Could someone give me some advice on the best way to deal with this. I need to replace two floorboards next to each other but I couldn't buy the original 22mm x 112mm floorboards so I had to buy 18mm x 108mm but now I have big gaps between each floorboard and I need to raise the floorboards 4mm. I did think of buying a piece of hardboard and fixing it to the new floorboards but will this look stupid.
Put the packing on each joist to raise the boards. Buy a length of 19 x 12 and pin it to one of the board sides.
You would be better keeping what you bought for another project and try and source some old style floor boards from a salvage yard or have a wander around the streets to see what is iin some skips
In the past I've lifted floorboards from a cupboard to patch an exsisting floor which was visible, & put down ply in cupboard.
A proper timber yard can cut to size and plane to thickness. A real work place with sawdust and men having a swear and a roll up. Not a shed with people in aprons.
I'm glad I'm not the only one on here who goes skip hunting! There is always some good stuff to be found.
As said already, a decent real timber yard will machine that for you in minutes. As you are fitting these in, T&G is not needed and will get in the way.
You can get timber that thick and even wider but you would have to do the t&g yourself. I came of the rollies 15 years ago, my lungs are still buggered.
I went to my local timber merchant today and they only do 18mm and 20mm floorboards so if I bought a thicker piece of timber would they be able to rip it down with the table saw they have.