How to calculate posts and bearers for a deck?

[felipenm]

Hey there!

I am thinking about building a DIY deck and I put together this decking design. I am now working on the decking frame plan. I know that for the posts it is better to use H4 treated and the bearers and joists can use H3 treated. As for the joists' spacing, I know that I should be spacing them no more than 450mm. Now, I've done some research and I can't get my head around those span tables to decide how many posts I need and the spacing between bearers. Does anyone out there would be able to help me with that?

Your help is much appreciated.

Thanks

Felipe

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[MitchellMc]

Hi @felipenm,

The usage of the term bearer and joist can cause some confusion. If you have a joist over bearer system that makes sense. However, their meaning can be lost in translation in a single-level deck. For a single-level deck, it's best to use the word joists, which supports your decking and not use the word bearer.

For a single-level deck, you have joists spaced every 450mm, as you've mentioned. Those joists need to be supported depending on their calibre and span. Say for 90 x 45mm joists you'd want to support them every 1000mm.

If you have enough height to build a joist over bearer deck, it can save on how many supports you need to install as you'd use a larger bearers that can span further and then lay your joists over them. 

If you can tell me if you are building a single level deck or a joist over bearer deck, I can assist with appropriate spacings. If you can't decide on which, let me know your available working height and we can go from there.

Please let me know if you have any questions.

Mitchell

[felipenm]

Hi Mitchell,

Thanks for your response. Your explanation about the joist and bearer makes total sense. The available working height is about 40cm. 

Regards

Felipe

[MitchellMc]

With that amount of room, you could easily fit 140mm bearers, 90mm joists and your 19mm decking @felipenm. You'd run the bearers at 1000mm centres, supported every 1800mm.

Mitchell

[felipenm]

Thanks Mitchell! When you say supported every 1800mm you mean having a post every 1800mm, right?

In terms of width and thickness, do these measurements sound about right?

Posts - 90 x 90 H4

Bearers - 140 x 45 H3 (or 140 x 90?)

Joists - 90 x 45 H3

Here are a few other questions I hope you can help me with:

How deep should the posts go? And what should be the diameter of the footing?

Do you recommend any concrete for proper footing?

Should I go with double bearers or double joists?

Do I need a ledger board (I’d rather not)? If yes, what are the recommended screws to use?

If using a ledger board, do I need flashing?

Do I need blockings for further strength and stability? If yes, where should they go?

By the way, I found this helpful deck calculator that can help those out there: https://www.blocklayer.com/deckcalculator#tblStumps

Here is an image of part of my deck using this calculator and the figures you suggest.

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[MitchellMc]

That looks great, @felipenm, and yes, a post every 1800mm.

The size and depth of your footers is something that needs to be determined by the soil type in your area. Only an engineer can answer that for you. 

Standard concrete is fine. 

There's no need to go with a double joist or bearer; you'd only do that if you wanted to span further distances than 1800mm. 

There's no requirement for a ledger board.

You could add additional blocking if you wished; in many cases, it's not needed. 

Please let me know if you have any questions.

Mitchell

[felipenm]

Thanks Mitchell, your help is much appreciated! It is all clear now and I will let you know if I have further questions!

[felipenm]

@MitchellMc, sorry to bother you again, but I have a question about which posts/stumps I should use. As far as I know, I could use one of those options:

• Stirrups bolted down on concrete footing;
• Stirrups set in concrete footing; or 
• H4 F7 treated pine 90 x 90.

Not considering budget, what would be the best / safest / easiest option?

If stirrups, should they be stainless steel or galvanised?

If H4, what is the best way to have the bearers attached to it? On top of it or the side of the post?

Regards

Felipe

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[MitchellMc]

Hi @felipenm,

The best option for durability and ease of installation would be stirrups bolted down on concrete footings. This method keeps the posts off the ground, reducing the risk of rot and moisture damage. Galvanised stirrups are typically sufficient for most environments, but if you're in a coastal or highly exposed area, stainless steel stirrups offer superior corrosion resistance.

If you choose H4 F7 treated pine posts, the strongest method is to attach the bearers on top of the post, providing direct load-bearing support. While side mounting the bearers is possible, it doesn't offer the same strength as the top method. 

Please let me know if you have further questions.

Mitchell

[felipenm]

Thanks @MitchellMc! That makes sense. As for side mounting, I refer to this method as shown in the picture.

I live in a coastal area, but not that close to the water. It is like 8km from the beach.