A lot of patios start with a slab or a paved area already there. It might be the alfresco slab from the house build, an old concrete patio or a paved entertaining area by the pool. The question we hear is whether the new posts can just sit on it.
Often they can. Sometimes they cannot. The answer depends on what is under the surface, not what it looks like on top.
What a post actually does
A patio post does more than hold the roof up. In a storm the wind tries to lift the roof off, and the posts have to hold it down. So a post needs a footing heavy and deep enough to resist that uplift as well as carry the weight. That is why the footing is engineered for your site’s wind rating and soil, and why a thin layer of concrete is not always enough.
Posts on an existing slab
Posts can be bolted to an existing slab on welded base plates when the slab is thick enough, reinforced, and in good condition where the post lands. The engineer sets the minimum, and the post usually needs to sit far enough in from the slab edge for the bolts to hold. When it checks out, this is a tidy result: no digging and no patching.
- We check the slab thickness, often by drilling a test hole or looking at the house plans.
- We look for cracks, settlement and old repairs near each post position.
- We check how far each post sits from the slab edge.
- The engineer confirms the base plate and anchor design for your wind rating.
When it needs new footings
If the slab is too thin, has no reinforcement, is cracked or the post lands near the edge, the post needs its own footing. We cut a neat hole through the slab, dig the footing to the engineered depth, pour it and set the post. The slab around it is made good. On a new house slab this is quick and the patch is small.
Sometimes it is simpler to place posts just off the edge of the slab, in the garden or lawn, where a new footing goes in without cutting anything. We look at both options at the measure-up.
Pavers are never a footing
Pavers sit on a bed of sand or crushed rock. They are not tied together and cannot hold a post down in the wind. A post on pavers needs a proper footing under them. We lift the pavers where each post goes, dig and pour the footing, set the post, then relay the pavers around it. Keep a few spare pavers if you have them, because older ones can be hard to match.
Levels and drainage
- An existing slab should fall away from the house. Once it has a roof over it, check where the rain from the new roof will run, as the downpipes need to go to the stormwater line.
- If the slab is close to the house floor level, keep the post bases and any edging clear of door sills and weep holes.
- Under a new roof, an old paved area that used to dry in the sun may stay damp, so check that it drains.
Sewer and stormwater under the slab
Slabs are often poured over the sewer line, and the backyard manhole can end up inside a paved area. The Queensland Development Code keeps footings about 1.2 m from small sewer mains and 1.5 m from the centre of a manhole, and the manhole must stay open and reachable. If a post must go closer, a build-over application goes to the council or water authority. We find the sewer on the plans before we set out the posts.
A new slab with the patio
If there is no slab yet, or the old one is past saving, it is often worth pouring a new slab at the same time as the patio. The posts can go into footings poured with it, the falls are set for the new roof, and you finish with one clean surface. The slab may need its own approval and a plumber for the drainage, which we work into the plan.
Our advice
- Find the house plans or the slab details if you have them, as they save a test hole.
- Do not lay new pavers before the patio goes in: the footings come first.
- Tell us if the slab has ever cracked or moved, as reactive clay is common in Logan and Ipswich.
- Expect us to check rather than assume. A post that looks fine on day one is the one that has to hold in a storm.
