Seahawk Highlights Video (206) 949-5683, Top Rated Plumber, Seattle

# Thread: Joist size for raised floor

1. ## Joist size for raised floor

I have an old bedroom I'm converting to a bath. The existing floor joists are 2x6s on a concrete slab. I'll be installing a raised floor directly over the existing floor, but...what joist size do I need to allow sufficient height for the in-floor traps for the shower, tub, toilet? I'll have a plumber do the work, but need to know what size framing to use. Thanks a ton.

2. Are you removing the 2x6 floor?

If the plumbing runs with the joist, then 2x8 works.
If you are crossing the joists, then 2x10
If you have 2x6, then you will need to knock a little concrete out for the p-traps.

3. We're leaving the 2x6 flooring in place, as well as the plywood. The new (raised) floor will allow the shower, tub and toilet plumbing to run in one joist bay. Will 2x8's be enough room for the trap as well as the necessary drop? The toilet is at the near end of the run - where the waste line will enter our cellar and meet up with the main, so that shouldn't be a problem. The shower is the furthest away - about 15 feet from where the waste line enters the cellar. So, with a 1/4 inch drop for every foot, that'd be about 4 inches of drop. Sound about right for 2x8's??

Thanks a bunch.

4. ## position of main

good question, about the 15' distance. Showers often have raised floors inside their space, so it wouldn't feel awkward or unusual if the floor were raised a half inch more just to accommodate the height needed for its drain, imho.

So far, I think a big, missing, factor in calculating the height, is all the information about the main, and its position compared to the slab. Inches above, etc. Vertical horizontal etc. Type of material etc. The coupling to be used will need a measurable and known amount of space above and below. This determines your overall numbers.

david

5. The main is in a cellar under the room next to the new bath. The main is about two feet lower than the point at which the waste line will enter the cellar. I don't see a problem there.

6. shower is limiting factor then.

A copper P trap is smaller than a PVC one, FWIW. It doesn't seem like you are interested in saving a fraction of an inch.

How will you vent it? It cannot run 15' unvented. It'll need venting before it reaches the main. This may get complicated; it may be easy.

david

7. I'm adding in wall studs all around the perimiter of the room, for the sake of insulation, etc. So..the vent can run up through the new stud wall. So...do the 2x8's work? The drain line coming out of the shower will be 2", I assume, so that gives me some added leeway over the 15' run of the shower.

8. ## instead of 2"x8" joists

A very small height half-step is a safety hazard. A height transition between two rooms is not.

So, personally I would work real hard at figuring out a way to keep the 2x6 joists. Since the room is so deep, maybe you can keep them until you get to the shower, or put a real full size half-step there. Work with them for as long as possible. Maybe you can keep them all the way right up to the shower and just add another 1/2" under the shower. The shower floor is often higher than the rest of the bathroom anyway. If I really needed the height at the shower, I would even give thought to sloping the entire bathroom floor 1/8"/ft so that the necessary height is there at that end. All that just to avoid a potentially invisible half step.

david
p.s. did you mean furring strips for insulation and not studs?

9. The current 2x6 floor runs perpendicular to the plumbing run, so....that doesn't work. I have a "non-trip" floor plan all drawn up that will work for the raised floor (2x8s), so we're good there.

10. Oh, and I'm actually putting in 2x4 framed walls around the perimeter of the room. It's a 160 year old brick house with zero insulation, and the 2x4 walls I'll frame within those brick walls will give me lots of room for electrical, insulation and supply plumbing.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•