Decking posts.

[bathjobby]

megawatt, looks like a great job and good pics. I am planning (thinking still) to put a deck on a slight slope and am toying between concrete blocks cemented in the ground vs posts to support the joists. In your deck6 pic, is the supporting post thats in the ground a wooden post that is concreted in ? as I thought you said that wasn't your preference ? I can see that posts will be easier to get level as they can be sawn off after the deck is attached and leveled vs having to get all the individual concrete blacks level up front.......
many thanks in advance.

 

[megawatt]

The wooden posts sit in metal post anchors which are set into the top of the concrete piers when the concrete is poured (slightly above ground level) ... The bottom of the wooden posts thus stay dry and (hopefully) don't rot

This arrangement is ideal for levelling as the posts may be removed (unbolted) from the anchors for cutting etc.

Don't forget to paint with bitumen and wet assemble for greatest protection.

 

[bathjobby]

Ah, think I have it, aka Meta post holders. Thanks.

 

[megawatt]

The very same (though I used Jewson's own brand as they were a bit beefier).

 

[grantm]

I dug 24" holes intending to use posts set in concrete, but they filled with water. Under the topsoil is a heavy clay and the garden slopes towards it, so I'm getting alot of water running over the clay into the holes. I'm worried timber won't last long in such wet conditions.

So to keep timber above ground level, are metpost concrete-in 4" suitable for decking supports? I can't find where to get the concrete version.

Or would timber posts last as long as the rest of the decking?

Grant

 

[megawatt]

The metpost concrete-in anchors are fine you simply need to make sure that the concrete pier extends above the level of any likely water and set the metpost anchor in the top whilst wet i.e. Keep the wood out of the water.
In the US they use a product called sonotube (like a large toilet roll holder) as a concrete form which enables the pier to be set above ground level. You can do the same thing by making wood forms though (say 8" high) which contain the concrete until it is set.
Also liberally paint bitumen over the concrete pier above the ground, the metpost and the post where it leaves the metpost anchor.

 

[grantm]

Do you think I said avoid posts-in-concrete ? Just been to B&Q and they said posts would be fine, especially if I used presure-treated fence posts.

 

[megawatt]

Just been to B&Q and they said posts would be fine, especially if I used presure-treated fence posts.

And, of course, B&Q know best

If you speak to timber yards they will tell you that tanalised timber is good for it's guarantee period above ground (great for the decorative parts of a deck) because it is able to dry out naturally after being soaked but they would not be so confident for timber permanently exposed to moisture.

Plenty of people set their posts directly in the concrete as they would generally do for a fence ... Difference is, you can see the fence posts and easily change them when (not if) they rot and, of course, you aren't walking on top of the fence

If the deck is low to the ground and you don't plan on being in the house in 10 years time set the posts in concrete. If you do so and you want to do the next occupant of your home a favour paint the buried parts of the post in bitumen and let it set before sinking them, raise the concrete pier above ground level as previously described and ensure that the top of the concrete slopes away from the post.

If you plan on staying in the house longer or the deck is more substantial in structure (or high off the ground) do like the experts in the US with decades more experience and keep the posts out of the ground

Incidentally ... When they do sink posts in the piers in the US they generally use 6x6" Hardwood which is far more resilient than the softwood posts generally used over here (tanalised or not) ... The bitumen helps to prevent capilliary damage from the end-grain on softwood posts though so be sure to use it.

A judgement call really.