DIY Home Improvement Forum banner

Securing plank subfloor to joists?

27K views 83 replies 10 participants last post by  duffman56 
#1 ·
I have purchased some #10 2" screws to attach our t&g plank subfloor before tiling.

How often do I need to screw the planks into the floor joists? Joists are 16" oc and planks are 3-1/4" wide. So, screwing every other plank would mean a screw every 6-1/2". Is this adequate, overkill, or a good plan?

1/2" ply to go over t&g, attached with #8 1-1/4" screws, then Ditra, then the tile. I've received advice on amount/spacing of the 1-1/4" screws, but not the 2".

Oh yes, I consulted the Ditra handbook, which was helpful regarding the method, but said nothing about how often to screw the t&g.

Thanks to everybody for your help!
 
#54 ·
At this point, I think you need to have a professional come in and look at this. Yes it will cost you some money, but doing this install on inadequate support the entire thing is going to be a disaster.
 
#56 ·
I'm wondering if the screws aren't stripping out. They will continue to turn, but aren't going any deeper. This surprises me, as the t&g planks they're going into are in good shape.

As far as the technique - those are a lot of helpful tips. Based off of what you said I think I'm doing it correctly. When I run into one that doesn't countersink I've just been moving it (usually 2-3 times) until I find a spot it will countersink.

The Ditra Handbook said to space the ply sheets 1/8" for expansion joints. Just curious - won't these fill in with thin set when I go to put in the Ditra? I'm sure it will work, as this is what they suggest, just wondering how the joints will expand if thin set gets into the gap.
 
#58 ·
The Ditra Handbook said to space the ply sheets 1/8" for expansion joints. Just curious - won't these fill in with thin set when I go to put in the Ditra? I'm sure it will work, as this is what they suggest, just wondering how the joints will expand if thin set gets into the gap.
Good question...I'd like to hear the answer to that myself...never thought much about it.
 
#57 · (Edited)
Are you using fine threaded screws...if so, you need coarse threaded screws...other than that...you may occasionally hit a space...like the T&G, but moving the screw over should have taken care of this, as long as you're not moving them along the line of the T&G.

Fine threaded screws don't have enough bite in wood...and can strip out...
 
#61 ·
Ha! I would LOVE to have Holmes come in and redo everything, as long as DIY was paying for it...

Given the fact that I am worried, should I go ahead and sister the joists (at least every other one)? Would it take a special grade of 2x6s to accomplish this? Would this be extremely pricey?

Or would bridging be a better option?
 
#62 ·
duffman56 said:
Ha! I would LOVE to have Holmes come in and redo everything, as long as DIY was paying for it...

Given the fact that I am worried, should I go ahead and sister the joists (at least every other one)? Would it take a special grade of 2x6s to accomplish this? Would this be extremely pricey?

Or would bridging be a better option?
If I was in your position I would sister the joists,
A 2x4 or 2x6 perpendicular between the joists will stiffen up the play you have.
 
#63 ·
Is this "sistering" or "bridging"? I thought sistering was basically attaching another joist alongside the pre-existing joist (parallel, not perpendicular)?

Just took a look and there is a lot of stuff to work around down there (ducts, electrical, gas lines), so installing boards perpendicular may be easier in some places. I was under the impression that attaching boards to the joists provided more strength than perpendicular.
 
#64 · (Edited)
Just looking into bridging - Anyone heard of "IBS Engineered Bridging"? Their site claims it provides significantly more floor stiffness than standard bridging options. Just wondering if there's any truth to their claims.

Looks like it's a product from Canada - it looks like minimum joist height for them may be 7.25" though. Also, I'm not sure but it looks like they may be designed specifically for engineered I-beam joists, and not 2-by's?
 
#66 · (Edited)
JetSwet - everything I've read states that the described method is the least effective in reducing deflection. Please tell me this is wrong, as this would likely be the easiest method to use.

Yet another couple methods I just saw - gluing and screwing a 2x4 to the underside of the joist. The main disadvantage of this is reduced headroom, but since that I-beam is under the joists anyway this wouldn't be a problem.

Also, Ideas 5 at the link below, which I have never seen nor heard of. It's a very interesting idea, but I wonder about effectiveness:
http://www.finehomebuilding.com/PDF/Free/021184090.pdf

I'm starting to wonder if securing the joists (though I don't know how) to the I-beam may be beneficial? Any tips on how to accomplish this? Will try and post a photo or at least detailed description tonight.

Thoughts?
 
#67 · (Edited)
Geee, lots of catching up for me to do with this thread. :wink:

In post # 56, Duffman, you said.

I'm wondering if the screws aren't stripping out. They will continue to turn, but aren't going any deeper. This surprises me, as the t&g planks they're going into are in good shape.
Are you telling us you can't tell if the screw is just spinning? :eek:

ED911 asked;

Are you using fine threaded screws...if so, you need coarse threaded screws..
I hope you're using coarse, that's elementary for soft woods.:thumbsup:

The Ditra Handbook said to space the ply sheets 1/8" for expansion joints. Just curious - won't these fill in with thin set when I go to put in the Ditra? I'm sure it will work, as this is what they suggest, just wondering how the joints will expand if thin set gets into the gap.
Several theories about that. The latex modified mortar will bond the ply together while still compressing a bit. You want a monolithic substrate otherwise you will get movement causing cracks in the tile work. Some people on the other hand will tape over the joint or apply some caulking in it to keep the gap free.

