I am going to finish part of the attic in my house. When the previous renovation was done we added a 3rd floor as “storage” with an eye towards eventually converting one of the rooms to a bedroom. I have 2 situations to deal with.
#1 – The floor is now 1/2″ cdx that was attached only at the 4 corners so I could easily remove the sheets to access the joist bays. The finish floor will be carpeting. I feel that the subfloor should be 3/4″. If I secure the 1/2″ and add a layer of 1/4″ or 3/8″ over it is that as good as 3/4″ plywood? I would rather not hump 3/4″ sheets up 3 stories and also would not want to hump the 1/2″ back down if I can avoid it. Or is 1/2″ ply good enough for a bedroom subfloor?
Should I glue down the 1/2″ and glue down the top layer or is that overkill?
#2 – Since the building codes will not allow 3rd floor living space (I’m in NY) the prior construction included insulation in the joist spaces between the 2nd floor ceiling and the 3rd floor floor. I will now need to insulate between the roof rafters. The roof was built with a ridge vent and soffit vents. Can I leave the insulation between the heated spaces or should I remove the existing floor insulation? The insulation is fine where it is but I was under the impression that you should not insulate between heated spaces due to vapor considerations. Is that true?
Thanks for the help.
Replies
glue it...
can the building hold the weight???
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No, 1/2 + 1/4 does not equal 3/4. You could rip the new ply into 2 ft x 8 ft pieces, which would make them lighter and easier to maneuver up the stairs. If you insisit on adding a layer, I would use at least another 1/2" ply, probably 5/8".
You should pull up the existing ply and then reset it in a bead of PL or liquid nails, and screw it to the floor joists. It will help to make it stiffer, and eliminate floor squeeks.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
If you glue&screw the second layer at right angles to the first, avoiding coincident joints, you'll have a much stronger assembly than if you use a single layer. Gluing down the first layer will help a lot too. It's important to use a glue that sets vs remaining pliable.
There's no harm in leaving the existing insulation there.
Thanks for the responses. It seems like if I glue down the existing 1/2" and laminate another 1/2" at right angles I should be good.
By sets versus remains pliable I assume you mean it gets hard like glue instead of flexible like caulk. Do you know if the subfloor adhesives all do this or do I need to look for a certain brand or do a test first. I would like to avoid squeaks. Although, since it is my 16 year old daughter moving up there maybe some tell tale squeaks would not be so bad although I doubt Romeo would have a 35ft tall ladder.
Many of the wall/floor adhesives are primarily to prevent squeeks and don't really "set". I don't know which (if any) of the caulking gun type glues that would work well for the joists. Between layers I'd use a carpenter's glue, spread with a finely notched trowel.What you don't want is a glue that remains tarry/rubbery and will flex a lot or tend to creep. (You also don't want something that's glass-like and brittle, of course.)
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use PL Premium.
PL Premium is the best glue chjoice. Glue the first ply down too.I would NOT run the second ply perp to the first. The strength axis should run perp to the rafters and joists. What are the joists and what lyout - what span?????Same Q for rafters.Don't forget that you must have legal egress windows in any bedroom. Makes it easier for Romeo to get in, but if the house burns, she would be trapped in the most vulnerable spot.
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I just checked the layout - the joists are 2x10's 16" centers and they span 13ft.
If I don't put the new plywood perpendicular to the existing what is the best way to stagger the joints? Should I start with a 4ft piece and over lap each side by 2ft so that no joint falls on another joint and work it out that way?
The rafters are 2x10's also on 16" centers and the height of the ridge is 13'. It is a 12in12 pitch roof and the collar ties are now set at 7ft from the rough floor. This was to pass inspection due to the no finished space on the 3rd floor rule. I am going to move the collar ties up to about 9ft (I believe I can go higher if I double them on each side of each rafter) which should give a nice airy feel to the space.
As far as fire safety - an architect friend just did a similar job and he spec'd sprinklers instead of a 2nd egress. He said it was not that expensive and looked better than a fire escape on the outside of the house. I am considering this as well. Any thoughts?
Thanks for the input.
Yes, start with a short piece, but rather than offset 2 ft, try one floor joist or 16". that way the edges always fall on a joist.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
my thought,smoke rise's.if the first floor kitchen catches fire, goes up the stairway to the second all the sprinkler heads in the world on the third floor are not going to get your daughter out of there.you have to come up with a way that worse case senerio she can get out. just a side note i had a smoke detector go off at 3 in the morning.i'm flying up outa bed running through the house,nothing. my 16 yo daughter was in the second floor bedroom with a smoke detector with a light blinking 11' from her bed ,never heard it.i'm still freaked out. moral of story they need all the time they can get to wake up and get out.make a computer room outa the attic,at least your awake then. larryhand me the chainsaw, i need to trim the casing just a hair.
Sounds doable
Any random staggering of the second ply so the joints are offsest 16" works.Sprinklers in the whole house are a fine idea. but if in this room only, you canhave her dying of smoke inhalation and steeaming in the heat 'till parboiled while the house burns out from under her if there is no legal egress window.
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that gets the point across..
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!<!----><!---->
Forget the primal scream, just ROAR!!!
Why NOT run the plywood sheet layers perpendicular to each other? The individual layers inside a single sheet of ply are already perp to eachother (alternating) anyway. Isn't that how plywood gets it's strength?
I'm curious about this as well. I would have thoroughly glued, and screwed down another layer going the opposite direction. And for the same reason. The plys in the plywood are glued in opposing directions as well.I wonder if ply still has a primary shrink swell direction regardless of the way it is glued up ? That would make a difference.If there is a good reason not to go the opposite direction, I need to know. I was actually going to do this at some point on a building I am repairing.
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Because the strength axois in plywood should run perp to the joists. He is trying to add thickness and stability to this floor. The direction he needs more strength and les deflection is perp to the joists. If he runs the other way he will get the equivalent of adding only an eighth to the 1/2 existing.
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Strength axis...Is that because of the length, or the plys ? Or something I haven't thought of ??
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All plywod has a strength axis. I've even seen advantec and/or osb have a label/stamp denoting the direction of the axis.Typical CDX 1/2" has three plies. Two outer run same direction and one inner is perp.
Four ply has two outer directional, with two iner alternating on an angle through middle
Five ply - I don't remeber exactly since its been a long time since I've seen it, but I think it has three running grain with the 8' length and two on the bias.
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I suspected it was because of the plys. (Usually an odd number. But I have seen 4-ply before.) Sounds like that was correct.Thanks Piffin.
A good heart embiggens even the smallest person.
"Why NOT run the plywood sheet layers perpendicular to each other? The individual layers inside a single sheet of ply are already perp to each other (alternating) anyway. Isn't that how plywood gets it's strength?"
Yes, OSB and plywood have directional strength depending on the thickness ie. 3,5,7 ply. I believe that we are all on the same page regarding what way the ply or osb should be laid, at right angle to the first. but the first sheet should be laid prep. to the joist. To me it makes no sense to run the sheets in the same direction of the joists regardless of a good stagger. Perhaps I'm missing something?