Weep holes and cavity venting
Every rainscreen wall ends at a row of small openings most people have never noticed, and a fair number have caulked, painted, or landscaped shut. The weeps drain the cavity and the vents dry it, and together they are the difference between a wall that manages Metro Vancouver's wet season and a wall that stores it. This guide covers how the path works, the four ways it gets blocked, and how to check yours without opening anything.
The exit and the air supply
Our guide to how a rainscreen works follows water down the cavity to the base of the wall. The weeps are where that story ends: openings at the cladding base and above every cross-cavity flashing, sized to pass water out while the cladding edge above shelters them from wind-driven rain. On panel and plank claddings they read as a consistent gap, often dressed with a perforated strip. On brick veneer they are the open head joints your eye slides past in the bottom course of mortar.
The same openings do a second job between storms. Air enters at the base, moves through the cavity, and exits at vents near the top of the wall, carrying residual moisture with it. Drainage handles the litres; venting handles the dampness that drainage leaves behind. In a climate that delivers 1,189 mm of precipitation a year at Vancouver International Airport (1991 to 2020 normals), the drying half of the job is what keeps the assembly young.
Four ways the path gets blocked
Weeps rarely fail on their own. They get closed, usually by someone doing a different job in good faith.
Paint and coating bridges
The most common blockage we find. A spray or roller pass carries coating across the gap at the cladding base, or fills the perforations of a vent strip, and the drainage opening becomes a painted-shut seam. It happens a little more with each repaint cycle, so a wall on its third coating can be fully sealed without anyone having decided to seal it. Masking the openings belongs in every painting contract.
Sealant in the wrong place
A resident or handyman sees a gap at the bottom of the cladding, reads it as a defect, and caulks it. Multiply by every ground-floor perimeter and every balcony interface on the building. Sealant across a weep converts the drainage exit into a dam, and the water that should have left the wall now sits against the barrier through the wet season. Education beats repair here: councils should tell owners what the gaps are for.
Blocked or clogged insect screens
Vent strips and weep screens keep wasps and mice out of the cavity, and they belong there. They fail by clogging: lint, spider webs, efflorescence, cottonwood fluff in June, and corrosion products all accumulate in the perforations. A clogged screen throttles both drainage and airflow. The fix is cleaning or swapping the strip, minutes of work per metre when caught on a routine walk-around.
Grade changes and hard landscaping
New planters, raised beds, added pavers, or a re-graded walkway bury the base of the cladding, and the weep path with it. Soil against cladding also wicks ground moisture up into the wall, adding water while removing the exit. Keep visible clearance between finished grade and the cladding base, and treat any landscaping proposal that touches the building perimeter as an envelope question, worth a walk with the plan in hand.
What a blocked wall does about it
Nothing dramatic, at first, which is the trap. A rainscreen with sealed weeps still sheds most of its rain off the cladding face, so the building looks fine. Meanwhile the fraction that enters the cavity has stopped leaving. It pools on flashings, wicks into strapping and sheathing paper, and holds the assembly damp through the winter. The failure announces itself years later as staining at the base of the wall, coatings blistering from behind, a musty ground-floor suite, or soft sheathing found during unrelated work. By then the cheap problem has compounded into an envelope repair scope.
Coating blisters deserve a special mention because they get misread. When an elastomeric or acrylic coating bubbles off a wall that also has sealed weeps, the coating takes the blame, but trapped cavity moisture pushing outward is often the real driver. Our article on elastomeric wall coatings covers when those products earn their keep, and none of it works over a wall that cannot breathe.
Checking the path, and keeping it open
The check costs an hour a year on most buildings. Walk the perimeter with a flashlight: daylight or a clean vent strip along the cladding base, open perforations, no paint film, no sealant beads, no soil or paver contact. Probe gently with a zip tie where you suspect a bridge. In heavy rain, watch the base of the wall and confirm water actually drips clear of the weeps. Photograph what you find and file it with the rest of the annual inspection record. Upper-storey vent lines on towers are rope access territory, quick work for a certified crew.
Brick buildings get one extra line on the checklist. Weep holes in veneer are single open head joints, spaced along the bottom course and above shelf angles, and they are small enough that one mortar smear or one enthusiastic bead of caulk closes a whole run. Efflorescence blooming near the base of a brick wall is often the first sign its weeps stopped keeping up.
Keeping it open is mostly contract language. Painting specs that mask the openings, sealant scopes that name where caulking must never go (our sealant and caulking guide covers the rules), landscape plans that respect the clearance at grade, and a walk-through before final payment on any of them. The rainscreen hub keeps repeating one idea, and this page is its purest case: the wall survives on its details, and the details survive on someone checking.
Quick answers
What are weep holes on a building wall?
Weep holes are the openings at the bottom of a wall assembly where water drains out. On a rainscreened building they appear as a continuous gap or a perforated vent strip at the cladding base and above flashings; on brick veneer they are open head joints left in the bottom mortar course. They are the exit of the drainage path, and everything upstream in the wall depends on that exit staying open.
