Posts for raised decking: timber or plastic?

[Lift span]

Hello,
I’m designing a 6m by 2.4m deck which will be fixed via a ledger board to my house along the 6m length. The other 6m side will be supported by a beam which will be fixed to posts.

What posts are best? I definitely don’t want to put timber in the ground.

I’m thinking 

1. Plastic posts - but these only come 100mm square and I don’t see much evidence of anyone using them.
2. Timber posts 100mm but can’t see any where these are recommend for a deck raised above 600mm
3. Timber posts 140mm as recommended in “TDCA Code of Practice Raised Timber Deck Structures on New Homes”, which is a great guide!

The timber posts above to be fixed to a steel bracket with a concrete footing formed from some kid of tube (as in Quickcrete style in the USA).

For the 100mm posts I don’t see any specs for spacing where as for the 140mm posts the TDCA guide has good info.

Any thoughts?

[Alan Ambrose]

I’ll throw in some other options - green oak beam on ground screws?

[Lift span]

That’s a good idea. I had a very quick look at oak and also laminate posts. Also even just bolting 150x47mm timbers together. I’m just looking for best practice really.

 

How much are ground screws? I presume these have to be done professionally so it would go against my strict (mostly)  DIY policy!

[Alan Ambrose]

The screws are cheap and DIYable for decking, sheds etc.

 

e.g. https://www.groundscrewcentre.co.uk

 

p.s. I bought a bunch of green oak from these guys during covid:

 

https://www.uk-timber.co.uk/58-structural-green-oak-beams

 

Oak is more expensive but much longer lasting than treated softwood. Use A4 stainless fixings.

Edited June 5, 2024 by Alan Ambrose

[Iceverge]

Concrete piers? 

 

Something like Sonotube they use in the US. 

[Conor]

I did my brother's deck of similar size with 100mm posts. Spaced them at 1200mm. No issues.

[JohnMo]

Flat steel plates embedded in concrete or on concrete pads.  The sandwich in wooden framework

[Lift span]

4 hours ago, Alan Ambrose said:

The screws are cheap and DIYable for decking, sheds etc.

 

e.g. https://www.groundscrewcentre.co.uk

 

p.s. I bought a bunch of green oak from these guys during covid:

 

https://www.uk-timber.co.uk/58-structural-green-oak-beams

 

Oak is more expensive but much longer lasting than treated softwood. Use A4 stainless fixings.

 

Wow! That’s a great find thanks.  Will definitely explore that option. I didn’t realise you could DIY them.

[Lift span]

3 hours ago, Iceverge said:

Concrete piers? 

 

Something like Sonotube they use in the US. 

I was looking for some cardboard tubes here in the UK and they seem remarkably expensive for what they are. I have some plastic barrels though which I thought by cutting the top and bottom off I could achieve similar.

[Lift span]

3 hours ago, Conor said:

I did my brother's deck of similar size with 100mm posts. Spaced them at 1200mm. No issues.

I’ve no doubt it’s a good job! I’d be interested in any photos!
 

I would like to find some info that would have these as part of an official spec. The Timber Decking and Cladding Association https://www.tdca.org.uk say 140mm posts. If I could find a spec for 100mm I’d do that as they are obviously much easier to come by.  

[Lift span]

3 hours ago, JohnMo said:

Flat steel plates embedded in concrete or on concrete pads.  The sandwich in wooden framework

I did wonder about a steel option. At 1.5 m high I’d be looking at a beam rather than a plate though. Again, just looking for best practice.

[JohnMo]

A snippet of our drawings for our deck structure, we moved the hill about instead of a big support structure.  Fairy simple steel, galvanised and wood between.

 

[JohnMo]

36 minutes ago, Lift span said:

At 1.5 m high

You really should be getting a structural design done for the decking and the balustrade structure. Didn't realise the fall was quite that much.

 

You have several issues

 

Decking lift in the wind

Balustrade wind and protection loadings

Decking collapse due to loading or pulling away from the ledger board 

 

To name a few

[Iceverge]

How tall are the legs of the piers lightly to be? 

 

Will they be visible?  What's the ground surface underneath like? 

[Iceverge]

Ah, I see the 1.5m.

 

Whatever about getting an engineer I would be inclined to decouple it from your house and discard the ledger board. 

 

That way if it slides down the hill of moves in a gale at least it won't pull the wall of your house with it.

[Conor]

Ah, I'd assumed a wee bit off the ground. For what you want, you will need planning permission, a structural design and building control. Not randomers on the internet. Best of luck.

[Lift span]

Building Control have signed off the plan already. They were only interested in the Balustrade height. 
Doesn’t need planning permission. Lots of other work did but when the planning officer visited he confirmed that the 30cm rule was from the ground adjacent to the start of the deck.

 

Even with a structural engineer I’ve found in the past the conversation starts with “How do you want to do it”

So I’m just looking at options so thanks for all your ideas!

 

 

[Lift span]

16 hours ago, JohnMo said:

A snippet of our drawings for our deck structure, we moved the hill about instead of a big support structure.  Fairy simple steel, galvanised and wood between.

 

 

 
The more I look at this the more I like it! What’s the maximum height of those columns in your deck?

 

Thanks.

Edited June 6, 2024 by Lift span

[JohnMo]

I think some of the where about 3 to 4m.  But we have a similar arrangement holding down our roof overhang and they must be about 7 to 8m.

2 hours ago, Lift span said:

 
The more I look at this the more I like it! What’s the maximum height of those columns in your deck?

 

Thanks.

 

[Lift span]

21 hours ago, Iceverge said:

Ah, I see the 1.5m.

 

Whatever about getting an engineer I would be inclined to decouple it from your house and discard the ledger board. 

 

That way if it slides down the hill of moves in a gale at least it won't pull the wall of your house with it.

That’s a good idea. Something I’d not considered. Maybe some kind of hybrid where it’s self supporting but with a ledger that’s “weakly” connected just get the level threshold.

[Lift span]

Here are some photos just to give some perspective.

 

Vertical supporting posts would be around left hand side of that scaffolding.

 

[Iceverge]

A picture paints 1000 words. 

 

Ok, the slope isn't mega. I had visions of a cliff like fall. 

 

Also your access is pretty good. 

 

Looking at your sketches I would be concerned about the maintenance of the wood that near to all that vegetation.  Also I would be worried that the crawlspace would be a haven for undesirable wild life. 

 

Could something like building up the ground into a patio work instead? 

 

 

[Lift span]

The underneath is going to be open like below with a weed barrier and gravel directly under the deck.

 

 

Yes, the maintenance of the wood is a concern.  I’ve just seen you can get 100mm square plastic posts so maybe they are the answer after all!  That way I’d be able to dig footings to be level with the lower level of ground and simply concrete those posts in therefore eliminating any issues with the slope.

 

[Lift span]

I found a guide from the USA which gives the 7ft rule for footings on a slope.

 

[Lift span]

As far as post size the guide says this: