How to plan bearers for low level deck?

scw

I’ve been learning lots from the many deck posts but I’m still a bit stuck on how many posts I need.

The deck is an extension to link a current deck to a new sauna. The current deck is a bit odd- brick piers on concrete footings with 100x50 bearers and 100x50 joists.

The decking I build will be 3m long and about 1800wide on one side then cuts in a little to be 1600wide for the last 1200 to meet the existing deck around a garden bed.

I’d like to have the fewest posts necessary, and keep airflow under the deck. Here’s a couple of options I can see:

1. traditional joists on bearers, using 140x45 bearers and 90x45 joists (all I saw in Bunnings that was structural was h3- is that correct? I’d be happy to use structural h4 for both). Bearers spaced and posts at ends and in middle. The down side is i think this needs three rows of bearers. And it goes below 150mm gap for airflow. So not preferable.

2. A single layer deck, with 140mm x 45mm for joists and bearers (is it a joists and rim joists/trim boards?) Could I just do that, and would that mean I can just have 6 posts? I’d put the posts in the corner and then either in the middle of the bearer or at 1800 to match the end of the sauna deck.

I've read quite a few posts and tried to include those into my design; I'd love to double check a couple of things on where I ended up.

• If I use posts into concrete footings, are my designs ok with span tables? I find it hard to understand span allowances for single layer deck.
• As above, if I go with single layer deck and I use 140x45 h3 mpg10 timber, for joists and the edge (trim board?) can I can reduce the number of footings and posts to 6, and should I simply have them in the corners and then halfway down each side (or should I put this at 1800?) my plan was basically to make the whole thing 1600 x3000 and then address the gap to the 1800wide deck with joists on joist hangers to between the two ledger boards (or the screws I saw you mention are options elsewhere)
• How far down do the concrete footings need to be- minimum? I'd use a stirrup into concrete and 90x90 h4 posts.
• In a single layer with rim joists like this do you just notch them into the post on both sides? Or is it better to out the post back from the absolute corner and support the long edge (what would usually be the bearer/what the joists will hang off)?
  

Sorry for all the long winded detail and questions. The 'doing' side doesn't really faze me- it's the planning. I have not ever had to design footings or structural frame, or set a post. I have helped labour and done what I'm told.

JacobZ

Hi @scw,

Welcome to the Bunnings Workshop community, it is wonderful to have you with us.

Firstly, our stores stock structural H3, which is perfectly acceptable for use in this situation as it is raised above the ground and there is good airflow beneath the deck, however there is structural H4 that can be special ordered. It is not 100% necessary, but it can be used if you'd like some extra peace of mind that your deck is going to stand the test of time. If you'd like to explore this, check out the range from https://www.bunnings.com.au/brands/w/woodhouse-terraframe .

You are correct that if you were to use 90x45 joists, which have a maximum allowable span of 1400mm, then you would need a third row of bearers, which is obviously not ideal. So, I would suggest you use 140x45 for your joists, which have a maximum allowable span of 2600mm when acting as a joist. To do this with the height restriction, I'd suggest a single tier deck frame, which utilises https://www.bunnings.com.au/pryda-joist-hanger-to-suit-45-x-140mm_p0294197 and https://www.bunnings.com.au/pryda-timber-connector-screw-12g-x-35mm-hex-head-box-250_p0084753 to connect the joists to the bearers.

Now, a 140x45 acting as a bearer has a maximum allowable span of 1400mm, and your bearer runs are 3000mm, so with the dimensions of the posts taken into account, you are right on the limit. It is within the allowable limits to have the posts at the four corners with one additional post at the centre, but it is right on the limit so you may want to bring up to a 190x35 or 190x45 for the extra span capacity. Entirely up to you.

Now, there is an important distinction between a rim joist and regular joist. A rim joist is not a structural member; it is a finishing piece that is attached to the ends of the structural joists to add rigidity and close off the ends of the joists for aesthetics. They are typically used on two-tiered decks because the joists run over the top of the bearers, they do not butt into them. Your deck does not have rim joists. All the joists are acting structurally including the two at the ends. This means that they need to either have a structural connection to the bearer through a hanger or bracket or they need to sit directly on the posts. In my eyes, the cleanest finish would be to have them sit on the posts like in the render below.

In this setup, the joist is directly supported by the post and so is the bearer. You can then screw back through the bearer into the joists and sideways through the joist into the post to lock it in place. You can then bolt the bearer to the posts.

With that small section between the new and old decks, I wouldn't bother with joist hangers. It is a very small span, so the load it is bearing will be minimal. You can just drive screws through the bearers into the joists. The span is so minimal, that it will be fine without hangers. Due to the mismatch in timber size, you may consider using 90x45 here.

Your deck frame will end up looking like this -

For your post anchors, there is no well-defined minimum size for concrete footings as they are designed for use in a wide variety of situations and it is all relative to the soil in your location, the weight the anchor is supporting and the loads they are likely to face in that particular scenario. In saying this, for a small deck, a concrete footing which is 300mm in diameter and at least 600mm deep is common.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob