Building a deck that floats on the ground rather than dug in?

[d000hg]

We've had a quote to build a decked area against our log cabin, and it details digging in/concreting big posts (possibly 6x6" from memory) as you would gate posts.

However I was wondering is this really necessary? Our cabin itself sits on plinth foundations (https://swiftfoundations.co.uk/swift_plinth/) buried barely 6" and relies on spreading the load over greater surface area.
This is a whole building so could the same principle be applied to decking which will have very low loads? Or in fact, maybe this IS a common way? The ground is pretty stable on site.

I could buy some actual plinth things but given the low loads what I was actually considering was each post resting on something simple like a concrete paving slab, probably atop some pea gravel.

Has anyone done this here or seen it done?

 

[freddiemercurystwin]

I did a deck with joists straight onto bricks onto a bit of pea gravel not even paving slabs as I had a minuscule budget at the time and it being a new house on a phased building site at night I helped myself to a few bricks and pea gravel off the site. It was fine and didn't shift in the decade or so I lived with it. The whole deck just sat there under it's own weight with no bounce. Surprising really when you consider the ground was made-up ground and was poor quality.

 

[JP_]

I do think that we often over-engineer things just to be on the safe side. OK if money is plentiful!

 

[cwhaley]

I built my decking frame resting on slabs. The slabs are in each corner and then support every span (so 9 slab sections in total on mine) and they are on a bed of compacted hardcore/sand and cement.

I just built the frame on top of those using 9" x 2" shuttering and the posts from 5" x 4" posts (all reclaimed). There's lots of air flow underneath and checking under there throughout the winter shows no signs of rot or movement -- it's rock solid. I did think about posts piled into the ground but worried about them rotting. My method results in no timber touching the soil at all.

Whole thing cost about £200 not including the 18mm ply roof.

 

[JohnD]

I'ver seen a number of slabs laid on top of the ground rather than dug in, for workshops, sheds and the like. Commonly the ground is eroded at edges and corners, undermining the slabs. Water running out of downpipes or unguttered roofs, and rat nests, accelerate it. A slab with a thickened edge dug into the ground can prevent this.

I put up my carport using modest concrete pads dug into the ground and rising 150mm above it, with the posts in metal sockets on top. This keeps the posts dry, and the pads secure from disturbance. Like cwhaley, no timber touching the ground. If I was doing decking or a light shed, I'd do the same again.

 

[cwhaley]

Not seen that before, but I'd imagine water from a down pipe would erode the soil. Best putting the slabs on a bed of sand/cement/hardcore and ensuring there are no down pipes discharging water at the slabs.

 

[d000hg]

I built my decking frame resting on slabs. The slabs are in each corner and then support every span (so 9 slab sections in total on mine) and they are on a bed of compacted hardcore/sand and cement.

I just built the frame on top of those using 9" x 2" shuttering and the posts from 5" x 4" posts (all reclaimed). There's lots of air flow underneath and checking under there throughout the winter shows no signs of rot or movement -- it's rock solid. I did think about posts piled into the ground but worried about them rotting. My method results in no timber touching the soil at all.

Whole thing cost about £200 not including the 18mm ply roof.

Thanks for sharing @cwhaley, looks a tidy job. If you don't mind me asking what are your spans? 9x2 throughout is pretty heavy duty but I was a little surprised looking online how close together people put joists.

Your frame on top reminded me that I want to put a barrier around ours, I think the plan is basically to build a 3' fence. Clearly bolting this onto the side of the deck is not going to be as sturdy as postcreting them into the ground but my gut feeling is I should be able to attach fence posts on to an 8x2 / 9x2 satisfactorily as long as people aren't going to be swinging off the fence
I can picture the sort of fittings I might use but I have no idea what they're called?
I am picturing our deck's 'feet' will be very stubby so I wondered would I be better having the same posts run from the ground up as fence posts? Seems to me this would give them a bit more sturdiness and I could even bolt them then onto concrete slabs if it didn't seem quite strong enough.

Basically thinking the main frame from 8x2 with smaller joists (4x2? 6x2), and the feet/posts on the outside of the frame then build a fence inside the posts so the nice side faces in.

 

[cwhaley]

Thanks for sharing @cwhaley, looks a tidy job. If you don't mind me asking what are your spans? 9x2 throughout is pretty heavy duty but I was a little surprised looking online how close together people put joists.

