Weep holes are small drainage openings placed near the base of retaining walls, compound walls, wing walls, and other underground structures so trapped water can escape. A typical weep hole is a round or square opening about 0.1 m, or 100 mm, in diameter that lets water drain from the backfill side to the exposed face. The complete drainage layout, from weep hole positions to the pipes that carry water away, is explained in this walkthrough of retaining wall drainage, weep holes, and installation steps.
Rain soaks into the soil behind a wall, groundwater rises after storms, and capillary action pulls moisture up through masonry. When water stays trapped behind the wall, it builds hydrostatic pressure that grows with depth. A metre of saturated backfill can push several kilonewtons against each square metre of wall face, enough to crack brickwork, tilt panels, or push a poorly designed wall over. Weep holes relieve that pressure by giving the water a low exit point, which is why they sit in the lower part of the wall.
What Are Weep Holes in Retaining Walls?
A weep hole is a drainage opening formed in the lower course of a masonry, concrete, or block wall. In brick construction the opening is usually a vertical joint left open between bricks, while in cast concrete walls it is a formed pipe stub or a drilled hole. The same principle applies to compound walls, wing walls, cavity walls, and window frames. The way these openings behave in different wall systems is covered in more detail in our article on weep holes in masonry and retaining walls.
Weep holes do two jobs at once. They drain water out of the wall, and they vent the cavity behind it. Air moving through the openings dries the back of the wall and reduces dampness, which slows efflorescence, moss growth, and freeze-thaw damage. In brick masonry the openings double as small ventilators so air circulates behind the wall.
Why Location Matters
The height of the opening controls how much water can drain. Holes at the base let the water behind the backfill fall to the level of the opening, so the full height of retained soil sheds water. Holes halfway up leave the lower half of the wall standing in water, and trapped moisture causes damp patches and spalling. In retaining walls the drainage row should sit just above the footing or ground level, not buried in soil where it can clog.
- Located in the lowest course of the wall, close to finished grade
- Spaced evenly along the wall length rather than clustered in one spot
- Backed by gravel or filter fabric to stop soil washing through
Types of Weep Holes
Weep holes are classified by how they are formed and what keeps them open. Three types are common in practice: open head joints, cotton rope wicks, and PVC tubes. Before choosing one, study how the wall is drawn, because the weep hole detail must match the wall section. A retaining wall construction section drawing shows the footing, backfill, drainage layer, and weep hole positions in a single view, making the three options easier to compare.
Open Head Joint Weep Holes
In brick or block walls, an open head joint weep hole is made by leaving the mortar out of a vertical joint between two bricks. These joints are formed at 21 inch spacing, about 530 mm, in the lower part of the wall. The open joint gives water a direct path through the wall thickness. Open head joints cost nothing extra because they use no additional material, but they are the easiest to block with mortar droppings during construction, so builders stuff the joint with gravel and remove it after the wall cures.
Cotton Rope Wicking Weep Holes
Wicking weep holes use a cotton rope up to 16 inches, about 400 mm, long inserted into a joint. One end sits inside the cavity behind the wall and the other hangs out through the opening. The cotton absorbs water from the damp backfill and carries it to the outside by capillary action, where it drips away. This process is slow, so wicking suits walls with light or intermittent moisture rather than walls holding back saturated ground.
Tube Weep Holes
Tube weep holes are formed with PVC pipes set through the wall during construction, usually at about 10 feet (3 m) horizontal spacing. Each pipe slopes slightly downward toward the outside face so water runs out freely. Tube weep holes handle large water volumes and are the standard choice for retaining walls, where a 50 mm to 100 mm pipe beds into gravel at the wall base.
Choosing Between Types
- Open head joints suit brick walls with light to moderate drainage needs and cost nothing extra.
- Wicking suits cavity walls where moisture is intermittent and appearance matters.
- PVC tubes suit concrete and block retaining walls where water volumes are high.
| Feature | Open head joint | Cotton rope wicking | PVC tube |
|---|---|---|---|
| Typical detail | 21 in (530 mm) spacing | 16 in (400 mm) wick | 50-100 mm pipe at 10 ft (3 m) |
| Drainage speed | Fast, gravity driven | Slow, capillary driven | Fast, gravity driven |
| Material cost | None | Low | Low to moderate |
| Best use | Brick compound walls | Cavity walls | Retaining walls |
| Clogging risk | Medium | Low | Low with filter fabric |
Placing and Spacing Weep Holes Correctly
Placement rules follow the water, not the wall. If the groundwater level sits at the same height as the base of the structure, water will not accumulate behind the wall and weep holes may not be needed. They become necessary when the wall is built below the water table or when backfill holds rainwater, because pressure then builds up behind the face. Ignoring that pressure difference causes most wall damage, and the measures for preventing retaining wall distress and failures start with the drainage detail.
