Who this guide is for
- Homeowners noticing small gaps at the base of brickwork
- People who sealed weeps and now have damp
- Renovators planning a drained masonry or veneer wall
- Anyone briefing a mason on cavity drainage
Why walls have a cavity
A cavity or drainage gap behind the outer face gives water that penetrates the brick, stone or cladding somewhere to go, draining down and out rather than into the structure. The cavity is part of how a drained wall stays dry. Understanding it explains why the weeps exist.
What weep holes do
Weep holes are small openings near the base of the wall that let collected water drain out of the cavity, and they often double as low-level ventilation. They look like gaps to fill but are doing essential work. Keeping them open is the whole point.
- Weeps drain water from the cavity at the base
- They can also provide low-level ventilation
- They look like gaps but must stay open
- Blocked weeps trap water in the wall
Keeping the cavity clear
The drainage only works if the cavity itself stays clear; mortar droppings or debris during construction can bridge it and let water cross to the inner wall. This is a construction-quality issue planned in from the start. A clear cavity is as important as open weeps.
The mistake of sealing weeps
Filling weep holes with mortar or caulk, often to keep pests out or tidy the look, blocks the wall's only drainage exit and traps water. This is a frequent, damaging error. Never seal a weep without understanding its role.
Drainage detailing and professionals
Cavity drainage, weep placement and flashing at the base are weather-critical masonry details, so a qualified mason or builder should plan and execute them, and damp linked to blocked drainage should be assessed. Requirements vary by location and project, and this is not a place to improvise with sealant.
Cavity drainage planning checklist
- 1Recognise the cavity is the wall's drainage path
- 2Identify the weep holes near the base of the wall
- 3Keep weep holes open, never sealed
- 4Plan to keep the cavity clear of debris and droppings
- 5Coordinate weeps with base flashing
- 6Consider pest screening that still lets water out
- 7Have damp linked to blocked weeps assessed
- 8Use a qualified mason for the drainage detailing
Common mistakes to avoid
- Sealing weep holes to tidy the look or block pests
- Treating the cavity as empty space to fill
- Letting mortar droppings bridge the cavity
- Assuming the outer face keeps all water out
- Ignoring base flashing that works with the weeps
- Sealing over damp instead of finding the blocked drainage
When to involve a professional
- Use a qualified mason or builder for cavity and weep detailing
- Treat weeps and base flashing as weather-critical
- Have damp linked to blocked drainage assessed by a professional
- Plan any pest screening so it still lets water drain
- Requirements vary by location and project; verify with your professionals
Frequently asked questions
Questions readers ask about this topic
What are weep holes for?
Weep holes are small openings near the base of a wall that let water draining down the cavity escape, and they often provide low-level ventilation too. They look like gaps to fill but are doing essential work, so keeping them open is the whole point.
Can I seal up weep holes?
No. Sealing weeps with mortar or caulk, often to block pests or tidy the look, blocks the wall's only drainage exit and traps water. It is a frequent, damaging mistake, so weeps should never be sealed without understanding their role.
Why does a wall have a cavity?
The cavity or drainage gap behind the outer face gives water that penetrates the brick, stone or cladding a path to drain down and out rather than into the structure. It is part of how a drained wall stays dry, which is why the weeps at its base matter.
I have damp and think the weeps are blocked. What now?
Have it assessed by a qualified mason or builder rather than sealing over it, because blocked weeps or a bridged cavity trap water. This is a weather-critical masonry detail, and requirements vary by location and project.
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