Your frame on top reminded me that I want to put a barrier around ours, I think the plan is basically to build a 3' fence. Clearly bolting this onto the side of the deck is not going to be as sturdy as postcreting them into the ground but my gut feeling is I should be able to attach fence posts on to an 8x2 / 9x2 satisfactorily as long as people aren't going to be swinging off the fence
I can picture the sort of fittings I might use but I have no idea what they're called?
I am picturing our deck's 'feet' will be very stubby so I wondered would I be better having the same posts run from the ground up as fence posts? Seems to me this would give them a bit more sturdiness and I could even bolt them then onto concrete slabs if it didn't seem quite strong enough.

Basically thinking the main frame from 8x2 with smaller joists (4x2? 6x2), and the feet/posts on the outside of the frame then build a fence inside the posts so the nice side faces in.

Thanks -- loved doing the project. The span between the slabs was a concern at the time, but I just used my own initiative with it. The span is never longer than 5'6" but probably could have been longer as the whole thing is rock solid. Last August I had 9 people on it and all the levels are still bob on. I even used the 9x2 boards as floorboards for it. I've put a rough sketch of the plan below.

My 5" x 4" posts are just secured with bolts into the frame -- they sit level on the slabs. I put the slabs at a slight angle to allow rainwater run off, but with the roof on it doesn't see much rain. Once the posts were all linked with the 'hand rail' barrier they don't move at all. I thought about concreting them into the ground as you suggest but the weight of the frame and the posts and the roof together is so great it doesn't move -- keeping the sides open so that wind can flow through is very important.

I think your proposal is sound design wise. As you say use the 8x2 for the main frame (into a St. George cross shape) and then secure them together with the smaller joists to strengthen the frame's structure. If you can make the whole frame from 8x2 though I'd do that. Use old shuttering boards from reclaim yards.

 

[Munroast]

Only minor caveat I would add - if the decking is attached to posts concreted into the ground then any upward movement through warping of the decking would be resisted. It can happen

 

[d000hg]

I hadn't thought trying to get hold of used timber, interesting.

When I mentioned spans I was actually meaning the longest unsupported distance under the decking boards themselves?

 

[cwhaley]

I hadn't thought trying to get hold of used timber, interesting.

When I mentioned spans I was actually meaning the longest unsupported distance under the decking boards themselves?

Definitely consider it. It's available in so many places and can be sanded and preserved nicely.

The boards I used are 2" thick (same timber as the joists) and based on my plan above, the longest unsupported span would be around 2 ft - ish? If I'd used decking boards as you buy them in the shops (which would have cost more!) I'd have needed more support as they're only thin.

 

[d000hg]

Ah I'd missed your deck itself was using thicker timbers, not regular decking boards. Makes sense.

 

[geof worrall]

4M x 2.6M approx deck

I hammered in sharpened 50x50 posts at 2.4m centres front and back around 350 long into the earth as supports.
I hung 150 deep decking joists on galvanised joist hangers across the posts
I fixed a 150 joist to the ends of the cross joists with 20mm coach bolts with to act as a front trimmer with cross trimmers at each end to infill the voids.
the decking was then attached one length of 2.4m with one length of 1.2m joints, each row staggered joints.
there are 17 rows of decking planks the outer ones with a slight overhang to act as a drip.
the garden has a slope so i fitted a ground level fascia in decking joist to fill in the void.

P1050079 by geof worrall, on Flickr

ps few more photos if required...

 

[mrcrow]

4M x 2.6M approx deck

I hammered in sharpened 50x50x550 posts at 2.4m centres front and back into the earth as supports.

Then levelled the earth under where the deck would be, sprayed root and branch killer and then covered the area with a plastic membrane held down with stones.

I fitted 150 deep decking joists on galvanised joist hangers across the posts, leaving 50mm gap to the underside of the joists to allow ventilation.

I fitted 150 joists with 20mm coach bolts to the ends of the cross joists to act as a front and rear trimmers with cross trimmers at both ends to infill the voids there, fitting them with brackets to the inside edge of the front and rear trimmers.

The decking was then attached one length of 2.4m with one length of 1.2m joints, each row staggered joints.

there are 17 rows of decking planks the outer ones with a slight overhang to act as a drip.

the garden has a slope so i fitted a ground level fascia in decking joist beneath the front timmer to fill in the void.

P1050079 by geof worrall, on Flickr

P1050079 by geof worrall, on Flickr

ps few more photos if required...

 

[Tigercubrider]

There are plastic screw feet that are designed to make decking easier?
I would be tempted to dig out some soil, and create a concrete pad for each foot