Using untreated wood outside

There are 2 local timber merchants near me. One is out of stock with the current pandemic. The other has stock but its untreated. I have a framework at the back of the garden holding some gates up. The square perimeter and the posts are all treated. However the pieces going across every 500 are what will be untreated.

Would this be OK or would it swell more and put strain on the outside pieces? The wood will be well treated and painted every year or two

As long as the wood is well protected you will be fine

Could you not wait until the timber you require is back in stock?

I could but the other shop has everything I need and I can't warrant paying for delivery for 4 lengths of treated 4x2! The other shop doesn't seem to be doing a preorder either. I'm guessing that's only for the big customers/businesses

Yeah you will probably find they will look after the big orders first then you as the little guy goes to the back of the queue

Fingers crossed that now lockdown is over supplies will start to improve as people won't be so bored and turn to diy to ease the boredom

do they have any other options like ripping 8x2" treated in half??
or treated cls or even 5x2 treated if you only need 4 then look at any oversized option ??

When wood swells with moisture, it gets thicker, but not longer.

For garden work I often buy decking boards, which are quite cheap and pre-treated.

You can buy timber-preservative chemical mixtures. Typically about £25 per 5 litres. I prefer the solvent-based ones because I believe they penetrate better than water-based.

You need to soak timbers that will be in the ground (or better still, don't do it) but you can treat above-ground timbers with several flowing coats, each applied before the previous one has fully dried, so it penetrates deeper. If you stack them in a polythene tray, any drips or run-off will be soaked up by the ones beneath, and not wasted.

End-grain and knots need extra treatment because water can get in.

I don't understand what these cross-pieces are doing. Can you post a pic or photo?

Flat horizontal surface should be avoided, that’s why arris rails are used .