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Thread: Subfloor type? Thickness? Drywall?

  1. #1
    DIY Junior Member ceresia's Avatar
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    Talking Subfloor type? Thickness? Drywall?

    Greetings everyone!

    To give everyone an idea of my project and subfloor ideas here is what I have failed at so far:

    I just opened my second floor with a set of stairs which gives me access to my 600 sq ft attic. The original 1x4 planks were still up there on top of 2x8 joists. The floor seemed level and since I have never tackled a new floor I figured I would buy some cheap OSB panels (1/4 inch to be exact) and go crazy, the finished product would be carpet and pad on top of the OSB.

    I installed all the OSB boards and figured I would stop and wait for winter to pass before I start doing more. Well as I continue to check my attic I noticed the lovely cheap OSB boards sort of bowing or bubbling, and the floor is uneven, it seems the 1 by planks didnt like some of the screws. Well, now that I am frustrated at myself for taking the cheap route I have decided to rip up the 1/4 in OSB and the planks and start from joists up. I will be planing the joists to make them all level and my real question is sort of in two parts.

    Part 1, the main space upstairs will have carpet, so from the joists I would assume to use some type of OSB or plywood, followed by a carpet pad and carpet.
    What thickness and what type of boards would you guys suggest?

    Part 2, there is a 12x10 space off to the side of the main room that I will be attempting and most likely failing at making a bathroom, full. It seems I have read more about this aspect than regular carpet aspects, I am thinking 1/2" plywood, with 1/4" cementboard and then my tile. I want to build a tiled shower as well, so cementboarding the entire room is no issue for me.

    Also, the attic being upstairs and right below the roof, I will be putting insulation in both the walls and ceiling, right now its a bare room, but would one suggest greenboard instead of basic drywall? I will most likely be using the greenboard in the entire bathroom regardless since its a bathroom, but not sure if I want to spend the extra $$ on a basic room that will most likely see no moisture.

    Thanks all in advance!
    Last edited by ceresia; 02-02-2012 at 12:51 PM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    While dimmensional wood (your planks) are strong, they move or change shape a lot with the seasonal changes in moisture content. Normally, you need at least a 1/2" sheet of plywood to overcome that movement. It must be made with C or better faces (i.e., no D faces), and be assembled with exposure I or exterior rated glue. First, the planks need to be properly anchored to the joists (screws work best), then, when you install the ply, do not install it (screw it) into the joists, but only into the planks. It works best if you install the ply the same way as you'd normally do, across the joists, but place the end of the sheet about 1/4 the way over the joist bay and do not line up the corners of the sheets. no glue under the ply and against the planks, only the screws to hold them together.

    By code, a bedroom requires TWO means of egress. Typically, that's the 'normal' doorway and a properly sized window. The window needs to be at least the minimum required by code, and you need to be able to get down to the ground as well.

    The floor needs the same minimum ply underneath the tile. Then, assuming you're using cbu, you'd need to bed it in thinset, then tape the seams with the proper mesh tape, then install your tile. The thinset underneath is required to provide 100% support with no voids...voids mean potential movement, and tile hates movement. Often, the floor of an attic is not designed for the load of living space. You should check that first. You may need to add new joists, which will raise it up, and might compromise your overall height. Changing space into living space if it wasn't designed to be, takes a lot of work. Then, in addition to all that, the codes require special wiring accommodations (AFCI), and GFCI (for the bathroom), and smoke detectors that will also trigger the others in the house (and be triggered by them).

    Greenboard is generally no longer approved or dictated for damp areas - some codes have not been updated as the localities can choose when or if or which version they approve for local controls. Plumbing will be an issue, as you can't just drain it down the vent from below...you need to connect to the drain down there, and then tie into the vent for the new stuff. Insulation and vapor control is a whole other topic, and will need some review and engineering.
    Last edited by jadnashua; 02-02-2012 at 05:37 PM.
    Jim DeBruycker
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    How far do the 2x8's span? You may not can build on that......

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    DIY Junior Member ceresia's Avatar
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    Thanks for the replies, the 2x8's are around ~25 ft. The contractor I had help build the new set of stairs said that it would hold and would work great for living space. Luckily the bathroom that I want to install is above the main floor bathroom, and I have access to all of the water lines and drains with the shower, there should be a large enough gap between the joists above to run my copper and 4" drain for the toilet, I will have to run through another wall for the bathtub drain, I will be looking into all of the bathroom after I figure out the flooring in the main room.

