How to design low-level deck?

Deeken

Hey everyone, I’ve been reading around a lot trying to get my head around the design required to build a high quality lasting deck.

I’ve viewed the other DIY decks you (Eric + Mitchell) suggest to most but none have been my situation.

I’m looking at a 5.5m x 5.5m deck built off the back of our house abutting 2 walls to fill the back corner of our house in, eventually with a roof too.

My goal and problem here is that I am wanting to level the deck off our back door height, but currently we have a paved patio with only 200mm to step height.

Looking at the designs I think I’ve come up with a design but would like input from more experienced minds. I am thinking of framing the substructure with 140x45, using double 140x45s for bearers, and 90x45 joists. I’m unsure of bearer layout/spacing and reading the span table confusing myself a bit and would like some guidance.

This will give me a 140mm+19mm deck thickness, and I’ve I remove the pavers (50mm thick) to allow more air space, I should have about 90mm air gap under the bearers.. This will then require building over dirt and hence I decided on double 140x45 bearers to reduce the amount of footings I would need.

Thank you for any guidance.

MitchellMc

Welcome to the Bunnings Workshop community @Deeken. It’s fantastic to have you join us and many thanks for your question on decks.

With only around 200mm between the existing patio and the door height, this will certainly be a low-level deck, so keeping the framing as compact as possible will be important. Removing the 50mm pavers sounds like a sensible option as it will give you additional clearance and airflow beneath the structure.

For your proposed 90 x 45mm joists, I'd work on a maximum span of around 1200mm between bearers. That means your bearers should be no more than 1200mm apart, although I'd suggest bringing them closer to around 1000mm centres where practical. This should produce a nice solid deck with minimal movement.

For the 140 x 45mm bearers, I'd look at supporting a single bearer approximately every 1400mm. With your proposed double 140 x 45mm bearers, you could potentially extend the footing spacing to around 2000mm.

Given the limited height, rather than placing the joists on top of the bearers, I'd install the 90 x 45mm joists between them using suitable joist hangers. This allows the joists and bearers to occupy the same 140mm overall framing depth. With your 19mm decking on top, you'd be around 159mm overall, leaving approximately 40mm beneath the framing based on your current 200mm height. Removing the pavers would then provide considerably more clearance.

So, as a starting layout for your 5.5 x 5.5m deck, I'd be looking at double 140 x 45mm bearers at approximately 1000mm centres, 90 x 45mm joists running between them at 450mm centres, and footing/support points along the double bearers at approximately 2000mm centres.

It's also important to contact your local council before settling on the design. At 5.5 x 5.5m, you're looking at a deck of just over 30m², which could require approval and submission of plans depending on your location. Given you're also intending to add a roof eventually, I'd mention that to the council now as well. It would be much easier to incorporate any requirements for the future roof into the deck and footing design from the outset rather than having to modify it later.

Please let me know if you have any questions.

Mitchell