Most brick walls in the UK are built with water in mind, not to keep it out completely. Yes,
Rain gets in, leading to condensation forming & moisture freely traveling through the mortar joints. The trick isn’t to stop it cold, it’s to get it out safely. That’s where weep holes come in.
They’re teeny gaps, often ridiculously easy to overlook, yet utterly vital to the health of our cavity walls.
When they’re missing, blocked, or dodgily spaced, Damp takes a firm hold & once that’s happened, you can bet your life repairs are going to cost an arm & a leg in no time.
This guide covers the lowdown on weep holes: just what they do, what the UK building standards say about spacing them, and more importantly, how to safely & effectively retrofit them into walls that already exist.
What Are Weep Holes & Why Should You Care?
Weep Holes are small gaps left in the mortar of your brickwork, usually at the bottom of your walls or above your doors & windows.
And guess what their job is? It is pretty simple, really. Let trapped water have somewhere to go from the cavity of the wall.
Every wall in the UK has two sides, the outer brick face that gets absolutely soaked, and the inner blockwork leaf that should stay bone dry.
The gap between them is designed to help moisture drain away. When the rain gets to the outer leaf, it trickles down the inner face of that brickwork & the cavity trays catch it. The weep holes then chuck it out.
If you don’t have them… well, the trapped moisture has nowhere to go, simply seeping inwards & causing damp patches to appear on the plaster, rotten skirting boards, & those tell-tale white salt stains ( efflorescence) creeping right across your wall surface.
Weep Holes aren’t just about letting the water out, either. They’re also a way of getting a bit of airflow in there, which is especially handy to have when the wet weather is hammering down & the blustery wind is making the whole country feel damp & miserable.
How Weep Holes Work Within The Wall Cavity
Imagine the cavity as a secret drainage system built into the wall. Above the openings, a cavity tray is installed, a sloped, damp-proof membrane that collects any water that enters through the brick face. The tray directs the moisture towards the outer leaf.
Right where that tray meets the outer brickwork, a small gap, the weep hole, allows the collected water to drain out. Gravity does the rest.
A well-designed cavity system has several layers working together:
- The damp proof course (DPC) near the base stops moisture rising from the ground.
- Cavity trays above openings, catching rain that drives through the wall.
- Weep holes, usually above every lintel, give water a route to escape.
If one part fails, the others are forced to take the strain, and they rarely cope for long.
UK Building Regulations And Spacing Requirements
The role and spacing of weep holes in the UK are covered in a few documents:
- Approved Document C of the Building Regulations (Site preparation and resistance to contaminants and moisture).
- NHBC Standards 6.1.14 for masonry walls.
- BS 8215:1991 and BS EN 1996-2 for cavity construction practice.
The general rule:
- Install weep holes every 450mm along cavity trays or lintels.
- At least two weep holes above any opening, regardless of width.
- Align them with brick courses for neatness and consistency.
Most new builds use plastic weep vents, colour-matched to the brickwork and designed to block insects and debris.
Older buildings might just have open perp joints (the vertical mortar gaps between bricks), left unfilled.
Building Control officers check these details closely during inspection. Missing or unevenly spaced weep vents are a common snag list item, not just for moisture reasons, but because they indicate corners cut during bricklaying.
Common Problems With Weep Holes
Weep holes fail in two ways: they get blocked or they get hidden.
Fresh mortar falls into the cavity during construction and blocks the vents before the wall is even finished.
Over time, render, paint, or silicone sealants cover them completely. Homeowners fill them to “stop draughts” or “tidy the wall,” not realising they’ve just sealed off the wall’s drainage system.
A few telltale signs of weep failure:
- Damp patches above skirting boards or lintels.
- White salt staining (efflorescence) below window sills.
- Crumbling or flaking bricks on the first few courses.
- Mould growth on the inner wall near ground level.
And once frost gets involved, things get worse, trapped water freezes, expands, and starts popping off the brick faces.
Diagnosing Missing Or Ineffective Weep Holes
Spotting missing or blocked weep holes takes more attention to detail than a PhD, but doesn’t need loads of training – just a bit of a keen eye and a flashlight.
Take a close look above every door and window, particularly the lintel; you should be able to see some narrow vertical slots, roughly 450mm apart.
Now, if you see long stretches of brick with no signs of vents, or a wall that’s showing signs of damp but no sign of any vents at all – that’s more than a bit worrying.
Getting a peek inside the wall is where inspection cameras or one of those borescope things come in handy.
They can help you figure out if there are mortar droppings or some other blockage in there. Surveyors use them all the time before recommending any retrofit work.
If you’re in doubt, it’s worth getting a damp expert or building surveyor on the case. Surface symptoms like damp patches and hairline cracks can sometimes suggest there’s something more serious going on, like structural movement.
Retrofitting Weep Holes: Adding Or Unblocking
Retrofitting newer weep holes into existing walls is just one of those jobs that sounds easy but actually needs a bit more care than you might think.
Adding new weep holes in existing walls can be a real doddle if done right. You just need to get a masonry drill and then make a hole through the perp (that vertical mortar line between the bricks) at the right height, usually just above the DPC or the cavity tray.
Then you just insert one of those small plastic retrofit vents into the hole, and it’s flush with the brick.
Accuracy is vital – drill too low and you risk messing the DPC, drill too high and water won’t even make it through to the hole.
That’s why most pros will use an inspection camera to find the cavity tray or the damp course line first.
Blocked weeps can sometimes be freed up with some compressed air, a flexible rod, or a narrow brush, but only if the tray is properly lined up and the drainage route is in good nick.
A few safety bits to bear in mind:
- Always wear some eye protection and a dust mask.
- Don't, I repeat DON'T, drill through render or sealant, it's easier and safer to remove and replace a brick if needed.
- And finally, if you need to use scaffolding, make sure it's properly secured. Ladder accidents aren't confined to big jobs, so brush up on your workplace safety know-how.
Retrofit systems these days come in all sorts of colours and materials, terracotta, brown, even black to match the brickwork, and they’re not expensive, but they can save you a small fortune in the long run.
Alternative Moisture Management Strategies
If weep holes aren’t enough or can’t be installed easily, there are other options to control water ingress:
- Repointing using lime-based mortar, which breathes and helps walls dry faster.
- Installing new cavity trays during refurbishments if the originals have failed or never existed.
- Improving drainage around the base of the wall and simple landscaping changes can prevent splashback saturation.
Moisture problems often connect with structural or material issues, too, bowed brick faces, corroded wall ties, or failed render. In older cavity walls, addressing one usually means assessing all.
Training And Simulation Tools For Trades
Practical learning matters in construction. Knowing the theory of weep holes is one thing, spotting, cutting, and testing them on-site is another.
Tools like Tradefox let tradespeople, surveyors, and apprentices practice realistic repair and inspection tasks through safe simulation. It’s a good way to learn drainage principles and fault detection before working on live builds.
Conclusion
Weep holes are one of those details most people never notice, until they fail. A gap of just a few millimetres can be the difference between a dry, healthy wall and one riddled with damp.
Following UK spacing rules, keeping vents clear, and inspecting them regularly are simple steps that pay off. And when retrofitting, precision and safety matter just as much as the hole itself.
In short, brickwork doesn’t need to be watertight, it just needs a way to breathe and drain. Weep holes give it that breath. Ignore them, and the wall will eventually remind you why they were there in the first place.