How does cavity venting help a rainscreen wall dry?
Cavity venting lets air move through the gap behind the cladding, in at the base and out at the top on well-detailed walls. That airflow carries away the moisture left after drainage: the damp film on the back of the cladding and the face of the barrier. Between Metro Vancouver storms, venting is what resets the wall to dry. A cavity that drains but cannot vent stays damp far longer, and damp time is what ages an assembly.
What blocks weep holes most often?
Four blockers cover nearly every case we find on Metro Vancouver buildings. Paint or coating carried across the openings during repaints. Sealant applied over the gaps by someone who read them as defects. Insect screens and vent strips clogged with debris, webs, or corrosion. And grade changes, planters, pavers, or soil raised against the cladding base by landscaping work. All four are preventable, and three of them arrive on an invoice the strata approved.
Weep and venting questions
What are weep holes on a building wall?
Weep holes are the openings at the bottom of a wall assembly where water drains out. On a rainscreened building they appear as a continuous gap or a perforated vent strip at the cladding base and above flashings; on brick veneer they are open head joints left in the bottom mortar course. They are the exit of the drainage path, and everything upstream in the wall depends on that exit staying open.
How does cavity venting help a rainscreen wall dry?
Cavity venting lets air move through the gap behind the cladding, in at the base and out at the top on well-detailed walls. That airflow carries away the moisture left after drainage: the damp film on the back of the cladding and the face of the barrier. Between Metro Vancouver storms, venting is what resets the wall to dry. A cavity that drains but cannot vent stays damp far longer, and damp time is what ages an assembly.
What blocks weep holes most often?
Four blockers cover nearly every case we find on Metro Vancouver buildings. Paint or coating carried across the openings during repaints. Sealant applied over the gaps by someone who read them as defects. Insect screens and vent strips clogged with debris, webs, or corrosion. And grade changes, planters, pavers, or soil raised against the cladding base by landscaping work. All four are preventable, and three of them arrive on an invoice the strata approved.
Can painting a building block its rainscreen drainage?
Painting blocks rainscreen drainage more often than any other trigger. Spray and roller work bridges the base gap and fills vent strip perforations, and each repaint cycle adds film thickness until the openings seal completely. The prevention is contractual: painting specifications should require masking of weeps and vents, and the post-job walk-through should include a check that daylight still shows at the drainage openings before final payment.
Are insect screens in weep openings a problem?
Insect screens belong in weep openings; the problem is forgetting them afterward. A perforated strip keeps wasps, bees, and rodents out of a cavity they would happily colonize. The trade-off is that perforations clog over the years with lint, webs, efflorescence, and seed fluff, throttling drainage and airflow. Screens need occasional cleaning or replacement, a quick task per elevation, which is why they earn a line on the annual inspection checklist.
How do landscaping and grade changes block wall drainage?
Landscaping blocks wall drainage by burying it. Raised beds, new pavers, added topsoil, and re-graded paths lift the finished grade until it covers the cladding base and the weep path with it. Soil contact also wicks ground moisture up into the cladding and cavity, adding water at the same time it removes the exit. Any site work near the building perimeter should preserve clear separation between grade and the base of the cladding.
How do I check weep openings without opening the wall?
Checking weeps needs a flashlight, a feeler as simple as a plastic zip tie, and patience. Walk the cladding base and the lines above windows and floor flashings, confirming a consistent open gap or clean vent perforations, with no paint film, sealant, or debris. During heavy rain, watch for water actually dripping from the base openings, which proves the path works end to end. Note and photograph anything sealed, and check the same spots each year.
Should weep holes ever be sealed to keep water out?
Weep holes should never be sealed, and the instinct to seal them gets the physics backward. The openings are exits, sheltered under the cladding edge where wind-driven rain cannot meaningfully enter. Sealing them traps whatever water the cladding admits, holding it against the barrier and sheathing through the wet season. If water appears to be flowing out of a weep, that is the wall working as designed, and the volume is worth investigating upstream.
Do weep holes let water or pests into the wall?
Weep holes let almost no water in, because they sit at the drained bottom of the assembly, shielded by the cladding above, and drainage flows outward by gravity. Pests are the honest risk, which is why openings are screened with perforated strips or mesh sized to admit air and water while excluding insects and rodents. A properly screened weep gives up nothing to the wall’s enemies and gives the cavity its exit and its air supply.
What happens when a rainscreen cavity cannot vent?
A cavity that cannot vent turns from a drying chamber into a humidor. Drainage may still remove flowing water, but the residual dampness on the cladding back and barrier face has no airflow to carry it off, so the assembly sits wet between storms, and Metro Vancouver offers long runs of them. Over the years that extended wet time degrades coatings and sheathing paper, feeds mold on organic surfaces, and quietly shortens the life of the wall.
Not sure your wall is draining?
We check weep paths, vents, and flashings on strata and commercial buildings across Vancouver, Burnaby, and the North Shore, including the elevations only reachable by rope.