In outer compound walls, weep holes are commonly kept at 450 mm centre-to-centre spacing. For retaining walls, common practice spaces holes 600 mm to 1200 mm apart horizontally and adds a second row at mid-height when the wall is taller than 3 m. The lowest row should sit 150 mm to 300 mm above finished grade so surface runoff and mud do not clog the openings.
- Lay the drainage layer: place 100 mm to 200 mm of clean gravel directly behind the wall base, wrapped in geotextile fabric where the soil is fine grained.
- Set the weep pipes: bed each pipe in the gravel with a 2% to 5% downward slope toward the outside face.
- Form the openings: leave head joints open in brick walls, or cast short pipe stubs into concrete walls before the concrete sets.
- Protect the outlets: keep the outside openings clear of mortar, paint, and landscaping mulch.
- Verify drainage after backfilling: pour water into the backfill and confirm it exits each weep hole.
Spacing Rules of Thumb
- Compound and garden walls: 450 mm to 600 mm spacing
- Retaining walls up to 3 m: 600 mm to 1200 mm spacing, single row
- Retaining walls over 3 m: add a second row at mid-height
- Window and door frames: one opening near each corner of the sill
Purpose and Function of Weep Holes
The primary purpose of a weep hole is pressure relief. Water behind a wall creates hydrostatic pressure that acts on the full face of the structure, and by giving that water a low escape point the openings reduce pressure and protect the wall from cracking, tilting, and overturning. The openings act as gates that let moisture from rainwater, capillary action, or leakage drain out instead of soaking into the masonry.
Weep holes also protect the wall from everyday dampness. Rain running down window sills and door heads is collected by small holes along the bottom edge of the frames, which work like a gutter system. If those holes are blocked by paint or debris, water backs up in the frame and finds its way into the wall or the room. Keeping weep holes clear is routine maintenance, and the wider set of fixes for leaking windows, damp walls, and cracked panels is collected in this practical guide to essential home building solutions.
What Weep Holes Protect Against
- Hydrostatic pressure that can push walls out of plumb
- Uplift pressure under retaining walls and footings
- Dampness and efflorescence in masonry
- Freeze-thaw spalling in saturated brick
- Corrosion of reinforcement in concrete walls
Advantages and Disadvantages
Advantages of Weep Holes
The advantages of weep holes are straightforward. They reduce water pressure on the wall, protect the structure from moisture damage, improve air circulation behind the wall, and cost very little to provide. They are passive devices: once installed they work with no moving parts and need no attention beyond keeping openings clear.
Limitations to Plan Around
The limitations come from the simplicity of the detail. Weep holes are small, so mortar, paint, insects, or landscaping material can block them. They let cold air and pests into cavity walls unless fitted with fine mesh or filter fabric. If the drainage layer behind the wall is missing or too thin, weep holes can wash soil out of the backfill and create voids, so filter fabric and gravel are specified alongside them. Matching the hole type to the wall material and water load avoids most of these problems. The trade-offs between wall systems, materials, and drainage needs are compared in our overview of retaining wall types, materials, and applications.
Retaining Wall Drainage Design Considerations
A weep hole is only one part of a drainage system. The complete system includes a gravel drainage layer behind the wall, a perforated drain pipe at the base when water volumes are high, filter fabric to keep soil out of the gravel, and weep holes to vent the system to the outside. Each part relies on the others: weep holes without a drainage layer clog quickly, and a drainage layer without them simply stores water.
Matching the Drainage System to the Wall
Design decisions start with the water load. A garden wall on free-draining soil may need only open head joints, while a basement retaining wall below the water table needs PVC weep pipes, a gravel blanket, and a subsoil drain.
Whatever the wall type, the checklist stays the same: keep the lowest weep row near the base, slope every opening toward the outside, back them with gravel and filter fabric, and clear them after construction and during seasonal maintenance. The wall type shapes how the drainage detail is drawn, and the functions and design considerations of cantilever retaining walls are a useful starting point for walls that resist large soil pressures. Walls that drain stay dry, and dry walls keep their strength for decades.