DIY Home Improvement Forum banner

1 - 16 of 16 Posts

·
Registered
Joined
·
103 Posts
Discussion Starter · #1 ·
OK I’ve been bouncing around this forum about my floor, figured it’s time to get my questions all in one place, here they are. Sorry if this is too long to read but there are lots of details I am getting hung up on.

1970’s house, one level over a post and beam floor supporting car deck subfloor. All of this is over a 4-5 foot deep crawl space that is well vented and has new black plastic over the soil. The beams are 4x8 and have plenty of posts. I leveled, replaced a few posts, shimmed some, added some new footings and posts etc. The beams are spaced 4 feet apart and the car deck is 1 1/2 inches thick, T&g 2x6. Screws it into the beams and sanded all the leveling compound off it. I want to put down hardwood floors, probably engineered but maybe solid. I’ve got about 1200SF to do, plus two small bathrooms which will be tile. Most of the hardwood has to be parallel with the car decking.

1. Planning to stiffen and smooth the subfloor with a 1/2” underlayment. CDX plywood good for this? The local lumberyard suggests 1/2” SCUF but I have not figured out what that is.

The beams run N-S and the car deck runs E-W. What direction to lay the 4x8 sheets, E-W long way across the beams? Can the hardwood be laid the same E-W direction as the car decking with this 1/2 inch underlay laid in the same direction?

Acclimiting the plywood and flooring...don’t really know what to do. I’ve got the heat turned off since the house is stripped down to the studs, no insulation yet. Is it OK to put the plywood down before I get the heat turned on? This is in Seattle area, it’s wet outside.

I am going to insulate to R30 under the floors. Should a vapor barrier go on top of the underlayment?
 

·
Naildriver
Joined
·
12,853 Posts
One question and one request. You refer to this as a "car deck". Do you intend on using it to support a vehicle, or is it designed to be a living space? Can you post pictures of what you have so we can see what you see. It always helps.
 

·
Registered
Joined
·
103 Posts
Discussion Starter · #3 ·
One question and one request. You refer to this as a "car deck". Do you intend on using it to support a vehicle, or is it designed to be a living space? Can you post pictures of what you have so we can see what you see. It always helps.
The car decking is the old subfloor. It is Doug fir 2x6 milled to tongue and groove. It is in good shape, pic below. This will be living space if I ever finish renovating this house.
633455
 

·
Registered
Joined
·
2,836 Posts
I have used car siding before, good stuff, but only 1” thick. Acclimating: i dont think you need to do anything with the plywood, it’s essentially fully dried and the cross laminations pretty much make it stable.
 

·
Registered
Joined
·
103 Posts
Discussion Starter · #5 ·
I have used car siding before, good stuff, but only 1” thick. Acclimating: i dont think you need to do anything with the plywood, it’s essentially fully dried and the cross laminations pretty much make it stable.
I am also thinking about 3/4 Advantech...any idea if it will be stiffer than the 1/2 plywood? Seems like it will be
 

·
Registered
Joined
·
3,856 Posts
I am also thinking about 3/4 Advantech...any idea if it will be stiffer than the 1/2 plywood? Seems like it will be
Leaving aside the minor differences due to it being a different material, 3/4" thick panels will be about twice as strong, and more than 3 times as stiff, as 1/2" panels. Strength of panel is calculated using the square (power or exponent of 2) of the thickness. Stiffness is the cube (3rd power) of the thickness.

Unfortunately, the same applies with regard to the 2x6 subflooring and the 3/4" panels - adding the 3/4" Advantech or whatever only increases the stiffness of the subfloor system by about 12%. If you want the floor to be substantially stiffer, you'd need at least 1 1/8" plywood (40% stiffness increase) or another layer of 2x lumber (100% increase). Adding a 1/2" would not even be a noticeable change.

The other option, which I would recommend, is decreasing the span of the 2x6 carsiding subfloor, by framing in joists between the beams. This would consist of 2x8 cross beams spanning the 4' between posts (assuming they're not already there), and framing 2x8 joists into the cross beams, running parallel halfway between the 4x8 beams. Cost-wise it's about the same as 3/4" OSB, overall a few hundred dollars cheaper than the Advantech for 1200 sq. ft, but far more effective at stiffening up the floor.
 

·
Registered
Joined
·
103 Posts
Discussion Starter · #8 ·
That all makes sense. My current plan is to do what I can atop the cardecking. I'm doing everything solo (pandemic) and don't want to kid myself that I am going to muscle too thick of a sheet around so I'm leaning towards the SCUF 1/2 inch underlay or maybe 3/4. If there is a problem after that, I can go underneath and add joists. I might do it a little different by adding some footings and posts in order to support some new beams with the reconfigured joist arrangement, but that is all down the road. Probably would do spot upgrades under main walkways and open rooms and leave some areas as is like under closets and under small rooms.

Still trying to decide if the subfloor bounces too much or am I being overly sensitive to it? Here is how I checked: first I levelled the floor underneath, got all the posts and beams up to snuff. Then I set up my laser level (floating laser) on its tripod in a few spots, walked around normally on the 2x6 subfloor and saw the laser jiggling. If I jumped I could make it jiggle a lot. My gut feel tells me that might happen on a modern subfloor with 16" joists and good plywood subfloor. My gut feel also tells me a good 1/2 inch underlayment and a good nail down wood floor, 5/8 thick for engineered wood or 3/4 for solid wood, will stiffen it up.

But the big question is: will the wood floor be stiff enough as kitchen, dining room, living room living space? I'm not always wearing my slippers when I stomp around after dinner.
 

