How to build a low-level deck over existing patio?

Question

hi there,

I’ve been having a look through previous questions & guides but looking for advice on how best to build a timber deck over our existing patio while extending it out.

the current patio is 4m in length by 2m wide and is 70mm high, we are looking to extend it out to be approx 6.5 x 2.8m made of timber.

The current idea is to dig out the area to the front & side to level & flatten, add cracker dust & pavers (to be used as support for pedistals) & use H3 timber for the joist’s running from the house to run the decking horizontal - this is what I’ve seen from the guides but want to check I’m not missing anything or need to do differently?

MitchellMc · Answer

Thanks for confirming @iswally. At 85mm, your proposed arrangement starts to make more sense. With the 90 x 45mm joists laid flat, 19mm decking and around 10–15mm clearance underneath, you'd be sitting at roughly 74–79mm overall.

The important consideration is that laying the 90 x 45mm timber flat dramatically reduces its ability to span between supports. You'd therefore need supports at much closer intervals than you would with the joists standing vertically. Over the existing patio, that should be achievable as you can provide regular support underneath them.

Using bearers across the soil section could certainly be an option too, but remember that adding a bearer underneath your joists also adds substantially to the overall framing height. You may still need to excavate sufficiently for the bearer, joist, decking and ground clearance while keeping the finished deck level with the section over the patio.

I'd work backwards from your intended finished deck height and draw a simple side profile showing the decking, joists, bearers and supports. That will quickly establish how much excavation you'll actually need and whether the bearer arrangement saves you enough digging to make it worthwhile.

Mitchell

iswally · Answer

hey @MitchellMc

thanks for getting back to me, the gap is around 85mm from the base of the patio to the door still, so the idea was to use 90x45 on their sides, with the deck & raising the H3 about 10-15mm off the patio to make sure it wasn’t touching and help ventilation.

We have also thought about putting bearers across the soiled area & at the front instead of the pedestals to support & made the extraction of the soil a little more achievable.

MitchellMc · Answer

Welcome to the Bunnings Workshop community @iswally . It is great to have you with us, and many thanks for your question about building a low-level deck.

The main issue I can see is the available working height, particularly between the existing patio and the door sill.

At a minimum, you would typically need around 90mm for your joists, 19mm for the decking and at least 10mm underneath the joists for airflow. That means you'd need approximately 119mm between the existing patio and the door sill, whereas from your photo it appears you might only have around 60–70mm available.

Unfortunately, that's going to make decking over the existing patio very difficult. You could look at more extreme low-profile arrangements, such as laying 90 x 45mm joists flat so they're only 45mm high, but even then, once you add the decking and allow some clearance underneath, it looks like you'd be extremely tight for space. Laying joists flat also significantly reduces their spanning capacity and would require substantially more support.

Because of this, I'd consider decking directly over the existing patio impractical due to the lack of available height. Your main options would be to remove the existing patio to gain the required depth or leave the patio as it is and extend the deck out from its edges.

You'll have a similar consideration over the soil area. To accommodate your cracker dust, pavers, pedestal feet, joists and decking while maintaining adequate clearance and drainage, you'll likely need to excavate a significant amount of soil. Depending on the levels and components you use, you could easily be looking at removing 100mm or more.

Running the joists from the house towards the lawn would allow your decking boards to run horizontally as you've proposed. H3 treated Pine is suitable for above-ground framing, provided it is kept clear of the soil and you maintain adequate drainage and ventilation underneath.

I'd start by taking an accurate measurement from the top of the existing patio to the underside of the door sill. That measurement will tell you exactly what you're working with and whether there's any practical framing arrangement that could fit.

For the sake of others reading the discussion or in case you missed them, here are some helpful guides: How to build a deck and How to build a low-level deck. Also, here's out Top 10 most popular deck projects for inspiration.

Let me mention @gtvone, @RAllan, @Bcour8, @hrd.patel, @AdamCook_93 and @Sryan97, who recently contributed their own decking projects, to see if they have any thoughts.

Please let me know if you have any questions.

Mitchell