Look at the bottom of almost any face-brick house in Cape Town and you will see small gaps in the mortar every few bricks along. Those are weep holes. They look like a builder forgot to finish the joint, so homeowners often fill them in, or a plasterer closes them up during a paint job. That small mistake is behind a lot of the damp walls we are called out to fix. This guide explains what weep holes do in a cavity wall, what the building regulations say, and what happens when they are blocked or left out.
What Are Weep Holes?
Weep holes are open vertical joints (called perpends) left without mortar in the outer skin of a brick wall. They are usually at the base of the wall, just above the damp-proof course, and sometimes above windows and doors. Their job is simple: let water that has got into the wall drain back out.
They only make sense once you understand how a cavity wall works. Most newer Cape Town homes, and many extensions, are built with two separate skins of brick (the outer leaf and the inner leaf) with a gap between them. That gap is the cavity. Wall ties hold the two leaves together across it.
How Weep Holes Work in a Cavity Wall
Face brick and mortar are porous. In heavy, wind-driven rain, water soaks through the outer leaf and runs down its inside face into the cavity. The cavity is there to stop that water reaching the inner leaf and your interior walls. But the water still has to go somewhere.
At the bottom of the cavity, a damp-proof course or membrane catches it and directs it outwards. The weep holes in the course of bricks just above or below that damp-proof course are the exit. Water runs out of them instead of building up in the bottom of the cavity. Weep holes also let air move through the cavity, which helps the wall dry out after a wet spell.
What the Building Regulations Say About Weep Holes
In South Africa, walls are covered by SANS 10400 Part K. For a masonry cavity wall, it requires a damp-proof course in each leaf, or a membrane across the cavity that sits higher on the inner leaf than on the outer leaf, so water is directed outwards. Where necessary to stop water building up in the cavity, it requires weep holes in the external leaf spaced not more than 1 metre apart, in the course of bricks right next to that damp-proof course.
The same standard sets cavity widths at between 50 mm and 110 mm, and says no horizontal damp-proof course may be installed less than 150 mm above the finished ground outside. That last rule matters for weep holes too: if the paving or garden bed outside is raised over the years, the weep holes and the damp-proof course can end up at or below ground level, where they stop working. Our guide on building plans approval explains when new work has to be inspected against these rules.
Why Weep Holes Matter So Much in Cape Town
Cape Town is a winter rainfall area. Between May and September we get long spells of rain driven sideways by north-westerly winds, often for days at a time. That is exactly the weather that pushes water through face brick and into the cavity. Coastal suburbs add salt to the mix, and older homes on low-lying ground can have high groundwater as well.
So in this climate, weep holes are not a detail you can skip. A cavity wall in Cape Town that cannot drain will hold water every winter, and that water has to go somewhere.
Problems When Weep Holes Are Blocked or Left Out
When weep holes are missing, filled with mortar, plastered over or buried under paving, the water in the cavity has no way out. Over time we see the same set of problems:
- Damp on interior walls, usually a tide mark near the skirting, bubbling paint and salts coming through the plaster.
- Water bridging the cavity where mortar droppings have collected at the bottom, carrying damp straight to the inner leaf.
- Mould and musty smells in cupboards and rooms against the affected wall.
- Efflorescence, the white powdery salts on the outside face of the brickwork.
- Rusting wall ties over the long term, which weakens the bond between the two leaves.
Many of these symptoms look like rising damp, and they are often treated as rising damp. Injecting a chemical damp-proof course will not fix a wall that is simply full of water with nowhere to go. Our article on rising damp causes and signs explains how to tell the two apart, and cracks in walls covers the structural side.
Weep Holes Around Windows, Doors and Extensions
The base of the wall is not the only place weep holes are used. Above a window or door, water running down inside the cavity hits the lintel. Builders often fit a cavity tray (a strip of damp-proof membrane that slopes outwards) above the lintel, with weep holes in the outer leaf just above it, so the water is pushed out instead of dripping onto the window frame inside.
Extensions need extra care where the new wall meets the old one, and where a new roof or patio roof meets an existing cavity wall. If a flashing or new structure blocks the existing weep holes, the old wall can start showing damp a winter or two later. Our home extension structural checks guide covers these junctions.
Is It OK to Fill or Cover Weep Holes?
No. Weep holes should stay open for the life of the wall. A few common ways they get blocked by accident:
- Plastering or bagging the outside of a face-brick house without leaving the weep holes open.
- Paving, concrete or garden beds raised against the wall above the damp-proof course.
- Filling them with silicone or mortar to keep insects out.
- Painting over them with thick elastomeric paint.
If insects or rodents are the worry, use a proper weep hole insert or vent made for the purpose. These let water and air through but keep pests out. If you are planning to plaster or paint a face-brick house, tell your contractor to keep every weep hole open.
Fixing a Wall With Blocked or Missing Weep Holes
The fix depends on what we find. Where weep holes have been filled, we clear them out and check that water drains. Where the ground outside has been raised, the paving or soil needs to come down to at least 150 mm below the damp-proof course. Where weep holes were never built in, we can cut the mortar out of selected joints in the right course to form new ones. Where the bottom of the cavity is packed with mortar droppings, a few bricks may need to come out so it can be cleaned.
Once the wall can drain, the inside usually needs time to dry before the damaged plaster and paint are redone. Doing the plaster first and the drainage later just means doing the plaster twice. If you are dealing with damp and are not sure of the cause, ask us for a site visit through our quote page and we will find the source before we price anything. For the wider picture, see our waterproofing services.
Weep Holes: Frequently Asked Questions
How far apart should weep holes be?
Under SANS 10400 Part K, weep holes in the outer leaf of a cavity wall should be no more than 1 metre apart, in the course of bricks right next to the damp-proof course.
What happens if a house has no weep holes?
Water that gets into the cavity cannot drain out. In a wet Cape Town winter it builds up at the bottom of the wall and often shows up as damp on the inside, peeling paint and salts on the brickwork.
Can I fill weep holes to stop insects getting in?
No, filling them traps water in the wall. Use weep hole inserts or vents designed for the job, which keep insects and rodents out but still let water and air through.
Do solid walls need weep holes?
Weep holes are mainly needed in cavity walls, where there is a gap for water to collect. Retaining walls also need drainage holes so water behind them can escape, but those work differently.
Can weep holes be added to an existing wall?
Yes. A builder can rake out the mortar from selected vertical joints in the right course to form new weep holes, as long as the damp-proof course position is confirmed first.
