Why does my wall have gaps in the mortar joints, and can I fill them?
The open vertical joints above windows, doors and at the base of a wall are drainage, not defects — what they do, and the three other gaps people confuse them with.
- Difficulty
- beginner
- Time
- 8 min
- Read
- 3 min
- Safety
- caution
Short answer
They are weep holes. A cavity wall has trays above openings and at its base that catch water crossing the cavity, and the weep holes let that water back out. Blocking them sends the water into the house instead. Leave them open, keep them clear, and never point them up.
Perfectly regular gaps in the vertical joints, usually just above a window or a door and often at the very bottom of a wall, look exactly like a bricklayer who ran out of mortar. They get filled by decorators, by people repointing, and by anyone trying to stop draughts or insects. They are drainage, and filling them causes damp that is then blamed on everything except the actual cause.
Safety
Step by step
- Recognise a weep hole.An open vertical joint, or a small plastic vent insert, at regular spacings in a bed joint course — commonly just above a window or door head, above the damp proof course at the base of the wall, and above any cavity tray. They come in a row, evenly spaced, which is what distinguishes them from a joint that has fallen out.
- Understand what they drain.In a cavity wall, water that penetrates the outer leaf runs down inside the cavity. Where something interrupts the cavity — a lintel, a tray over an opening, the damp proof course at the base — the water is caught on a tray and has to be let back out. That is the weep hole's only job.
- Leave them alone.Do not point them up, do not fill them with sealant, and do not let render or a repointing job close them. If they are blocked, water backs up on the tray and eventually crosses to the inner leaf and appears inside, usually above a window.
- Keep them clear.They block with mortar droppings during construction, with paint, with render, with spider webs, and with insect nests. Clearing a blocked weep hole is a job for a piece of wire and a moment of care, and it is worth checking them when investigating any unexplained damp above an opening.
- Do not confuse them with other openings.Airbricks are much larger and ventilate a suspended floor void, and they must also stay clear. Wall vents serve rooms or appliances. Ventilation bricks in a garage are different again. All of them have a purpose and none should be blocked.
- Do not confuse them with a failed joint.A joint that has weathered or fallen out is irregular, ragged and in no particular pattern. A weep hole is clean-edged, deliberate and part of a regular row.
- Use insect-proof weep vents if there is a real problem.If bees or wasps are getting in, plastic weep vents allow drainage while excluding insects. That is the correct solution — filling the hole is not.
- Check them when diagnosing damp above a window.Damp appearing on the inside above a window head is a classic symptom of a blocked or missing weep hole, a failed cavity tray or a bridged cavity. It is worth checking before anything more expensive is considered.
Questions people ask
Can I fill the gaps in my brickwork to stop draughts?
Not if they are weep holes. They drain water out of the cavity, and blocking them causes damp inside. Draughts are dealt with at the inner leaf and at the openings, not by sealing the outer wall's drainage.
How do I tell a weep hole from a missing bit of pointing?
Regularity. Weep holes come in an evenly spaced row along a course, with clean edges. Failed pointing is irregular, ragged and randomly located.
Do solid walls have weep holes?
No — there is no cavity to drain. Regular open joints in a solid wall are more likely to be putlog holes from old scaffolding, or ventilation. Find out what they are before filling anything.
Wasps are coming out of my weep holes. What should I do?
Deal with the nest rather than the hole. Once the nest is gone, fit plastic weep vents, which keep insects out while still letting water drain.