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!!

[AWolff]

Thanks so much. By foundation blocks are you thinking of the type such as mentioned above that are made of concrete and sit on the ground? If I used those, how many could I get away with? for example would it be possible to use half blocks and half stumps?

'You could also convert the framing at each end of the joists so they are actually functioning as a bearer' - yes please I'm very curious how I'd achieve this. 

thanks!

PS: could I mix the concrete with aggregate to need less of it? Or do they already come pre mixed? (hence being called concrete not cement?)

[JacobZ]

Hi @AWolff,

Yes, I mean the DEKO 250 x 150 x 150mm Instant Concrete Foundation Block that @AlanM52 mentioned above. You could use them for any of the supports, or all of them if you wanted to, but they would move with the earth if it did happen to move. I can't see this being a huge issue, though I can't know what your soil is going to do, so you would need to evaluate the situation. Ultimately, using concrete piers with stirrups for every support would be better, but it might not be economical. 

To make the framing at each end actually act as a bearer, you would need to connect every joist to them using 140 x 45mm Pryda Joist Hangers and connector screws and then support the bearers in line with the bearer span tables. This means they would need to be supported at no more than 1400mm centres, meaning each bearer needs 4 supports. As the joists still have a maximum span of 2600mm, you would still need a support in the centre of each joist, but this would reduce the number of supports to 15 instead of 21. Knowing how much work it is to dig the holes, then mix and pour concrete, in my opinion, it would be worthwhile.

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

Jacob

[AWolff]

Hi,

yes that does sound worthwhile. I was planning on using joist hangers so I'm glad that it's not more complicated than that. If each length of wood on the outer edge of the frame then becomes a bearer, and each bearer needs 4 stumps, would the number not be 16 instead of 15? Maybe I've misunderstood. If I choose this method I assume using the instant foundation blocks for the joists in the centre where there are no stumps would still be wise?

[JacobZ]

Hi @AWolff,

There are only two bearers, the timbers at each end that run perpendicular to the joists.  Therefore, you would need 8 stumps between those two joists. You would then need a single support near the centre of all 7 joists.

8 + 7 = 15

You could certainly use the instant foundation blocks in the centre if you would like to. You can decide whether you use stirrups or foundation blocks wherever you would like to use them.

Jacob

[AWolff]

thank you that makes sense now. i dont know why i thought all the outer sides where bearers! 

[JacobZ]

Hi @AWolff,

That's alright, I totally understand. One of the biggest problems I had when learning about deck construction was understanding what makes a bearer a bearer. I just assumed it was the framing that ran around the outside edge of the deck, like I imagine you did.

It's why I wrote the article How to understand deck components. I'd encourage you to have a read if you haven't already.

Regardless, we are here if you need further support when reinforcing your deck.

Jacob

[AWolff]

Are there stirrup anchors that fit this size of joist? 

[JacobZ]

Hi @AWolff,

There is no real need for a full stirrup in this situation, so you could just use half stirrups. As long as the joists sit on them and are fixed in place with bolts, washers and nuts, this will provide adequate support.

Jacob

[AWolff]

thanks. the sections that are bordered by concrete mean that I won't be able to get the stump hole as wide as 300mm. is it ok if the stumps are narrower there? Or do i need to somehow dig under the concrete and fill it up with concrete so its definitely 300mm wide?

[JacobZ]

Hi @AWolff,

How wide can you make them? On which part of the deck is this a problem? How many stumps would it affect?

It's not ideal, but as long as there is a good amount of concrete in the hole and the stirrup is encased on all sides, it should be ok.

If the stirrup is likely to poke out the side of the concrete, it wouldn't be ideal, and you'd want to look at an alternative solution.

I'll wait for further details.

Jacob