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Cavity Wall Weep Hole Spacing Calculator

Estimate the number of weep holes needed along the base of a brick veneer or cavity wall.

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  • Every formula cited
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SettingsSettings for this calculationUS
Market
Imperial · sales tax
The total length of cavity/veneer wall base needing weep holes.

Measure along the base of the veneer directly above the flashing, not around the building. Weeps drain a flashing run, so every separate run needs its own: a wall stepped down a sloping site is several runs, and each window head has a flashing over it that also needs weeping. Add those lengths together rather than taking the perimeter, which counts wall with no flashing under it at all.

The on-center spacing between weep holes.

24 in (610 mm) is a common spacing for open head joint weep holes; wick or tube weeps are sometimes spaced closer — 16 in (405 mm) — for reliable drainage.

Weep holes needed

18 weep holes

Medium confidence

The plus-one term already places a weep at each end of this run, so do not add two more by hand — and where elevations meet, counting each run separately books the shared corner position twice. What is NOT counted: weeps above every door and window head, at shelf angles, and at any other flashing discontinuity, each of which needs its own row at this spacing.

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • Standard cavity/veneer wall practice places weep holes at regular spacing along the wall base directly above flashing, typically every 16-33 in on-center depending on the weep hole type, which is the 406 to 838 mm this page accepts

Inputs used

Wall Length
33 ft
Weep Hole Spacing (on-center)
24 in
Final result18 weep holes

Confidence note: The plus-one term already places a weep at each end of this run, so do not add two more by hand — and where elevations meet, counting each run separately books the shared corner position twice. What is NOT counted: weeps above every door and window head, at shelf angles, and at any other flashing discontinuity, each of which needs its own row at this spacing.

What this calculation does not cover

  • One straight run along one flashing line, and nothing above it. Every cavity tray higher up the wall — over each lintel, at each cill, under each shelf angle, at every roof abutment — is a separate drainage compartment needing its own weeps in the course directly above its own flashing, and none of those openings are in this figure.
  • Nothing here checks the spacing you typed against the code you build under. The field accepts 406 to 838 mm (16 to 33 in), which is the North American band; British practice is tighter, commonly around 450 mm centres, and other markets differ again. This is arithmetic on the figure you enter, not a verdict that the figure is permitted.
  • The weep itself is outside the model — opening size, free area, and whether the type you have matches the spacing you chose. Wick and tube weeps move far less water than an open perpend, and cellular inserts and insect screening cut free area below the raw size of the hole, so the same count of openings is not the same drainage capacity.
  • Position decides whether any of them work and position is not counted. Weeps belong in the perpends of the course immediately above the flashing, formed as the bricks go back; one course too high and water sits permanently below its own outlet, and an opening with a mortar dam behind it drains nothing while still counting here.
  • Vents are not in this number. Openings at the head of a drainage compartment — under soffits, under shelf angles, immediately below each cavity tray — are what let the cavity dry rather than merely drain, and they are a second set of openings on top of the ones counted here.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

2 ft33 ft18 at 2 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Part of bigger jobs

This trade is one line of several job takeoffs. Run the whole job and every other trade comes back with it, off the same measurements.

Or plan the space itself: measure it once, doors and windows included, and this figure comes back worked out on that space, with what goes with it.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1
  1. Standard cavity/veneer wall practice places weep holes at regular spacing along the wall base directly above flashing, typically every 16-33 in on-center depending on the weep hole type, which is the 406 to 838 mm this page accepts
Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Tools and safety for this job

To lay brick and lay block. Generic types, no brands, no prices.

Protection this work requires

  • Cutting or grinding concrete, masonry, screed, tile or fibre-cement board releases respirable silica: cut wet or extract at the tool, and wear a P2/N95 respirator at minimum — a nuisance dust mask does not filter it.
  • Wet cement and lime burn skin and eyes painlessly until the damage is done: waterproof gloves to EN 374, safety glasses whenever the mix can splash, and never kneel in wet mix in permeable trousers.
  • Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
  • Breakers and grinders cause permanent nerve damage: limit continuous trigger time, keep hands warm, and stop if fingers tingle or blanch.
  • Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
Show the 9 tools this job needs

Essential

  • Brick trowel and jointer

  • Mixing bucket or tub

  • Spirit level

  • String line and pins

  • Tape measure

  • Wheelbarrow

Recommended

  • Angle grinder

  • Block splitter

  • Cement mixer

Also needed as materials: cutting and diamond discs.

what each masonry tool is for, and the spec that decides which to buy where one does.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Called something else where you work? Cavity wall / drainage cavity · Wall ties and veneer anchors · Weep holes and cavity drainage · Cavity tray and through-wall flashing — the term in each market, how close the equivalence really is, and the standard that governs it.

Already gone wrong? There is a white powdery deposit on my brickwork · The brickwork above my window is cracking or lifting

How to calculate cavity wall weep hole spacing in 3 steps

  1. Wall LengthThe total length of cavity/veneer wall base needing weep holes.
  2. Weep Hole Spacing (on-center)The on-center spacing between weep holes.
  3. Weep holes neededThe tool computes the weep holes needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Weep holes needed by wall length

Page defaults, not your figures above.

Wall LengthWeep holes needed (weep holes)
20 ft11
30 ft16
40 ft21
50 ft26
60 ft31

Frequently asked questions

What decides the spacing — the wall, or the water?
The water, and specifically how far it will run sideways before it finds a way out. A weep hole is an exit, not a vent: water that has crossed the cavity lands on a tray or a DPC and travels along it under gravity and surface tension until it meets an opening. Space them too far apart and the tray holds a standing head of water that eventually finds the inner leaf, which is the failure people blame on the tray rather than on the holes. That is also why every tray needs its own weeps — one above a lintel, one at the base, one at each cill — rather than a single set at the bottom of the wall, and why a weep sitting even one course too high leaves water permanently below its own outlet.
Why do cavity walls need weep holes?
Water that gets past the brick veneer collects on the flashing at the base of the cavity — weep holes let that water drain out instead of accumulating and potentially causing efflorescence, freeze-thaw damage, or interior moisture problems.
What types of weep holes are common?
Open head joints (simplest, just an unfilled vertical mortar joint), plastic or mesh weep vents (better pest resistance), and wick/rope weeps (draw water out by capillary action) — spacing requirements vary somewhat by type.
Do weep holes need to align with the cavity drainage mat?
Ideally yes — weep holes should be positioned so they connect directly to the drainage path created by any cavity drainage mat or mesh, not blocked by mortar droppings or other debris.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.