A weep vent — also called a weep hole vent or vermin-proof weep — is a small grille or insert placed in an open perpend (head) joint of a cavity wall. It keeps the opening clear for air and water while closing it to mice, wasps, bees and birds. Its size matches the perpend joint between two brick headers.
Why those openings are there
Open perpends are not sloppy workmanship but a technical requirement. They let the cavity ventilate and they allow any water that gets behind the outer leaf to run out over the cavity tray. They are therefore always placed where the masonry is interrupted or closed off horizontally:
- at the base of the wall, above the damp-proof course
- above and below window and door openings, and over lintels and soldier courses
- at the top of the outer leaf, below the eaves
UK guidance (NHBC Standards and PD 6697) puts weep holes over cavity trays and openings at a maximum of 450 mm centres — roughly every second brick — with at least two per tray run or lintel, and the first and last within about one brick of each end. Always follow the designer’s detail as well.
Application
- Never seal them. Do not close the openings with mortar or sealant to keep vermin out — that cuts off the drainage path and leads to damp penetration and efflorescence. Fit a vent instead
- Sizing. Vents are made in heights matching common brick formats. Measure the joint height before ordering
- Material and colour. Plastic vents come in black, anthracite and brick colours; stainless steel is the strongest option where rodents are genuinely a problem
- Fitting. In new masonry the bricklayer slides the vent in as the wall goes up. Retrofitting is possible with a thin blade or a joint raker; do not force it, because a damaged joint lets water through again
- Checking. Inspect the vents at every maintenance round: blocked or missing vents are one of the most common causes of mouse infestation and a damp cavity
Related terms
- Perpend joint
- Cavity wall
- Cavity tray
- Ventilation
- Brick
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