Replacing deck joist - board width mismatch

[tomsteve]

I think we lost him. But I'd just notch the joist too and be done with it.

thats too simple. this requires  sketchup, hours of investigation, scientific load tests, and a thesis.

[Stwood_]

I didn't believe it until I looked it up but you are right, a nominal 2 x 8 has an actual width of 7.25".  Always thought it was supposed to be 7.5".  Now I know better.  Thanks. 

John

All the standard, required sizes were changed several years ago.

[AUswimKC]

Notching is typically not allowed per code as it invites a split along the grain

[Snipe Hunter]

Notching is typically not allowed per code as it invites a split along the grain

It's allowed but has limitations

R502.8.1 Sawn lumber. Notches in solid lumber joists, rafters
and beams shall not exceed one-sixth of the depth of the member,
shall not be longer than one-third of the depth of the member
and shall not be located in the middle one-third of the span.
Notches at the ends of the member shall not exceed one-fourth
the depth of the member. The tension side of members 4 inches
(102 mm) or greater in nominal thickness shall not be notched
except at the ends of the members. The diameter of holes bored
or cut into members shall not exceed one-third the depth of the
member. Holes shall not be closer than 2 inches (51 mm) to the
top or bottom of the member, or to any other hole located in the
member. Where the member is also notched, the hole shall not
be closer than 2 inches (51 mm) to the notch.

[K. L. McReynolds]

I am working on replacing decking and found a couple of rotted joist I decided to replace as well.

The joist size is 2x8-12. I purchased the replacement joists however when I took one rotten out and compared with the new one, it turned out the new one is 1/4" wider. The label on both old and new joists says 2x8-12, but the exact width of the old one is 7" and new one is 7 1/4".
Did I get wrong product? The old deck may be 10 years old -- is it just result of the wood shrinking over years? If so does it mean I can't replace a few joists and should do it for all of them?
Technically I could cut off the extra piece but if it is shrinking, then it will become uneven in a few years.

Any guidance would be much appreciated

Lots of factors are in play here. The original boards have probably compressed due to rot/moisture/etc. New growth lumber(PT or not) has a less dense grain(because it is grown faster) and therefore shrinks more. New construction lumber is also often shipped sooner after cutting/processing(including kiln drying) and has not stabilized as much as it will after sitting exposed for a year.

The issue is now whether the new boards will fit and not cause humps. And how dry the new PT board is. If it is dripping wet(have seen that way too often), take it back and do not use it(them).

I would not necessarily notch a new joist, simply because if it shrinks, now there is a dip. A 1/4" hump may or may not be a problem.

Point being, we cannot give the correct advice for your situation on line. There are too many variables.

[fixtureman]

Can you move one of the old ones from the side and use the new one on the edge

[Bill Wilson]

I've run into this problem even when all the boards were new.  Built a pergola for my daughter and SIL earlier this summer.  A 1/4" difference in width was found in both 2x8's and 6x6 posts.  All the lumber was bought at the same time, from the same place.

[Cooler]

Also, if there are any doubled joists, top them with self-adhesive rubber flashing.  

The worst rot on my deck occurred where the water got trapped between abutted joists.  The flashing will prevent that.  It is fast, easy and cheap to do.

This is a 3" wide roll:  https://www.homedepot.com/p/Quick-Roof-1...267104-_-N

4" wide would be better if you can find it.

[Tapper]

Lots of factors are in play here. The original boards have probably compressed due to rot/moisture/etc. New growth lumber(PT or not) has a less dense grain(because it is grown faster) and therefore shrinks more. New construction lumber is also often shipped sooner after cutting/processing(including kiln drying) and has not stabilized as much as it will after sitting exposed for a year.

The issue is now whether the new boards will fit and not cause humps. And how dry the new PT board is. If it is dripping wet(have seen that way too often), take it back and do not use it(them).

I would not necessarily notch a new joist, simply because if it shrinks, now there is a dip. A 1/4" hump may or may not be a problem.

Point being, we cannot give the correct advice for your situation on line. There are too many variables.

No personal offense intended here, but this is clearly "paralysis by analysis" IMO. This is a pretty simple matter. Notching these few joists is NOT A BIG DEAL. If the new joists shrink a little later, just shim them to the level of the old ones to keep the deck surface level.

BTW, if you choose to trim the new joists to the original size, you run into the same problem, i.e. they may shrink later and have to be shimmed. I would notch them and be done with it. If they eventually split at the top of the notch you still haven't lost any strength, as they will be at the same load bearing level as the old joists.

Doug

[Snipe Hunter]

"paralysis by analysis"