What timber size for 8m x 4m deck?

Hi there,

I'm planning to build a free-standing deck in my back yard, however I'm having trouble working out the appropriate sized timbers to use.

I've been referring to Alan Staines - Deck and Pergolas Construction Manual, but can't make heads or tails.

Deck size will be 8m x 4m and roughly 500mm off the ground.

Decking will be 19mm merbau and the bearers and joist will be hardwood in the same plane to maximise space underneath.

Posts will also be hardwood and cast in to the ground.

My questions...

1. What size hardwood posts should I use and at what spacings?

2. What size hardwood bearers should I use?

3. What size hardwood joists should I use? These will be sitting in joist hangers between the bearers.

Appreciate if anyone can help me out.

Cheers, James

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MitchellMc

Welcome to the Bunnings Workshop community @_Jimmy_D_. It's wonderful to have you join us, and many thanks for your question about building a deck.

There is no one answer to your question. It all comes down to how many posts you're willing to install. The more support posts there are, the smaller the calibre of joist you can use. If you were to use 90 x 45mm joists, then you'd need posts every 1000mm along them. For 140 x 45mm hardwood joists, you should be able to get away with one mid-post across the 4000mm width. You'll need to check the timber suppliers span tables to see whether 2000mm is within the span capabilities of your joist timber.

I've put a basic rendering together to give you some idea of a possible layout. Note the two bearers down the middle that support the joists midway. Without these bearers, you'd need a middle post for every joist. That would be another 11 holes and posts.

The first thing you need to do is decide what calibre joists you'll be using. I'd recommend 140 x 45mm for a deck of this size. You then need to work out the max span of that timber through timber span tables. Your spacings for the posts must be within the span capabilities of the joists and bearers.

Here's a helpful guide: How to build a low-level deck.

Given the deck size, you'd likely need to apply for council approval. It would be best to chat with them about their requirements for your area and if plans for the deck need to be submitted. If so, the engineer you enlist will specify all the timber sizes to be used.

Please let me know if you have any questions.

Mitchell

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