Signs your retaining wall needs repair
Limestone walls fail slowly and give plenty of notice. The signs, roughly in order of urgency:
- A lean or a bulge in the face. Urgent. The wall has moved on its footing and will keep moving.
- Paving or lawn above the wall sinking. Urgent, because it means the backfill is going somewhere, usually out through the wall.
- Sand washing through the face and collecting at the base after rain.
- Water pooling behind the wall that takes days to drain.
- Cracked or missing mortar joints. Maintenance, if the wall is still plumb.
- Capping lifting or loose. Maintenance, and worth doing before water gets into the core.
The first two mean the repair is a rebuild. The rest can usually be fixed with the wall standing. Either way this is a retaining wall job, even when it is the paving that first caught your eye.
Why Perth limestone walls fail
Three causes account for almost every failed limestone wall we see. No drainage behind the wall, so water and sand build up pressure against the back of it. A footing too small for the height, so the wall rotates forward under load. And a surcharge added after the wall was built, such as a driveway, shed or pool near its top, that the wall was never designed to carry.
Perth sand makes the first one worse, not better. Sand drains well, but sand with water running through it also moves. Behind a wall with no ag drain, every winter washes more sand out through the joints, the backfill slumps, the paving above sinks and the pressure on the wall changes. That is the most common reason Perth limestone walls lean. The wall drain has to discharge somewhere, and soakwells covers that. The wider problem of water in the backyard is in backyard flooding in Perth.
There is also an age effect. The original timber retaining walls on 1970s lots in Duncraig, Greenwood and Girrawheen have rotted out and are being replaced in limestone rather than repaired.
Repair options
Which repair applies depends on how far the wall has gone.
- Repoint and re-cap. For a wall that is plumb and on a sound footing but has cracked, hollow or missing joints and loose capping. The joints are raked out and repointed, the capping is re-bedded or replaced, and the wall gets another few decades.
- Add drainage. For a sound wall that is wet behind. We excavate behind the wall, lay a slotted ag drain at the base wrapped in geofabric and surrounded by blue metal gravel, run it to a soakwell or lawful discharge point, then backfill in compacted layers. The wall itself is not touched.
- Rebuild the failed section. For a wall that has leaned or bulged. A leaning wall cannot be pushed back and repointed; the blocks have moved on the footing. The failed section comes down, a proper footing is poured, drainage goes in and the wall is relaid, reusing sound blocks where they match. The block options are on limestone retaining walls.
Our lifetime workmanship warranty covers IBP's own work and the section IBP rebuilds.
Do repairs need council approval
A like for like repair to a wall retaining 500 mm or less generally does not need a building permit. Rebuilding or replacing a wall that retains more than 500 mm generally does, with a structural engineer's design, because Perth councils state that walls over 500 mm, or walls that protect adjoining land, need a permit and engineering; the City of Fremantle and the City of Vincent both publish the rule that way. A boundary wall may need a permit regardless of height, and the neighbour is part of that conversation. The full set of rules is on retaining wall approvals in WA.
Source: City of Fremantle fences and retaining walls page; City of Vincent retaining wall building advice page.
What retaining wall repairs cost
Repointing, re-capping and adding drainage are quoted per job after we see the wall. For a rebuild, the indicative Perth industry ranges from the whatsthedamage.com.au Perth table are: demolition of the old wall about $105 per lineal metre, drainage about $52 per lineal metre, a new limestone block wall about 1 m high roughly $295 to $630 per lineal metre, and engineering about $525 to $2,100 where the rebuilt wall retains more than 500 mm. None of these are IBP's prices. The full breakdown is on retaining wall cost in Perth.
Source: whatsthedamage.com.au Perth retaining wall cost table. Indicative Perth industry ranges only.
Wall repair and the paving beside it
Paving above a failed wall usually sinks with it, because the backfill has washed out. The order is wall first, then paving: once the wall is drained or rebuilt and the backfill compacted, the paving is relaid on a fresh bed. That side of the job is covered under paving repairs and sunken paving repair. Where the wall sits beside a pool, remember that a retaining wall within 500 mm of a pool barrier is non compliant under WA's Rules for Pools and Spas (Diagram 11), so a rebuild is the moment to get the clearance right; see pool paving.
Mature 1970s walls are concentrated in Duncraig, Greenwood and Girrawheen. We work across Perth's northern suburbs from Landsdale; see the full list of areas.
