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My current mud/laundry room was once an exterior porch (enclosed decades ago). I removed all the layers of flooring down to the original planks thinking I would refinish it or do tile or something. But then I started thinking what I really want is to raise the floor to the height of the kitchen floor, which you currently have to step up into from the mudroom by about 8" through a doorway.Then I can widen the doorway and basically "expand" the kitchen a bit.

I've seen a lot of how-tos for building a floor on top of a concrete slab to raise the height, but nothing that specifically mentioned raising a wooden floor....So is the correct way to do this to install rim joists around the room then install new joists, then subfloor, etc Or, are there any other methods?

If the above method is the way to go - a few questions:

  1. Should the joists run parallel to the existing joists in the original floor (length of the room)?
  2. Should there be insulation between the raised floor and old porch floor? If so what kind is best (moisture is a factor from the basement--old cement floor, stone foundation, and the sump well/pump is below the mudroom. However there is some ventilation and we use a dehumidifier down there as well).
  3. Should the rim joists be bolted into the studs? Or screwed? I saw one post that used 3 wood screws per stud, another that used bolts instead.
  4. Would I just leave the existing drywall in place and install the rim joists over it?

More info: The mudroom sits above an unfinished basement with a stone foundation and a cement floor. There is an full bathroom (also added decades ago) on the second story above that matches the footprint of the mudroom. The room measures approx 6.5' x 19'.

The ceiling height of the mudroom matches the ceiling height of the kitchen, and there would be adequate ceiling height if the floor is raised. I would have to address the issue of the exterior door, (which opens onto a covered patio) because it would then be too low with the floor height change.

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What you’re talking about in this case is technically a ledger attached to the studs. I presume you’re going to need to pass an inspection, and in any case you want the floor to do its job, which means there are calculations to be done to determine how much load each stud ends up holding. Not all screws are appropriate for a job like this. The article I reference just below talks about Simpson Strong-Drive SDWS timber screws, and provides some computations as well. The screws have to have high sheer strength so they won’t break off, and you don’t want them to rust out and fail in the future. Here is an interesting page on the subject:

https://www.jlconline.com/how-to/framing/attaching-a-ledger-to-studs_o

Classic time-tested construction is post and beam. The posts are set on a foundation, and the beams sit on top of the posts (or a rim joist and the ends of the floor joists sit on top of a foundation). Basically, everything is stacked so that load is transferred straight down in a vertical column to the foundation. Gravity does all of the work. Fasteners like nails and screws (or holes and dowels) keep things aligned vertically so that nothing slips sideways and gravity keeps the structure up instead of pulling it down.

But in this case, you would obviously have to either build a parallel foundation to support the floor (a hard expensive job) or hang the floor off of existing structure. There is a good chance that attaching a ledger with the right fasteners and using joist hangers is legal where you live. It can definitely be strong enough if done correctly.

Remove the sheetrock high enough to attach the ledger in direct contact with the studs. You can leave the sheetrock in place higher on the wall.

Make sure you measure twice before attaching the ledger so that your subfloor and flooring don’t end up 1/2” higher or lower than your kitchen floor.

Run your joists across the shorter distance. This does at least two things. First, you can use narrower joists because each one carries a smaller load over a shorter span. Second, each of the studs that the ledger is attached to will carry a smaller load, because the load will be distributed across more studs down the longer wall(s).

Use metal joist hangers attached to the ledger with appropriate (high sheer strength) nails or screws (typically nails).

If you’re using dimensional lumber for your joists they might differ very slightly in width from one to the next. This can make your floor uneven. In that case use hangers with a flat face that do not hook over the top of the ledger. That way you can adjust the hangers individually so that the tops of the joists are exactly even with the top of the ledger.

If a joist has any bow to it, set it so that it arches up instead of dipping down. But get them as straight as possible. Lumber can twist and warp if it’s cut from relatively young, green trees and cut into boards before the wood has dried sufficiently.

Better yet, buy engineered joists. They’ll be identical in width and won’t have any bow to them.

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  • Would that method (attaching ledger to studs as you describe) be the only viable option? Would running new joists directly on top of the existing wood floor be simply too much load for the original joists to bear? – JLL Apr 23 at 3:35
  • I would probably lay them across the existing joists, perpendicular to them. What are the dimensions of the existing joists, how long a span do the cover, and how many are there? I presume they’re dimensioned lumber, not engineered joists? Are you just concerned with the short distance you’re trying to raise the floor and worried that there isn’t enough height for a ledger board and joists? What is the distance of the shortest wall to wall span on the room? – Craig Tullis Apr 23 at 4:49
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    I don’t really like the idea of just building the new floor on top of the existing floor because you might exceed the designed load carrying capacity of the old floor. If the height you need puts the joists right on the old floor, why not put ledgers right on the floor, tie them in to the studs like you would if you were hanging them, and use joist hangers to attach the joists to them. That way the joists are sitting on the old floor, but the load is also shared by the ledgers. – Craig Tullis Apr 23 at 5:07
  • That sounds like a good approach. The porch floor has been here since at least 1906, possibly earlier. The main house was built in 1884. The joists are 1.75" x 5.5", there are 5 of them. Two sets - first group spans about 8', then they tie into a 4" x 6" perpendicular beam, then another set of 5 continue on the other side of that beam for about another 11 feet, where they connect to 4" x 6" beam which is up against another larger beam, where the exterior door used to be. The distance between the joists is not consistent - 13" OC to 16" OC. – JLL Apr 24 at 3:08

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