Weep holes are one of the most important — and most overlooked — features of a concrete block retaining wall. Without them, water builds up behind the wall, creating hydrostatic pressure that can cause bulging, cracking, or even catastrophic failure. The good news is that installing weep holes is straightforward if you plan ahead during construction or use the right technique for an existing wall.
Why Weep Holes Matter
When rain soaks into the soil behind a retaining wall, it has nowhere to go unless you give it an escape route. That trapped water exerts enormous lateral pressure — far more than the soil alone. Properly spaced weep holes allow water to drain freely, protecting the structural integrity of your wall and the land it retains.
Under the NZ Building Code, retaining walls over 1.5 metres in height generally require a building consent. Even walls under this height must be built to a safe standard. Always check with your local council before starting, as height thresholds and consent requirements can vary by district and site conditions (e.g., proximity to boundaries or unstable ground).
Planning Your Weep Hole Layout
Before you lay a single block, plan where your weep holes will go. Key guidelines include:
- Spacing: Place weep holes no more than 1.2 metres apart horizontally along the wall. Many builders use every second or third block in a course, depending on block size.
- Height: Position the first row of weep holes in the lowest course of blocks just above finished ground level on the face side — typically 75–150 mm above the base.
- Additional rows: For taller walls, add another row of weep holes every 600–900 mm in height.
- Corners and ends: Always include a weep hole within 300 mm of each end of the wall.
How to Install Weep Holes During Construction
Installing weep holes as you build is by far the easiest method.
Option 1 – Leave the Perpend (Vertical Joint) Open
The simplest approach is to leave the mortar out of selected perpendicular (vertical) joints in the block course at the desired weep hole locations. This creates a natural gap approximately 10 mm wide. Do not fill these joints — leave them clear.
Option 2 – Insert PVC Pipe Sleeves
For a neater, more reliable result:
- Cut short lengths of 50 mm diameter PVC pipe (readily available at Bunnings or Mitre 10) to match the block width — typically 150–200 mm.
- As you lay each designated block, bed the pipe horizontally through the block cavity or through the mortar joint, sloping very slightly (2–3°) toward the face of the wall so water drains outward.
- Mortar around the pipe to hold it in place, keeping the pipe opening clear of mortar.
- On the back (soil) side, wrap the pipe opening with geotextile filter fabric to prevent soil particles from blocking the pipe over time.
Installing Weep Holes in an Existing Wall
If your wall is already built and showing signs of water pressure (bulging, efflorescence, or damp patches), you can add weep holes retrospectively:
- Use a rotary hammer drill with a masonry bit (50 mm diameter) to drill through the mortar joint or block at the correct locations.
- Angle the drill slightly downward toward the face of the wall.
- Insert a short PVC pipe sleeve and seal around it with a suitable mortar or masonry sealant such as a product from Sika.
- Clear any debris from the hole and test drainage with water.
Safety note: Always wear safety glasses and a dust mask when drilling masonry. If the wall is over 1.5 m tall or shows significant structural distress, stop work and consult a professional before proceeding.
Drainage Behind the Wall
Weep holes work best when paired with good drainage behind the wall:
- Aggregate backfill: Use clean, free-draining gravel (AP20 or similar) for at least 300 mm directly behind the wall.
- Drainage coil: Lay a perforated drainage coil (ag-pipe) wrapped in filter sock at the base of the wall, connected to a suitable outlet.
- Geotextile fabric: Line the back of the aggregate layer with geotextile to prevent fine soil from migrating into the drainage zone.
For guidance on drainage and moisture management, BRANZ is an excellent NZ resource for building best practice.
Common Mistakes to Avoid
| Mistake | Why It's a Problem |
|---|---|
| Spacing weep holes too far apart | Water pressure builds between holes |
| Placing holes too high | Water pools at the base before escaping |
| Blocking holes with mortar | Defeats the purpose entirely |
| No filter fabric on soil side | Holes block with silt within a few seasons |
| No aggregate backfill | Water can't reach the weep holes efficiently |
When to Call a Professional
Retaining walls are structural elements — getting them wrong can be dangerous and costly. You should engage a qualified Licensed Building Practitioner (LBP) or experienced retaining wall builder if:
- Your wall is 1.5 metres or taller (building consent is likely required)
- The wall is on a steep or unstable site
- There are signs of existing structural movement or failure
- You're unsure about drainage design or soil conditions
- The wall is near a boundary, building, or public area
A qualified retaining wall specialist can assess your specific site conditions and ensure the wall meets the NZ Building Code. You can find experienced local professionals on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
