[New Leaf Design]

Ok, at the risk of monopolising forums at the moment (apologies) I’m going to ask a question related to my other current forum discussion: what is the best gap between decking boards – and are there even tables for this relative to species and board width? I ask because i’ve been looking into this and found a lot of interesting discussion in the internet. This, although not UK based, was interesting and I think got to the nub of the problem:

http://www.mcilvain.com/what-size-gap-should-i-use-between-my-decking/

The ‘even a stopped watch tells the correct time twice a day’ analogy seems quite appropriate. FWIW, i note that both Champion Timber and Wickes (o.k., low-end suppliers I suppose) specify a 5mm gap on their softwood boards.

But surely the most relevant factor is at what time of the year are the boards being laid, and are they kiln-dried timber, or seasoned, or unseasoned.

E.g., i’m specifying English larch from a saw mill less then 10 miles from the site in question. The boards will be reasonably freshly sawn, and it’s fair to assume, stored outside in the english autumn. So, it’s highly likely that when laid, the boards will be at their most swollen, and will likely contract in the summer. Presumably, a 4mm gap with wet, fresh sawn boards, could open up to as much as 8mm in the summer sun, and a kiln-dried board laid in August with a 4mm gap could possibly see those gaps close up entirely in a long wet winter. I’m using 145x32mm boards. Any thoughts?

[Karl Harrison Landscapes Ltd]

Ivan, you are always welcome to post anytime, no matter how often.

It is easy for suppliers to offer advice on gapping and you can read as many books as you dare but I recon in reality for the absolute best gap is determined by a combination of the various texts out there and experience by decking installers.

Advice is usually best by TRADA but I do disagree on anything over 6mm, so there 10mm is a little excessive, even though for certain attributes of deck construction.

I do agree with your statement “But surely the most relevant factor is at what time of the year are the boards being laid, and are they kiln-dried timber, or seasoned, or unseasoned.” And of course your choice will be enhanced by using a moisture content metre. Again this detailed analysis is only useful on a surface fixed deck and not good what so ever on a side fixed deck as with the latter the gap is determined by the clip. Some clips allow movement for this to a certain extent but not all.

E.g., i’m specifying English larch from a saw mill less then 10 miles from the site in question. The boards will be reasonably freshly sawn, and it’s fair to assume, stored outside in the English autumn. So, it’s highly likely that when laid, the boards will be at their most swollen, and will likely contract in the summer. Presumably, a 4mm gap with wet, fresh sawn boards, could open up to as much as 8mm in the summer sun, and a kiln-dried board laid in August with a 4mm gap could possibly see those gaps close up entirely in a long wet winter. I’m using 145x32mm boards. Any thoughts?

May I ask why you have specified a timber not usually used for decking? One can’t be sure of its “best use” and I do know that this timber is rather soft – to the extent similar to pine and shouldn’t be confused with Siberian Larch which is more than double the hardness and completely different animal altogether. I would be certain this would be a lively product and I would recommend tight joist centres and also massive screws 5.5 x 75 mm (minimum)

[New Leaf Design]

Hello Karl, and thanks for your helpful comments. The reason for the English larch was really just a cost decision – the project was over budget, and I had to find every saving I could whilst keeping the overall design intact. At the time, English larch was £28/m2, and Siberian larch £35/m2. Having said that, if I now include for treating the English larch, whilst the Siberian can be untreated, I doubt there’s much of a difference. Maybe I should give you a call about supplying the larch Karl? I seem to remember you can supply Siberian Larch.

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