Leaving no gaps between decking boards (really that bad)?

I quite often dont leave a gap if I'm using "green" hardwood as I know that it will shrink as it dries. If you are using treated pine the opposite is true:IE it will expand and your decking will warp so you must use a bigger gap than say on seasoned hardwood.

Decking should be laid with a gap. Wood moves. When it's wet it expands, when it's dry it shrinks (this doesn't just refer to rain either, it refers to humidity). Decking is rarely completely dry when you lay it. Merbau in particular can be very wet still. It is most likely going to shrink. If you lay it with no gap, there will be one soon enough.

Victorian style verandahs are usually laid with tongue and groove flooring, are they not? The tongue and groove allows the gap to be concealed whilst allowing the boards to expand and contract.

I think you are opening the way for problems down the track.

Originally Posted by Bleedin Thumb

If you are using treated pine the opposite is true:IE it will expand and your decking will warp so you must use a bigger gap than say on seasoned hardwood.

I laid a TP deck for a client about 6 years ago now. The foreman told me to lay them tight, and I thought he was mad. The architect came along and told him to rip them up and re lay them with gaps and he said OK.
He did nothing, and before the end of the month they had reasonable and even looking gaps.
I built a TP fence here five years ago, and put all the palings hard up against each other. Now there's about 5mm gaps between them that started appearing a few weeks after installing them.

I suppose it depends on their MC when you receive them, but in my experience TP shrinks more than any other timber.

Yep your right John, The process of treating the pine makes it swell, so if your buying it "wet" it will shrink, though you shouldn't use wet TP for decking as it is more likely to warp, twist & cup that kiln dried.

Yes it is usually wringing wet when you buy it. And it shrinks quite a lot. But so does hardwood decking, and especially that has been my experience with Merbau, which is full of resin that leaches out as it gets rained on. It also shrinks in length.

The point is that if you don't allow for movement, you will have trouble down the track. All timber is going to shrink and expand throughout the year. If you pay attention to the time of year and the moisture content at the point of laying, you may get away with no gap, as it will shrink away and may never expand as much again. But sooner or later there will be a gap. The only way I know of to lay flooring that allows for this movement and also conceals the gap is tongue and groove.

So in short, don't lay decking with no gap and expect it to stay that way, and if it's really dry when you lay it, expect to have problems later on.