how to replace entire second level subfloor?


Old 04-25-05, 05:12 PM
Thread Starter
Join Date: Apr 2005
Posts: 42
how to replace entire second level subfloor?

hi all -

i'm gutting the entire second level of my dormered cape; this level used to have 2 bedrooms seperated by a bathroom. because all of the interior walls were non load-bearing, i've removed them. so aside from the rough plumbing and electrical, i basically have one large open space stretching from gable to gable.

the current flooring is 1/2 inch particleboard over a 1/2 inch plywood subfloor. due to some questionable plumbing in the bathroom, plus a carpet covering, much of the surrounding flooring is rotten. to start with a clean slate, i planned on removing ALL of the existing flooring on the second level down to the joists (not just in the bathroom), and gluing/screwing down some 3/4 inch ACX plywood for a subfloor.

my question is - the strips of existing subfloor underneath the sole plates of the exterior walls are impossible to easily remove since they're nailed through. is it ok to simply remove the existing subfloor down to the joists, but only up to the sole plates, then abut the new 3/4 inch subfloor up to the plates and the old 1/2 inch subfloor still nailed below them? i had read that the subfloor provides important structure in holding the joists together, and didn't know if doing it this way would somehow weaken things. if it's ok do it this way, would people recommend to replace small areas (i.e 4 x 8 sheets) at one time, or remove all subflooring first, then lay down the new one?

thanks in advance to everyone for any help
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Old 04-26-05, 06:35 PM
Join Date: Dec 1999
Location: South Dakota
Posts: 3,140

I answered this about 4:00 this morning, but noticed that it did not get posted. It is now 12:30 am, so have been up 20 hours and will answer again, but please excuse typing errors. First of all you are using the correct lumber, in putting down your new floor. Good for You. Next, do not take up all of your old sub floor at one time. Remember under your subfloor is your ceiling of the room underneath. I always take off 2 sheets and replace 1. This way, you always have a good platform to work off of. Next open up an area along your wall with a sawsall, so you can see your wall plates and the next floor joist. If it is 6" or less you will be fine. Your 3/4 sub flooring is very strong. If over 6", then just nail a sister stud onto your wall sill and plate for your sub flooring to sit on. Do not disturb your wall plates or sill. Sounds like a good project. Good Luck
Old 05-18-05, 09:23 AM
Thread Starter
Join Date: Apr 2005
Posts: 42
few more questions...

thanks jack...even though i think i have a pretty good idea how to proceed based on your answers, i have a couple more questions.

1. when i cut away the existing subfloor up to the exterior wall that's perpendicular to the floor joists, do i need to bother installing blocking? i was planning on laying sections of 2x8 flat between the joists, but don't know if that's overkill.

2. when i cut away the existing subfloor up to the exterior wall that's parallel to the floor joists, i definitely need to install some support b/c the distance to next joist > 6". should i basically sister a new joist to the rim joist for the entire length, or install sections of 2x8 standing up every 8"-12" between the rim joist and adjacent joist?

3. lastly, on a couple of joists under the bathroom, the prior owner notched about 6" deep in order to run the plumbing. should i sister these joists on both faces, or just one? should the sister joists run the entire span of the existing joist, or just a couple of feet on either side of the notch?

thanks for the help, i really appreciate it

Last edited by econguy; 05-18-05 at 11:02 AM.
Old 05-18-05, 05:06 PM
Join Date: Dec 1999
Location: South Dakota
Posts: 3,140
Replacing sub floor.

I will try to answer your questions simply. I could get carried away, but will not in this case.
Question 1: Yes, I would install blocking.

Question 2: Your idea is one of many that would work. Go ahead and use your method. It may end up being the easiest way.

Question 3: I would sister it on each side for a distance of about 4 feet. Use lots of nails.

It sounds like your project is right on track. If I could see it, I may have some more ideas, but it sounds like you are a thoughtful fellow, and are doing a good job. Good Luck
Old 05-23-05, 07:41 PM
Thread Starter
Join Date: Apr 2005
Posts: 42
what type of subfloor is best?

hi again jack -

even with all the help you've given so far, i still have another question...

the scope of my work encompasses two bedrooms separated by a full bath. i had planned on using 3/4" acx plywood for the entire subfloor, but then recently saw that my local home depot also carried both 3/4" t&g plywood (a single-layer combo of subfloor/underlayment, which home depot says is not exterior-rated) and 3/4" t&g osb (also a single-layer subfloor/underlayment, but home depot says this is moisture-resistant).

which one would you recommend to use, and why? is it necessary for the subfloor to be exterior-rated, or moisture-resistent?

i had planned on putting ceramic tile in the bathroom (after first raising the subfloor to the required height with cement backer board over the subfloor). should this fact, or the general fact that part of the subfloor would be in a bathroom, affect the choice at all?

i'm leaning towards the 3/4" t&g osb, except for the reviews i've read that it may warp at the edges, and that some tile manufacturers recommend against tiling on top of osb subfloor.

again, i appreciate your help. although it probably doesn't seem like it from my repeated posts, the project is moving along - albeit slowly - and i'm pleased with the progress to date.

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