I’ve got most of the vertical T&G cladding on my bungalow now.
It looks like this at the moment, I’m leaving the bits over the windows till last to use up the offcuts.

When I fixed the horizontal shiplap to the shed it was soaking wet, so I just tacked it on at first, allowed it to shrink over a few months, then moved the boards closer together.
I ended up with about 25mm shrinkage over 2m.

I’ve done the same with the bungalow, except this time I’ve bought a moisture meter.

This wall gets a couple of hours of direct sunlight every day.
Moisture content is 2% and the gaps have opened up.

This wall gets virtually no direct sunlight and is at 14%. The gaps have opened up, but not as much.

The plan was to pull all the boards off and bunch them up. You can see I’ve left the nail heads proud to make it easier.
Should I do that though ?
What if we get a monsoon and they all swell up again ?
The tongues and grooves are there to allow for expansion, so if it’s down to 2% moisture and the tongues haven’t pulled out of the grooves, should I just leave them as they are ?
Is there an official rule for moisture content and gap ?

Second question; Should I buy stainless nails at about £12 per kg or galvanised nails for about 1/6 the price ?

I have nothing useful to ad I’m afraid but it is great to see some more pictures of your place. I’ve not read anything about it since you were sorting out the foundations!

are you painting the cladding, treating it in any way?

the t&g is there to allow expansion and contraction if you pull it off and close them up you might get boards popping as there’s no where to expand.
SS nails are better and stop black staining that you’re starting to see on your other nail holes. you could get away with galvanised but you run the risk of them staining, degrading faster than the SS ones. up to you and depends on the final finish.

I’ve just done the sums and it looks like if I only put one nail in each board at each lath, 4kg will be plenty. <£60 is not significant in the overall cost, so I might as well use the best.

I was going to use some sort of coloured wood stain. Like Cuprinol, but better.
In fact, that will probably be my next question. Watch out for a What Wood Stain And Preserver thread coming soon. 😉

Yeah, boards popping off is my worry.
I’ve seen floor boards with the tongues pulled right out of the grooves so you can see through the gap where they’ve been nailed on wet then dried out.
I’ve heard of boards buckling where they’ve been nailed on dry, then swollen.
If I’ve got 2-14% moisture and the gaps are about half open, should I just leave it at that to allow for movement either way ?

It’s not something I know much about, but I wonder what numbers you’d get if you measured on the inside face of a few boards, and early morning. For the outside might vary quite a lot, depending, could be misleading you.

if the tongues aren’t open with that moisture content i’d leave them. make sure the stain you use is MICROPOROUS as this allows the construction to continue to breath. (ie no varnishes, or anything that seals the wood. it will lead to maintenance issues, reapplying frequently and potential damp issues). Sadolin is good but speak to the manufacturers direct to get the best spec for you particular situation then buy accordingly. cheap is most definitely not always good OK for a shed but not that.

Good point slowoldgit, I hadn’t thought of that.
I’ve got a layer of Tyvek Housewrap on the inside which is water vapour permeable.
The shaded side of the building is about 14% on both sides of the boards.
The sunny side is about 2% on the outside and 10% on the inside.

Give me your email and I’ll send you some BRE guidance information.

Thanks pjm84, that would be a big help.
Rearrange the following;
.com
gmail
@
vegangraham

Thanks again pjm84. That’s one of the documents I found while searching for information, but I couldn’t download it as I’m not a paid up member.

A hint about moisture meters – you really need to get the prongs well stuck into the wood if you’re going to get a reliable reading – just touching the points on the surface will give an unrepresentative reading.

Yeah, getting readings of 2% and 10% on opposite sides of a 19mm board shows how the sun only dries the outer surface.