Raised decking help.

geneticmaterial · 12 May 2020 at 22:27

Hi.
Looking to install a 11.5x2.4m decking at the bottom of our garden.
The 11.5m length is backed by a shared fence which uses 6 sunk posts.
The incline of that 11.5m length drops a foot or so from one end to the other.
At one side (the lower side) there's a garage wall. The other side is a retaining wall of the same brick type.
Fairly sure the ground is top soil then clay.
The deck will be 2.4m wide all along simply based on standard joist lengths and will be 400 to 600 apart depending on beam width. The decking boards will run the length of the 11.5m.
Just a few questions:
What spacing would you recommend for the footings? Some neighbours have done 1-1.5m apart and 300m deep with post-crete and 90mm posts.
Could I utilise the walls at each to anchor into? I've seen some videos on doing this and leaving gaps etc to avoid damp.
Can I use the fence posts at all to anchor the joist running the full 11.5m length too?

Any help greatly appreciated.

The green playmats are 2.4m from each of the fence posts.
Thanks. Dan.

Dis86 · 12 May 2020 at 22:59

First thing I'd ask is will the decking be in the sun or shade?

geneticmaterial · 12 May 2020 at 23:33

Full sun from first thing until 6pm

Dis86 · 12 May 2020 at 23:42

That's good. I'd look at no more than 2m spacing depending on the size of your timbers. Potentially down to about 1.5m.

geneticmaterial · 12 May 2020 at 23:46

1500mm deep?

Abyss · 13 May 2020 at 10:47

The spacing for the footings depends entirely on the size / configuration of the joists you are using. Deeper or doubled up joists will mean greater spacing.

In terms of footing depth, there's going to be little to no lateral movement so depth is entirely dependant on how deep you need to go to hit solid compacted ground (i.e. not topsoil). For my decking I dug in 4 posts for stability then for the rest removed the top soil and sat the posts on pads as it's far easier to replace in the future.

geneticmaterial · 13 May 2020 at 11:05

Ah ok that's a good shout.

geneticmaterial · 13 May 2020 at 11:07

Would it a good idea to bolt into the walls at each side and the available fence posts along the back length?

Abyss · 13 May 2020 at 14:45

I'd be tempted to use the walls but not the fence posts.

For the walls just bear in mind that 11.5m of decking is going to expand/contract through changes in the weather. You don't want to risk the decking expanding and damaging the wall, or contracting and pulling the fixings out. I'd be tempted to bolt timber to the wall then sit the decking on it so it's floating with an expansion gap between the decking frame and the wall.

For the fence posts, given they already take a beating from lateral force on the fence and often aren't installed that well I wouldn't want my decking to be reliant on it. It also makes it a bigger job if you ever need to replace a damaged fence post. I'd leve the two structures independant.

Bug One · 13 May 2020 at 15:14

geneticmaterial said:
Would it a good idea to bolt into the walls at each side and the available fence posts along the back length?

I did a similar build. I anchored onto the wall, but not fence posts (they're a neighbours posts anyway).

I had 10 posts concreted into the ground and used 40 of these on all the intermediate timbers.

geneticmaterial · 13 May 2020 at 16:08

Nice one guys. Appreciated.

Bug One · 13 May 2020 at 17:08

Actually, it was this type I used. Easily adjustable height. They're all sitting on spare paving slabs (it was paving underneath before the decking).

https://www.ebay.co.uk/itm/3-Grosen...are-Stelzlager-Terrassenunterbau/111914177899

Bug One · 13 May 2020 at 17:28

I used quite a simple construction. However I over specified all the materials. So used 6x2 for everything, used thick 35mm decking and had more legs than I really needed. The price of the materials was probably double what I could have got away with - however I want it to last.

I used 6m decking boards and joists - so could do a full span across without any joins - similar to how you describe you will be doing with 2.4m lengths.

(This is only half the adjustable legs installed.)

(Only half the adjustable legs installed)

Complete (lighting off)