Adding new joists to old deck: lumber size mismatch

Question

Very new to this forum, forgive me for any mistakes.

I am replacing the decking on an existing deck at my house, and the new decking is a composite PVC cap stock product. The old decking was cedar, and the joist spacing was 24". That is beyond spec for the new decking, so I am adding a 2x8 joist in between for 12" spacing.

The problem I'm having is that the old 2x8 joists are 7" actual width, whereas the new joists are 7.25". This causes the new joists to be proud of the old ones, including the rim joists. I don't think I can just hang them and then run the decking over uneven joists.

Has anyone seen this before? Can anyone comment on why the old joists are small? Could that just be due to shrinkage?

Answer 1

First make sure that your support posts and beam(s) are in good shape. You'll be adding a lot of weight doubling the number of joists and adding the heavier PVC decking.

Your choices are to either allow the new joists to hang lower than the others, or if you don't like how it looks, cut a 1/4 inch off the width before they are hung.

No matter which choice you make, the joints are still hung flush on top.

Answer 2

That is a problem! While all wood will shrink as it dries, pressure treated wood is the worst offender because pressure treated wood is puffed up with chemicals and moisture when new. If you measure the width in the middle and at the ends of new P.T. wood you will often find the ends are already a little narrower because the ends dry faster. So yes, your new wood is swollen, yes it will eventually shrink to the size of the others.I would notch the new wood to the correct width taking advantage of the slight narrowing on one end and keep the joists turned the same way since you are probably cutting several inches off the other end. Then in a year or so if the width difference bothers you you can raise the joists up with your hangers or maybe even glue shims under them.