·
Registered
Joined
·
3,856 Posts
The only way you'll get any significant gain in stiffness with adding 1/2" or 3/4" panels is by making it composite - connecting the layers together so that they act as single thickness. I think you'd need a more rigid connection than you'd get using screws, or even construction adhesive; I think you'd need wood glue, the PVA stuff (regular Elmers yellow wood glue). Getting it glued good and solid along over the beams and along the midspan between the beams is the most important. If you can accomplish some decent composite action, you could double the stiffness of the system by adding 3/4" plywood or OSB (not sure whether you can get good bond with the coated panels like Advantech, etc.)
 

·
Registered
Joined
·
103 Posts
Discussion Starter · #10 ·
The only way you'll get any significant gain in stiffness with adding 1/2" or 3/4" panels is by making it composite - connecting the layers together so that they act as single thickness. I think you'd need a more rigid connection than you'd get using screws, or even construction adhesive; I think you'd need wood glue, the PVA stuff (regular Elmers yellow wood glue). Getting it glued good and solid along over the beams and along the midspan between the beams is the most important. If you can accomplish some decent composite action, you could double the stiffness of the system by adding 3/4" plywood or OSB (not sure whether you can get good bond with the coated panels like Advantech, etc.)
I did a floor bounce test with my laser level in my house I live in (1984 vintage with conventional joists, not sure of subfloor thickness, plus 3/4 oak floor) and it seems about the same as the car deck. So not quite as worried about it after seeing that.

The plan now is 1/2” solid core plywood underlayment stapled to car deck subfloor with felt vapor barrier and 3/4 oak hardwood floor. If the floor bounces too much I can get underneath and retrofit one of the methods of adding joists midway between the beams, access is pretty decent underneath.
 

·
Registered
Joined
·
3,856 Posts
I did a floor bounce test with my laser level in my house I live in (1984 vintage with conventional joists, not sure of subfloor thickness, plus 3/4 oak floor) and it seems about the same as the car deck. So not quite as worried about it after seeing that.

The plan now is 1/2” solid core plywood underlayment stapled to car deck subfloor with felt vapor barrier and 3/4 oak hardwood floor. If the floor bounces too much I can get underneath and retrofit one of the methods of adding joists midway between the beams, access is pretty decent underneath.
Sounds like a plan. I just wanted you to realize that just adding 1/2" or even 3/4" panels wouldn't change the stiffness much.
 

·
Registered
Joined
·
2 Posts
Hi @Zipperje - sorry, just stumbling on your post. How’d it go? I’m also in Seattle with car decking sub floor, considering hardwoods. Appreciate any updates or info you can share!
 

·
Registered
Joined
·
103 Posts
Discussion Starter · #13 ·
Hi @Zipperje - sorry, just stumbling on your post. How’d it go? I’m also in Seattle with car decking sub floor, considering hardwoods. Appreciate any updates or info you can share!
Loaded question...it went about as well as expected considering what I started with! There were many problems with my subfloor including posts that had rotted on the bottom, foundations had settled, missing posts in a few places, etc. Top that off with a few areas where the original nails had missed the beams. It was a mess.

The 1/2 inch SCF plywood I used started at $55/sheet and peaked at about $100/ sheet by the time I finished so it would be smart to look at alternatives.
I am satisfied with the floor stiffness throughout the portions of the house where I leveled the floor beams and replaced or re shimmed the posts. There is a hallway where I have some squeaks but that area is still on the old posts as is, I am hoping I can stiffen it up underneath.

Have not put the hardwood floors down yet but I am now thinking prefinished engineered hardwood rather than 3/4 oak. I had a quote of $18/sf for 3/4 oak with finish.
 

·
Registered
Joined
·
103 Posts
Discussion Starter · #14 ·
Hi @Zipperje - sorry, just stumbling on your post. How’d it go? I’m also in Seattle with car decking sub floor, considering hardwoods. Appreciate any updates or info you can share!
Loaded question...it went about as well as expected considering what I started with! There were many problems with my subfloor including posts that had rotted on the bottom, foundations had settled, missing posts in a few places, etc. Top that off with a few areas where the original nails had missed the beams. It was a mess.

The 1/2 inch SCF plywood I used started at $55/sheet and peaked at about $100/ sheet by the time I finished so it would be smart to look at alternatives.
I am satisfied with the floor stiffness throughout the portions of the house where I leveled the floor beams and replaced or re shimmed the posts. There is a hallway where I have some squeaks but that area is still on the old posts as is, I am hoping I can stiffen it up underneath.

Have not put the hardwood floors down yet but I am now thinking prefinished engineered hardwood rather than 3/4 oak. I had a quote of $18/sf for 3/4 oak with finish.
 

·
Registered
Joined
·
2 Posts
Whoa, thanks for the crazy fast reply! Sounds like a big project - nice work! Any alternatives to plywood you’d recommend? And $18 per square ft was for material + labor?
 

·
Registered
Joined
·
103 Posts
Discussion Starter · #16 ·
Whoa, thanks for the crazy fast reply! Sounds like a big project - nice work! Any alternatives to plywood you’d recommend? And $18 per square ft was for material + labor?
Whoa, thanks for the crazy fast reply! Sounds like a big project - nice work! Any alternatives to plywood you’d recommend? And $18 per square ft was for material + labor?
yep,,
Whoa, thanks for the crazy fast reply! Sounds like a big project - nice work! Any alternatives to plywood you’d recommend? And $18 per square ft was for material + labor?
yep, installed and finished
 
1 - 16 of 16 Posts
Top