How to lay concrete slab for patio?

[aisac]

Hi everyone,

I’m planning to build a patio at the back of my house and want to start by pouring a concrete slab. Before I go too far, I’m hoping to get some advice from those with experience.

A few specific questions:

Depth of the Slab – What’s the recommended depth for a patio slab that will be used mainly for outdoor furniture and general entertaining? I’ve heard 100mm (10cm) is common, but I want to make sure that’s appropriate.

Obstacles in the Area – There’s a water heater system and some bathroom plumbing close by (possibly running under or near the proposed slab area). Would these affect the process in any way? Should I be organising a scan of the area to avoid damaging any underground pipes or utilities?

Drainage Considerations – Any tips for making sure water drains properly off or around the slab? I want to avoid pooling or water flowing back toward the house.

If anyone’s dealt with a similar situation or has suggestions (or things to watch out for), I’d really appreciate your input before I bring in any contractors.

Thanks in advance!

[MitchellMc]

Hi @aisac,

It sounds like you're planning this out well, which is great to see. You've raised some important points, and I’d be happy to share some thoughts to help you move forward.

To start with your slab thickness—100mm (10cm) is indeed the standard for a patio that's going to support foot traffic, outdoor furniture, and general entertaining. If you’re planning on putting anything heavier on it (like a spa or built-in kitchen), then you'd want to go thicker and potentially reinforce with mesh.

Regarding the plumbing and water heater nearby—yes, those are definitely things to factor in. I’d strongly recommend determining the location of pipework in the area, especially since you suspect there might be plumbing lines nearby. Damaging an underground pipe can be costly and cause delays, so it’s worth the peace of mind to check. You might find the location of the pipework on your home's plans, which are typically available from your local council.

For drainage, you’ll want to ensure the slab is poured with a slight fall—usually around 10mm per metre—away from the house so that water doesn’t pool or flow back towards your foundations. You might also want to consider installing a drain or channel grate if the area is prone to water buildup.

Will your slab be elevated above ground level or set down into the ground? An elevated slab can sometimes make it easier to work around underground services and help with drainage, so that detail could influence your approach. 

Please let me know if you have any questions.

Mitchell