-detailed work.

The openings are small by design and easy to miss. On some cladding types they are purpose-made vents. On others they are simply the open head joints between masonry units, or a deliberate gap at the base of a siding run.

Four ways they get closed

Paint. This is the most common cause in this region and the most frustrating, because it happens during work the strata commissioned and paid for. A roller passing along the base of a cladding run bridges the openings with coating. Nobody sees it happen, the finished job looks better than what it replaced, and the wall loses its drainage.

Mortar. On masonry and adhered veneer assemblies, mortar droppings during original construction can fill the cavity base and bury the openings from behind. This one is present from the day the building was completed and will not be found until somebody opens the wall or looks closely at the base.

Sealant. The repair described at the top of this post. It is applied by painters, by handypeople, and sometimes by residents trying to stop a draft in a ground-floor suite.

Debris and biology. Dust, pollen, insect nests, and organic growth accumulate in and around the openings. Cavity vents that include an insect screen are particularly prone to this, because the screen is exactly the surface fine material sticks to. Given the moss and biofilm loads typical of shaded North Shore and Burnaby elevations, this happens faster than most maintenance schedules assume.

What the trapped water does

Water sitting at the base of a cavity is in contact with whatever is there. On a wood-frame building that means bottom plates, sheathing edges, and the lower run of the weather barrier. On a concrete building it means the slab edge and any embedded steel near it.

Wood exposed to sustained moisture at temperatures typical of a Vancouver winter will rot. Not quickly, and not visibly from outside, which is the problem. The sheathing degrades behind cladding that still looks acceptable from the sidewalk, and the first external sign is often a soft spot found during unrelated work, or staining that appears years after the drainage stopped.

Steel exposed to sustained moisture corrodes, and the products of that corrosion occupy more volume than the steel did. The mechanism is the same one described in the post on rebar corrosion and concrete delamination, and the outcome on a wall assembly is a fastener or connection that loses capacity while nothing visible changes on the surface.

Finding the blockage

The honest answer is that you have to go and look at the openings.

There is no reliable way to assess cavity drainage from the ground on a building of any height. Binoculars will find gross staining and obvious paint bridging on the lowest storey, and nothing above that. The openings are small, they are in shadow, and their condition varies along a single elevation depending on where a painter started and stopped.

That means rope access or a lift, and a technician who checks the openings along a run rather than sampling one and assuming the rest match. The variation is the whole point: a wall where two openings out of forty are clear is functionally a blocked wall, and a single-sample inspection has a good chance of landing on one of the two.

While the technician is at the openings, the useful additional checks are whether a flashing is actually present behind each one, whether the opening connects to a cavity or to solid material, and whether there is standing water or staining at the cavity base.

What can and cannot be fixed cheaply

Where the blockage is paint over an otherwise sound opening, clearing it is genuinely straightforward. The opening is reopened, the drainage path is confirmed, and the wall resumes working.

Where mortar has filled the cavity base, drilling a new hole produces an opening into solid material. It looks like a weep hole and drains nothing. Fixing that case means opening the cladding to clear the cavity, which is a different order of work.

Where no flashing was installed behind the opening, water arriving at the cavity base has no surface directing it outward, and reopening the hole achieves little. That is an original construction defect and is handled as part of a larger remediation, using the assessment logic in the post on building envelope condition assessment.

The diagnosis matters more than the repair here, because three problems with identical appearance from the outside have three very different scopes.

The habit that prevents most of it

Inspect the drainage openings after any exterior painting.

That single practice catches the most common version of this defect at the moment it is created, while the painter is still available and the work is still recent. It costs a fraction of the painting contract and it protects the wall the paint was supposed to be protecting.

It is worth writing into the painting specification directly: openings are to remain clear, and the contractor confirms this at handover. A painter who knows this is a checked item will mask the openings rather than roll over them, which is all that was ever required.

Add the same check to the regular envelope inspection cycle described in the rainscreen inspection checklist, and the failure mode largely stops happening.

The buildings that end up with rotted sheathing behind clean-looking cladding did not usually suffer some exotic failure. Somebody closed the drain, and nobody looked.

Sources and further reading

• BC Housing, Building Enclosure Design Guide, rainscreen cavity drainage and ventilation provisions, referenced 2026
• National Research Council Canada, National Building Code of Canada, environmental separation and moisture provisions, referenced 2026
• Canada Mortgage and Housing Corporation, Best Practice Guide: Wood-Frame Envelopes in the Coastal Climate of British Columbia, referenced 2026
• Allweathercoating field inspection records, Metro Vancouver, 2026