Replacing the rim joist (beam) on a 50+ year old porch

Question

A 2x8 rim joist, that also acts as a beam, on a 50+ year old front porch is damaged and needs to be replaced. This rim joist is resting along its length on a cinderblock wall. I'm not sure if the existing rim joist and the regular joists are pressure treated; probably not as they are painted on all sides.

If I replace with a pressure treated joist, it will be taller (wider) than all the other joists because of chemical treatment/moisture content. If I rip the joist to be the same width/height as other joists, then it will work for now but, in a year, it will sit lower than the other joists as it dries. Even if I use untreated lumber and paint on all sides to prevent rot, there will be some shrinkage over time.

How do professionals address this problem?

Answer 1

It does depend on other criteria how the extra width is handled. If there are bearing points that will no allow the beam or rim joist to be picked up. which by the way, the joists will need to be re-supported by a temporary beam or plate with a number of posts to hold the joists for the time while the repair is going on.

That being said. it will be a matter of how high do you need to go to get the full size replacement in, and will anything keep it from being raised enough to do so.

If you are working with a home (as so it seems) built in 1965, maybe as late as 1970, to present day there should only be a difference of a 1/4" to 1/2" and that is being generous. There should only be a 1/4" since all 2X material has been milled to the same dimensions, and the only reason the wood would be bigger is from the water used in the treatment process. Therefore this is why I would jack the porch up and set the full size "beam" in place. The 1/4" shrinkage is nothing in the scheme of things as far as porches go, THAT IS, if it is a standard porch and not a porch with a room above.

The amount needed to get a new beam in is negligible, provided the beam hasn't dropped in any way due to rot and you need to jack it up extra to make up for that. If it isn't rotted, you will only need to jack up the floor joists and hopefully the roof goes up with it too (more on that) only an 1/8 ", but if the new wood is still wider, jack it up that much plus the 1/8, of more if it is easy enough and the roof isn't supported by the beam you are replacing. There is a good chance it is.

If so you need to support the roof separately anyway, you can do so from the joists which may be simpler, but more demanding of the temporary supports, at least under the porch roof support posts, there you will need to go the extra mile. You could support it independently from beyond the porch or do like I have done. remove the porch decking to a degree and run the temp supports straight up from the ground below the where the posts bear under the roof. A few things, the framing need to be determined at those areas so the temp supports are not on finished wood that really does no support at all. That could be a bad surprize.

I will leave this answer at this point, if you need more direction, I or others will give it to you, should you decide to take this on. More detailed direction depends on your findings.