Expert: Bruce E. Johnson - 5/10/2009

Question
Dear Sir, I wish to lay garden decking over & beyond my current solid concrete patio. Can you please advise me to make the frame, what size should I use ?, should it be treated to prevent rotting ?, should I use anything between the timber & patio to prevent the timber retaining/sucking moisture from the concrete?, any help would be appreciated, regards, Simon.

Answer
Hi simon, normally when doing a wood deck over concrete slab I use 2x4 joists laid on edge and 24" apart with 2x6 decking screwed to the joists using treated deck screws.  All material is pressure treated pine and shouldn't require any film or vapor barrier.  Where your deck extends beyond the concrete you can either pour a strip of concrete to support the edge of the 2x4 joists, they can span up to 4' so if you are going 8' beyond the concrete slab then you need to do a support strip every 4'..Another option is to bed a 4x4 pressure treated sleeper in the ground at the elevation of the concrete slab and use this to support the joists.I hope this information helps fell free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com