Decking Posts, Joists and DPM Questions.

21 Jul 2008
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United Kingdom
Hi there

I am about to embark on a decking project and I am a little confused on a few issues.

The decking is going to run across the whole of the back of the house (7.7m) and extend out 4.8m on one side and 3.2 on the other.

On one side there is a large concrete base that until recently supported a concrete shed. On the other side is grass. Between theconcrete and grass and the house there is a strip of tarmac.
The concrete plinth is the same height as the back door step which is only 4" below the back door sill (and the internal floor.)

My questions are: -

1.) I plan to use 3x2 joists on the side that's over the concrete as the concrete is flat and there is only a run of 1m from the concrete plinth to the house wall (over the tarmac) that is not supported, and even that can be supported with 'feet' down to the tarmac. Ideally I would use something a bit thinner (2x2) as once the decking (32mm) is placed on top it is going to be ever so slightly higher than the back door sill. On the side with the grass I plan to dig the grass out and use 6x2, attaching the 2 frames together at the edge of the concrete where they meet. Is this OK?

2.) I plan to attach a joist to the house (a ledger board?) and attach the joists to that. How much of an issue is it if this sits across the DPM of the house wall? I'm guessing people are going to say that it is a big deal, if it is then I can't use 3x2 and would have to use 2x2 (again they will be supported along their length, see above.)

3.) I intend to have a balustrade across the back of the decking with 2 90 degree angles in it and a gate. I want to set posts in concrete to use to attach the joists to, but also use them to attach the balustrades and the gate to. I can't forsee any problems with this but is there anything to take into account when doing this as I can't seem to find any examples of this way of using posts on the internet

Apologies for the very long list of questions but any help would be very gratefully received. Here is an artists impression of the 'site': -

) concrete )
) concrete )
) concrete Gate________
) concrete grass grass grass )
) concrete grass grass grass )


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3x2 and 2x2 for joists or a ledger board will not be strong enough and anyway how would you attach your 6 x 2 joists to the ledger board securely....the very minimum I would use is 5x2 but generally I always use 6x2. So get the spade and sledge hammer out and start digging a little to accommodate 6x2 joists, ledger board and for the whole framework!

As for the DPC you can place the the ledger board on the the DPC provided you leave a 10 - 12mm gap between the board and the house wall this has been covered many times in this forum!

There is no reason why you cant put a gate in at all, best to use 4" x 4" or 100mm x 100mm posts!

Look through the rest of the posts with regards to decking in this section and you will see that the answers you require have already been covered ;)
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Hi there, thanks a lot for your responses. That Ultimate Handyman website is very good, the pictures help a great deal.

With regards to the concrete that I was intending to place my frame on. What is the advantage of removing it in order to use 6x2 joists when it is flat and solid and would, therefore, provide a sturdier base than wood could? If the 3x2 sat directly onto the flat concrete then how do I lose out on stability?

I intended to effectively have two frames, one of 3x2 joists sat on the concrete and one of 6x2s, each frame would have its own ledger board of the same size and the two frameswould be attached together where they butt up against each other.

Also, I didn't ask my question about posts very well. I will only require posts along the front of the decking (the part furthest from the house) and I was hoping to use full length 4x4s so they could also double up as newel posts. (I have searched on here for similar posts but not yet found anything.) Is this OK to do?

Thanks again,


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