Val
I use tongue or groove 1/3 thickness of board in imperial measurement 3/4" board 1/4 groove

 

Hi Val.

Welcome to the forum.

Do you have a router table?

IMO, 6.35mm is close enough to 1/3 the timber thickness to do. (6.35 x 3 =19.05mm)

I believe the T&G is more to stop the timber twisting than for other reasons.

 

I believe the T&G is more to stop the timber twisting than for other reasons.

I agree with James. It's just a method to help with aligning the panels for glue-up. You could use biscuits, nails, dowels, a routed glue line or simply nothing.

If however you're after the V-groove look then again as James said, 1/4" is fine for 20mm stock.

 

THanks a lot James, et al

yes, just built my own router table - proud of it

I see your point. I was more considering it from the point of view of equalising strength of both pieces. I am joining them at 90degrees - crosswise boards fitting into 2 edges lengthwise boards.

 

In that case you need to be careful of wood movement. Research breadboard ends.

 

You can go a little thicker if you want. The 1/3 rule is probably mostly for flooring. If you had a gap under the groove side of a board and applied pressure to that edge then the weight would be held by the wood above the groove. Too thin and it would want to split. I would say that the 1/3 rule has less to do with the tongue than it does with the groove side.