Where should I put stumps under my deck?

[AWolff]

Unfortunately the previous owners of this house built a deck straight into the ground. the wooden stumps have rotted as there was no concrete or anything used. So I have taken off the decking boards and removed the built up soil and now am left with the above pictured structure that I would like to add stumps to. How many should I add, where, how deep? Should I use stirrups? The grey spots are locations I considered putting stumps. The frame is attached to the house and not very high off the ground (5cm) so getting stirrups under the wood is a bit tricky but possible if i dig the holes wide enough. Appreciate any help. Thanks!!

[EricL]

Hello @AWolff 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your decking project.

I suggest having a quick look at these discussions:

• Solved: What is the correct post hole depth for a deck by @eddy 
   
• How to calculate posts and bearers for a deck by @felipenm 
   
• How to understand deck span tables 

Generally speaking, your post positions are determined by the size of your bearers and the type of decking frame that you have. Footings are generally placed uniformly across the bearer at specific distances depending on the wood's length, width and height. I recommend having a look at these guides - How to build a low-level deck by @Adam_W and How to build a deck by @MitchellMc. These will give you a good understanding of decking frame types and how a deck frame is assembled. It will also show you a general idea of where the footings are to be placed. 

I also suggest looking at the Bunnings deck building guide: 

• How to plan out your deck
   
• How to set out a deck
   
• How to install stumps
   
• How to install bearers
   
• How to install joists
   
• How to lay decking
   
• How to add a step to your deck.

If you need further assistance, please let us know.

Eric

[AWolff]

Thank you Eric. 

I had a look at these resources before posting but struggled a bit to comprehend it all and apply it to my situation. But on second glance I get the impression that I can use standard cement in holes that are 400mm deep, sinking in metal stirrups so their length is at least halfway in concrete. Theoretically I only need supports at the corners if itss every 180cm, but then presumably I'd also need something in the centre? But as there's a storm pipe in the way how would I best decide where to put a central stump? And should the stirrups reach at least 200mm deep or is it mostly just important that their length is half in concrete?

Also, I have 2 stirrups that are wider than the actual wood they would support. can i pack out the stirrup with extra wood? 

Lastly, as it's only a single layer deck, does the rule about stumps being 180cm apart still apply or would I need more stumps to account for this?

many thanks!!

[EricL]

Hello @AWolff 

Let us say for example that you have a bearer size of 140x45. According to the bearer span table you can place footings at a maximum distance of 1400mm, that's 1400 between posts. In essence the distance between supports can be less as long as you don't go over 1400mm. For example, if I wanted 4 supports for 3600mm I would divide the space by three, (because dividing it by two would give you a result that is greater than the span limit). This will give you an answer of 1200mm. That's well within the 1400mm limit.

A support would therefore be placed at the very start of the bearer counted as one and the next support placed at 1200mm (No.2). The next support at 2400mm (No.3) and the last support at the end of the bearer at 3600mm (No.4). This would give you 4 supports and all of them within the limits of the span table. Please note that this is based on a single level decking frame, I suggest having a quick look at this featured project - Low-level composite deck over concrete and soil by @oninpena. They are using 90x45 for framing, it's a good example of where to place footings on your deck frame.

If there are not enough supports or footings or if they are placed too far apart your deck would become springy, which is not good for the structure of the deck. If the storm pipe is in the way, I prefer to over engineer things so I would suggest having a footing on both sides of the pipe. Please keep in mind that this all depends on the location of the footing in regards to the span table. You could totally miss this pipe and there would be no need for extra footings.   

The general rule of 1/3 for decking posts states that at least one-third of the total length of the post should be below ground level, with the remaining two-thirds above ground. While 1/3 is a common rule, some guidelines suggest that for stability, at least 450mm to 600mm of the post should be in the ground. Please note that for taller decks your soil composition would need to be analysed in order to calculate the correct footing size and depth.

Can I pack out the stirrup with extra wood? Yes, you can pack out the stirrup with extra timber. 

If you need further assistance, please let us know.

Eric

[AlanM52]

Hi @AWolff,

Also checkout Instant Concrete Foundation Blocks.

I suggest reading the Brochure under 'Guides & Documents'.

Cheers

[AWolff]

these look great but as the deck is attached to the house, my fear is that with clay soil where i am, the soil will shift under the blocks and might seperate the deck from the house. it's covered... so doesn't get too wet all over but water can still leach into the soil. there isnt any info in the brochure about how it deals with clay soils but perhaps they would be handy for the parts in the middle near the storm pipe so i don't need to dig too close to it. 

[JacobZ]

Hi @AWolff,

If shifting soil is a concern, then concreted-in supports would definitely be better than foundation blocks or pedestals.

Have you got any photos you can share of the deck? Seeing what you are working with will certainly help us to offer specific advice.

Also, what are the dimensions of the bearers and joists? Their allowable spans are based on their size, so we'd need to know the size of the framing to know where they need to be supported.

If you can upload some photos and some details, I could give you a more solid idea of where you'd need to add supports.

Jacob

[AWolff]

Hi Jacob,

140 x 45 is the dimensions of the joists and all the timber of the frame. 

I've mapped it out based on Eric's suggestion of 4 rows of 4 stumps. (it might be hard to see, I've just placed broken pavers and 2 stirrups i had in each spot). It does mean that some joists won't have supports under them however so not sure if they should be distributed differently or not?

How many bags of concrete do you think it would take per hole is they are 400mm deep? And do you have advice on the best concrete to use?

thanks!

PS: there is a roof over the deck. 

[AlanM52]

G'day @AWolff,

I am thinking if the blocks shift (subside) that likely means the water table has dropped and everything else will shift by the same amount - in the same direction. To put that another way if the house shifts all of the surrounding soil will be effected and will shift by the same amount - in the same direction

Anyway... I always lay those on a small bed of gravel.

Cheers

[JacobZ]

Hi @AWolff,

Your deck isn't technically using bearers, so you would have to treat everything like it is a joist, which means every single joist needs to be supported in the same way.

Assuming your joists are 140x45 MGP10 Pine, then they would be able to span 2600mm between supports. This means every joist needs 3 supports.

The amount of concrete is relative to the diameter of the pier, but generally, you would aim for a diameter of around 300mm. So, a diameter of 300mm at a depth of 400mm would be equal to 0.03 cubic metres, which is roughly 3 x 20kg bags of concrete per support. As you'd need 3 supports per joist, you would need 21 piers, which would mean 63 bags of concrete.

You can use pretty much any concrete, but post mix concrete or rapid set concrete are often used.

This is obviously a fair bit of concrete, so it may be worth considering using some foundation blocks or pedestals to reduce the amount of work. 

You could also convert the framing at each end of the joists so they are actually functioning as a bearer. If you'd like to look into this as an option, please let me know so I can explain the process.

Let me know any feedback or questions that you have. I am happy to assist further.

Jacob