Different New And Old Joist Sizes On Deck Remodelling Job

by Scratching our heads
(Midwest)

We are re-decking the flooring of our existing 15 yr old deck, but previous owners spaced joists at 24" OC. These 2x8s measure about 7 1/4" while the new joists we are adding are noticeably taller at 7 1/2".

Will the new ones dry to match?

If we rip to match, we are afraid the drying process will reverse the problem, but if we don't there is a noticeable difference in the bowing of the deck boards. We don't want to buy all new joists - saving the money was the point of the remodel.

The deck itself is very sturdy. Never heard of old lumber being smaller than new - usually vice versa.




Editor's Comments

This is a great question. What you are seeing here is the difference between new "green" lumber - not the color green but new or young, wet lumber from the mill or the yard. Your 2x8 joists are typically milled at 7-1/2" wide but are usually wet when milled.

They might have as much as 25% moisture content in them when they are milled. Therefore it is not uncommon at all to see a large 7-1/2" piece of lumber shrink 1/2" across the width over time.

Particularly with fast growing species like pine and southern pine.

What To Do?

To be absolutely sure, you could buy the lumber and let it sit in your garage and air dry for three weeks or so. This would bring the lumber down to about 12%-15% m.c. and you will most certainly see that the lumber has indeed shrunk a bit.

At least now you will be confident that it is very close to its final dimension in the field. Then if for some reason you wanted to return the lumber, you could do that. Any lumberyard will take it back if it is the same good condition it was delivered to you in.

That is your safest option.

A Great Lesson On Lumber Sizes And Cuts

While you are learning about this, you might want to read a short but pertinent article on how logs are milled and the grain orientation which affects the movement and shrinkage of lumber as it dries.

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