How to raise a cubby house off the ground for a sandpit beneath?

[globetrotter]

Hello,

Have purchased a cubby house and would like to raise it off the ground to a height of about 1m as I would like to incorporate a sandpit beneath. Have a few ideas in mind but not exactly sure which approach would be most ideal and wanting some advice/recommendations. The cubby house is 2.7m x 2.5m. My intention is to have 9 posts dug at a dept of 600mm and at a width of 180mm using 90mm x 90mm H4 treated pine posts. To build the subfloor I was going to have two bearers and about 5-6 joists. Am not exactly sure which way the bearers should run and how they should attach to the posts.

Also how should the joists be attached to the bearers, just nailed in on an angle, bugle screws or joist hangers?

And what would the recommended size of the bearers and joists be?

Tagging @MitchellMc for ideas.

Picture below is the idea that I have.

Thanks in advance

[MitchellMc]

Welcome to the Bunnings Workshop community @globetrotter. It's fabulous to have you join us, and many thanks for your question about elevating a cubby.

That sounds like a reasonably solid plan. I'd suggest constructing the frame from 190 x 45mm L/M Outdoor Framing MGP10 H3 Treated Pine, and the joists from 140 x 35mm Outdoor Framing MGP10 H3 Treated Pine. That's on the beefier side, but I believe it's good insurance on elevated structure for kiddies. I'd have a central beam of 190 x 45mm and have the joists connect to it and the fame with Pryda Joist Hangers - To Suit 35 x 140mm. They can be nailed or screwed in. I'd recommend bolting the frame to the posts. You can mirror the top frame down to the base, retaining your sand and adding rigidity to the structure. Instead of using the H3 framing timber, it would be best to swap out for 200 x 50mm H4 treated pine sleepers as that timber will contact the sand.

You could run two bearers if you'd like instead of the one and swap the 140mm joists to 90mm. My preference would be the design I've proposed. I've created some renderings for you to illustrate the construction steps better.

Remember, with an elevated structure that kids will use, installing rated shock pads in any potential fall zones is important. Although it's only one meter, that's enough to cause significant damage.

Please let me know if you have any questions.

Mitchell

[globetrotter]

Thank you so much for the detailed steps and drawings @MitchellMc . The drawings have certainly helped to visualise the final product.

One question I had was the attachment of the bearers to the posts, I had read elsewhere that using bolts would place significant pressure on them and it would be a stronger design if I rebated the post so that the bearer's load could be distributed on the post instead of by just two bolts. I saw this from @ProjectPete https://www.workshop.bunnings.com.au/t5/Outdoor/Decking-frame-bearer-stump-rebate/td-p/33659. Or would this be an unnecessary overkill for my intended purpose?

Thank you

[MitchellMc]

Great question @globetrotter. If you're capable of rebating the posts go for it! All the better. I believe those bolts will do an adequate job of transferring the weight and it's extremely unlikely that they would ever sheer off. You'd need all the bolts to fail at the one time. However, I love that you've picked up on this advice and I recommend you go that route if you're confident. It's much better to have the load transfer straight onto the post.

Good pick-up.

Mitchell

[globetrotter]

Thanks @MitchellMc , was good to read all the great advice on this site. I'm not sure I can rebate them as well as that illustration but I'll give it a go.

I was going to use 90mm x 90mm for the posts, would that be adequate or would you suggest 100mm? And also to dip the posts in stop rot, again which product would you recommend to slow down the timber from rotting?

[globetrotter]

I just realised would I comprise the corner post's structure if I rebated the two external facing sides ? @MitchellMc 

[MitchellMc]

You're not compromising the structure of the post @globetrotter. You could have the frame sitting on top of the post. You are creating a ledge that sends all the weight above directly into the post and ground by rebating. However, there will only be a 50 x 50mm square left in the corner to bolt through. I believe this is still adequate for the bolts to pass through, but you might also like to use Pryda plates for extra security. You also don't need to do a full rebate for the 45mm thickness of the frame; you can do a half rebate at 22.5mm. There is still a ledge to transfer the weight, and you'll leave a slightly more significant portion of the post top to bolt through.

Let me mention @ProjectPete to see if he has any thoughts.

Mitchell

[globetrotter]

@MitchellMc have dug out my post holes, what size posts would you recommend I use, 90mm or 100mm? I think I will just go with your approach of bolting the bearers to the posts rather than rebating.

[EricL]

Hi @globestrotter

I tend to over-engineer my builds hence, I suggest using 100mm x 100mm. 

Please keep us updated with your progress, we look forward to seeing your raised cubby house ready for use.

I'm sure @MitchellMc will let us know if he has additional advice.

Eric

[MitchellMc]

I'd be going with the 100 x 100mm post as well, @globetrotter.

Looking forward to seeing your results.

Mitchell