Weep Hole Spacing — Cavity Wall Drainage

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.

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.

Maximum 450mm centres (horizontally) is the standard requirement above cavity trays, lintels and other openings, per NHBC Standards 6.1.17 and cavity tray and lintel manufacturer guidance (Catnic, IG, Keystone). A wider spacing of up to 900mm centres is sometimes used for weep holes along a continuous run of base DPC away from any tray or opening. 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.

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).