Decking - partially over concrete

25 Mar 2009
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United Kingdom

I've had a look through some decking posts on the forum but can't quite find anything that matches my situation.

Background: I've removed an old conservatory from the back of our house, and in it's place we'd like to put some decking. We have left the concrete slab in place, as there is just enough clearance to get a 4x2" joist, a 32mm deckboard in and allowing around 10mm for the joists to sit above the concrete, keeping the decking board surface level with the sill of our back door. However, we want to extend the decking beyond the concrete - both in length and width. We want to build an L shape deck - with a 2.4m x 2.4m section directly outside the back door, and then a 4.8m x 3.6m section connected to this to form one large L shaped deck. Clearly, some of the decking joists will be supported by the existing concrete, and some will need to be supported using posts concreted into the ground. The joists will run parallel with the house so that the deck boards can fall away from it, and the first joist will be bolted to the house. I've a few questions...

  • The section supported by concrete. I saw these 'decking cradles' available online ( - has anyone used these? They look a bit flimsy - is there a better alternative? Ideally would need to be adjustable for speed of joist placement
  • The section supported by posts. From my research, I see that for 4x2" joists it's recommended that a post is installed every 1.2m. However, for all the examples I can see online, the recommendation is to also put two counter joists in either side of every other post and then sit the main joists on top of these. I can't do this as there isn't enough clearance. What should I do in this situation? Can this be overcome using noggins and if so what spacing? Is there a better way?
  • Joist size - is 4x2" adequate or should I really dig up the concrete so I have clearance for 6x2"?
  • Balustrades/rails - what's the best way to attach these to newel posts? I've seen various methods online but none that seem to make sense to me! In my head I figured the easiest way would be to make them up outside of the deck first in panels and then, after fitting the newel posts, slot the panels in and fix with screws? We're going for the modern square spindles so figured I could have one at the far left and right of each 'panel' and screw through these into the newels. Is there a better/easier way?
  • The deck boards will sit slightly higher than the level of our DPC, by about 1cm. Is this an issue? Again online I've seen conflicting advice - some saying a definite no, others saying OK as long as you make a gap between the 1st joist and the house wall of around 5mm, and others saying OK even without a gap.
Thanks so much in advance!

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