I suggest you follow the direction based on the method you're using. I believe most, if not all bags of thin set tells you fill the space. I have always filled the gap. :thumbsup:

Jet said;

If I was in your position I would sister the joists,
A 2x4 or 2x6 perpendicular between the joists will stiffen up the play you have.
Ahhhhhhhh, What? :no: again........:whistling2:

I meant to say I wouldnt sister the joist I would but "cats" in yes bridge them if you want but thats the x effect of the joists not slanting on you,
Running cats is a straight run from joist to joist not angled.
Please pay no attention to the above on the long shot that anyone understood what was said. :eek:

I see OH Mike's advice in #33 of another thread didn't help. :laughing: http://www.diychatroom.com/f5/enough-subflooring-support-137052/index3/

Jaz
 
#68 · (Edited)
Jaz, really?!?... You just just put all the so far advice for this op and threw it right out the window.
Stop criticizing every one Jaz!..not cool man
Now I'm whistling and laughing..... Lol

Course screws for flooring is not Elementry for a DIYer.

This really shouldn't be a issue, if all planks are screwed in then ply on top staggered right and screwed in correctly there shouldn't be any play.
Not shore what the issue is I think it's still the screw size.
 
#70 ·
Having trouble uploading the second photo. We are having someone come look at the floor, hopefully this week. He installs tile for a living, and is probably the best in town (though our town is quite small). Regardless, the fact that he's still in business hopefully means he can give us some answers, and I hope the answer is :thumbsup:
 
#71 ·
I wish there were a simple way to TEST the deflection... Is there?

Should I hold off on securing 1/2" ply until we've figured out the joist issue? I believe I took the majority of the sag out of them. I just want to keep this project's minimal momentum going...
 
#73 ·
I've used the deflecto-meter to enter my specs, but to me this isn't truly testing my floor. The fact that the deflecto says I'm ok but there is still a noticeable bounce means there must be another variable involved. I meant more a way to truly MEASURE the deflection in my floor.
 
#74 ·
duffman56 said:
I've used the deflecto-meter to enter my specs, but to me this isn't truly testing my floor. The fact that the deflecto says I'm ok but there is still a noticeable bounce means there must be another variable involved. I meant more a way to truly MEASURE the deflection in my floor.
Do you have room in the budget to replace your planks with ply? If I was to guess I would say that the reason you still have so much play or bounce is that your planks are to narrow.
 
#75 ·
Simple way? Screw a board to the middle of the joist and let it hang down towards the floor, attach a marker to something sitting on the basement floor (chair, bucket, etc). The board will move when the joist flexes and the marker will show that distance. It is not going to be much though!
 
#76 ·
JetSwet - I had thought about replacing the t&g with 3/4" ply, but would like to avoid that if at all possible (we spent a lot of time and maybe 100 bucks putting over 1000 screws trough the planks into the joists).

If I HAVE to do this, I will. I'd just like to avoid it if possible. It may get tricky, as there are a lot of cabinets to work around, but hopefully leaving the t&g under them and putting the ply UP TO the cabinets would suffice (again, if I have no other option)?

So based on ceramic tile's requirement of L/360, am I doing this (see below) right?

Joists are a little over 13' long, with foundation support on one end, I-beam on the other and I-beam in-between. Foundation to I-beam is 6'-3" (to edge, not center of I-beam), I-beam to I-beam is I think 6'-10", or 82". 82"/360 = .2278", or just under a quarter inch (closer to 7/32").

So based off of the longest span (6'-10"), the floor is allowed to sag a maximum of 7/32"? What is a realistic weight to expect this to hold up to? I'd read a good test is to stand on your toes and drop to your heels, but I only weigh about 160 lbs. If I perform this test and the joist sage is <7/32" can I assume it meets the L/360 requirement? What about heavy appliances and moving them across the floor? (I'm sure we'll want to replace our fridge before too long).

Also - could this test be done after the plywood is secure, as this will undoubtedly strengthen the floor and provide for less deflection?
 
#78 ·
So in Other words, if the deflection is <7/32" when I do one thing (for example run across the floor) but >7/32" when I do another (e.g, jump as high as I can and land) the tile will hold up to the first, but not the second activity. It sounds like although there is a standard deflection requirement for tile there is not a standard weight-to-achieve-this-deflection requirement?
 
#80 ·
Yeah I had wondered if three-quarter would work better. The thing is I already have all the half inch and I've started to attach it (though over a small area). I got the half inch at Home Depot for 12 bucks a sheet. Any idea how much more three-quarter inch would cost?
 
#81 ·
duffman56 said:
Yeah I had wondered if three-quarter would work better. The thing is I already have all the half inch and I've started to attach it (though over a small area). I got the half inch at Home Depot for 12 bucks a sheet. Any idea how much more three-quarter inch would cost?
Probably 18 bucks or so, I would go with that.
 
#82 ·
Duffman,

Have you confirmed that the movement is because of the joists and not the planks? If the joists are deflecting too much they must be a poor quality in poor condition. 2x6 spanning such a short span should be fine.

Adding the plywood is not gonna improve the joists, just the subfloor. You need to figure out where and how much deflection there is.

When you add the new plywood underlayment make sure you do not buy sheathing. You were just referring to $12 a sheet ply which is the wrong stuff.

Jaz
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top