What are weep holes for?

Weep holes allow water that penetrates the outer leaf of a cavity wall to drain out rather than accumulating in the cavity or being bridged to the inner leaf. Water enters cavities through wind-driven rain, condensation, and mortar droppings. Without weep holes, water can build up behind cavity trays and cause damp problems. Weep holes also provide some ventilation to help the cavity dry out.

Where should weep holes be placed?

Weep holes must be provided at every DPC level (base of wall), above every cavity tray (over windows, doors, meter boxes, air bricks), at sill level below windows, and at any point where a cavity tray collects and diverts water. A minimum of two weep holes should be provided above each opening. They should be at the lowest point of the cavity tray where water collects — not at the high end of a stepped tray.

What spacing should weep holes be at?

Maximum 900mm centres (horizontally) is the standard recommendation per BS EN 1996-2 and most cavity tray manufacturers. Some authorities recommend 450mm centres for improved drainage, particularly in exposed locations. At least two weep holes should be provided per opening (one near each end of the lintel). Perpend weep holes (open vertical joints) should be a minimum of 10mm wide.

What types of weep holes are there?

Three main types: (1) Open perpend joints — simply leaving the vertical mortar joint open in the outer leaf (cheapest but can allow insects); (2) Proprietary weep vents — plastic inserts that fit into the perpend joint and include mesh to exclude insects (e.g., Timloc weep vents); (3) Wicking materials — rope or wick placed in the joint to draw water out by capillary action (least effective, not generally recommended for new build).

Weep Hole Spacing — Cavity Wall Drainage

Weep Hole Spacing Requirements

Location	Max Spacing	Minimum Number	Notes
Base DPC level	900mm centres	—	At or just above DPC in outer leaf
Above window/door lintels	900mm centres	2 per opening	At each end of cavity tray
Below window sills	900mm centres	2 per opening	Where cavity tray catches sill drainage
At meter box openings	—	2	One each side of the opening
At air brick locations	—	2	Where cavity tray diverts above air brick
At abutment with roof (lean-to)	900mm centres	—	Along the full length of cavity tray

Weep Hole Types

Type	Description	Pros	Cons
Open perpend	Vertical joint left open (no mortar)	Free, simple, maximum drainage	Visible, insects/draughts can enter
Plastic weep vent	Proprietary insert with insect mesh	Neat appearance, pest-proof	Small cost, reduced drainage area
Weep vent with extension	Insert with extension into cavity	Directs water to outer face	More complex to install
Cotton/nylon wick	Absorbent cord in mortar joint	Hidden from view	Poor performance, blocks easily

Key Installation Points

Weep holes must align with the lowest point of the cavity tray — water must be able to flow to them
Cavity trays should have stop ends to prevent water running past the weep hole location
The cavity below the tray must be clean of mortar droppings — use a cavity board or clean out as you build
Weep holes in the base DPC course should be positioned just above finished ground level (min 150mm)
Never block or mortar over weep holes during pointing or rendering
Stepped cavity trays (over sloping abutments) need weep holes at each step

Note: Refer to BS EN 1996-2, Approved Document C, and cavity tray manufacturer guidance for definitive requirements. NHBC Standards Chapter 6.1 also provides detailed guidance for new-build housing.

See the Movement Joint Spacing, the UK Brick Dimensions, and the Brick Bond Patterns guide.

Weep Hole Spacing — Cavity Wall Drainage

Weep Hole Spacing Requirements

Weep Hole Types

Key Installation Points

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