Best method to fill void under patio?

Question

Hi, I'm looking into how I might fill a void under the perimeter of the patio created from washout, see below. May I please ask for some advise on a good method of filling in the void? Inspector suggested subsoil and recompacting edges but I'm thinking it might just continue to wash out, so maybe concrete or spray foam, I'm not too sure.

Dan0 · Answer

Hi Mitchell, thanks so much for your detailed reply. Yeah I think youre right I'll have a look around my area and see who might be able to do something about this.

MitchellMc · Answer

Welcome to the Bunnings Workshop community @Dan0. It's fabulous to have you join us and many thanks for your question on stabilising a concrete slab.

From the photos, that looks like more than just minor edge erosion. If there’s been enough washout to create a void under the slab perimeter, the proper repair would typically involve excavating the affected area, removing any loose material, recompacting suitable fill, installing formwork, and then placing concrete back under and along the edge of the slab to properly support it.

However, the key issue here is the post. Because that post is supporting the patio roof, and the slab is effectively supporting the post, this has now become a structural matter rather than just a cosmetic or landscaping repair. If the ground beneath the slab continues to erode or settle, it could affect the stability of the post and therefore the roof structure.

For that reason, this really isn’t something I’d recommend as a DIY fix.

Given the structural nature of the situation, it would be best to engage a licensed builder or a foundation/structural repair specialist. They can assess whether the slab edge needs underpinning, pressure grouting, concrete infill, or additional footing support, and they’ll also be able to address the drainage issue that caused the washout in the first place.

It’s always better to correct it properly now rather than risk movement in the slab or roof structure later.

Please let me know if you have any questions.

Mitchell