Decking Spacing

lmanning · 3 Jul 2008

Hi,

My friend has just put some decking down and left a 7mm gap between the boards. He is now worried that with shrinkage these will become excessive.

How much shrinkage/expansion would you expect to see across the gap during a season.

When I do a search on recommended gaps I see anything from 3-6mm stated. Having done a search on here I see that 5mm is the recommended amount.

My mate is so concerned that he is considering taking up the ones that he has screwed down and re-spacing them at 3mm. Is this wise? Will involve some work but he is more concerned about the finish.

I am just about to screw mine down having done the joist work so any advice for me and him would be gratefully recieved.

Mark Br · 3 Jul 2008

How green are the decking boards that you are putting down and what is the width?

lmanning · 3 Jul 2008

The green is still fairly evident.

Does this disappear over time as I am just about to ask a Staining / oiling question regarding this.

big-all · 3 Jul 2008

lmanning said:
The green is still fairly evident.

Does this disappear over time as I am just about to ask a Staining / oiling question regarding this.

l o l in this respect "green" means how wet where/are your boards

if your boards where stored out doors and where completly saturated you could expect 6 to 9mm shrinkage on a 150mm board

if your boards where wet but only superficaly then 3 to 5mm

if they where fully dry[coverd but exposed] 1 or 2mm

lmanning · 3 Jul 2008

No Sorry. Green as in the tanalising. The wood is fairly new and has been stored in the shade, but we have had a dry few weeks.

I intend to lay them all across the joists to dry out as I will not be working on it again for the next week or so. However it is now raining

The boards are 145mm finished size

cheers

WabbitPoo · 5 Jul 2008

In my experience the boards dont change much dimensionally across the width, just along the length. 5mm gaps look best to me. I've done so many that I do it by eye, but find a suitable spacer and use that (actually, use at least two) - get the first board straight and the rest is easy.

Marty74 · 7 Jul 2008

If it helps I layed a deck in March this year. I used 5mm spacing between boards. I'd now say the spacing is about 10-11mm. A bit too much for my liking. A bit late to worry about it now though.

I was hoping they might go back a bit, does anyone think they might?

^woody^ · 8 Jul 2008

I laid decking one June, with 5mm spacing and the timber was in the garden in shelter for a couple of weeks first

The joints opened up to about 10mm by the following year and have never got any narrower - even in winter.

I would say that a 3mm spacing (piece of hardboard) would be enough to give a more pleasing narrow joint when the boards shrink

Marty74 · 8 Jul 2008

^woody^ said:
I laid decking one June, with 5mm spacing and the timber was in the garden in shelter for a couple of weeks first

The joints opened up to about 10mm by the following year and have never got any narrower - even in winter.

I would say that a 3mm spacing (piece of hardboard) would be enough to give a more pleasing narrow joint when the boards shrink

Seems the same as mine, shame really but not the end of the world.
If I did it again I think I'd go for a 2mm gap. Shrinking was the only thing I didn't think about when building the deck!

megawatt · 9 Jul 2008

The key point was at the start ... What is the level of moisture content in the boards when they are to be laid.

If they have been recently tannalised as most are for pros as they order as required generally direct from the sawmill and the boards are tannalised just before they're delivered then I would use 2.5mm drills as spacers as the gap will only get wider by a few mm throughout the summer months.

If the boards are absolutely dry though when laid then this gap will be too small and the boards will warp during the winter months when they expand due to moisture retention and I would lay with 4mm drills as spacers in this case.

It's always best to buy freshly tannalised lumber direct from a sawmill IMO and use a 2.5mm drill

WabbitPoo wrote:

In my experience the boards dont change much dimensionally across the width, just along the length

I assume you've mis-written this WP? All the expansion/contraction of timber is across the grain generally not along the grain and anyone with a deck will have witnessed the obvious difference in board gaps between the middle of winter and height of summer (around 3-4mm for softwood at the extremes on large boards).

MW

ross smith · 8 Dec 2021

WabbitPoo said:
In my experience the boards dont change much dimensionally across the width, just along the length. 5mm gaps look best to me. I've done so many that I do it by eye, but find a suitable spacer and use that (actually, use at least two) - get the first board straight and the rest is easy.

ross smith · 8 Dec 2021

This is odd, as timber typically expands perpendicular to the grain? Typically only a very small percentage increase in length happens along the length of the board. My decking boards were green when installed and shrank about 8mm across the width, over the first summer. After that, they seem to expand and contract around 4mm across the width, seasonally. Knowing they were green, i left a 4mm gap when installing, which became 8mm in the first summer. (when the wood seasoned). Now the gap goes from about 8mm to about 6mm seasonally.

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Decking Spacing

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