Downpipes, grading and the clay under the slab
How water around the edge of a home ends up as cracks inside it.
In short: On reactive clay, the edge of the slab is where wet and dry meet. Control the water there and most cracking stories end.
The mechanism
Clay swells when wet and shrinks when dry. If one side of a house is wetter than the rest, the ground moves unevenly and the slab follows.
What causes uneven wetting
Downpipes emptying beside the footings, ground falling toward the wall, leaking taps and garden beds watered against the slab.
What we recorded
On a South Kingsville investigation, downpipes discharged next to the footings at the rear, where the cracking inside was worst.
On new Manor Lakes homes
At fixing and handover we often record grading not yet done and stormwater pipes with too little cover. Both should be finished before you move in.
What to do
Connect every downpipe to stormwater, grade the ground away and keep paving and gardens off the slab edge.
Written by our local inspection team from work carried out in Manor Lakes. Licensed building practitioners, Building & Plumbing Commission registration CDB-U 53425.