    To get a better understanding as well, a lot of the angles are sort of A framed, and quite tall at that, but I have already done my rough 2x4 walls and set everything at 8 FT, the main room where I want the bedroom I have already tied in 2x6x14's for the ceiling and done cross braces.

    I have wired almost everything in the main room and closet, nothing in the bathroom, but I do have all the GFI plugs bought, and extra wiring for the job. I will read up on local codes and be sure I follow them.

    I am headed to Lowes tonight, not to buy, but to see what is offered, measure exact thickness on OSB, and see the grading as you said above.

    I will keep the thread updated.

    Also, even though the joists run ~25ft, there are multiple load bearing walls below to support what I am doing.

  5. #5
    DIY Junior Member ceresia's Avatar
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    I found 23/32 OSB underlay with: Weather Exposure - Exposure 1.

    I hope this is what you are explaining, it is tongue and groove.

    Is this type of OSB decent enough for subflooring, or should I go with plywood, I keep reading pros and cons to each, and it does sound like a thicker OSB would be good for my project.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    Not all OSB is created equal...I've used Advantec, and liked it. There are probably other brands that are decent, but that's the one I have had good luck with. It's nice and flat, holds nails well, but can be a pain to start a screw into (depends on the screw) because it is so hard. Some OSB and plywood warps when you look at it wrong. This stuff is heavy, though. Because you're putting this over planks, the T&G isn't a big issue; it is important when it is the first (or only) subfloor layer. The planks will hold the edges just fine from deflecting, and 1/2 nominal ply would likely cost less. The goal of this layer is to decouple from the planks, not strength, and 1/2" has enough strength to do that over planks.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member mudcat's Avatar
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    You used 1/4" OSB?? That stuff is like tissue paper for a floor. You should always use a minimum of 3/4" on a subfloor on top of joists.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by mudcat View Post
    You used 1/4" OSB?? That stuff is like tissue paper for a floor. You should always use a minimum of 3/4" on a subfloor on top of joists.
    Where do you get 1/4"? 23/32 is the nominal thickness of 3/4" stock. Industry standards call for a minimum of nominal 1/2", but if you can afford the height, thicker isn't a problem.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Where is the home? Is the attic insulated properly?

    What about a vapour barrier?

    Seems to me that too much work is going on before the basics are covered?

    Can your roof dry if you insulate it? Is there cross strapping for ventallation?

    I think you need to put the brakes on this job and hit the re-set button!

    JW


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    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

  10. #10
    In the trades Dana's Avatar
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    Quote Originally Posted by johnfrwhipple View Post
    Where is the home? Is the attic insulated properly?

    What about a vapour barrier?

    Seems to me that too much work is going on before the basics are covered?

    Can your roof dry if you insulate it? Is there cross strapping for ventallation?

    I think you need to put the brakes on this job and hit the re-set button!

    JW

    What he said!

    Start with the roof- vented, unvented? Insulation type? How are you air sealing?

    With at least 1.5" of vent space between the insulation and roof deck and soffit-to-ridge ventilation the importance of interior vapor retarders falls off rapidly in Springfield's down-state climate, but the air-retardency of the insulation itself makes huge difference in actual thermal performance- if batts, only high-density "cathedral ceilng" batt will do, but it may make even more sense to cut'n'cobble 2' of unfaced EPS as the exterior baffle maintaining the 1.5" air gap (sealed at the edges with 1-part foam) to also serve as a semi vapor-permeable exterior air-barrier for the batts. If you can't get at least R25 at the roof deck (with 2x6 rafters you'd only have 3" of space to insulate with, leaving a 1.5" vent space.) you might consider sistering-on extensions truss style for greater depth and using blown insulation. With 6" wide OSB webs every 2' with an interior 2x3s parallel with the original rafter you can hit almost any arbitrary depth, and with the lower thermal bridging of the webs even 7" of any insulation between the vent gap and gypsum would beat the code-min R30 despite being only ~ R25-ish center-cavity. Or you could go unvented with 6-9" of mid-density spray foam, as long as the perm rating of the foam is less than 2 perms, but that's a more expensive proposition.

    Air sealing is more important than vapor retardency, whether vented or unvented. Even one square inch of air leakage moves more moisture to the roof deck in winter than a whole roof's worth of vapor diffusion through standard latex paint on the interior finish surface. Whatever you do, DON'T put any recessed lighting cans in the ceilng- it's a thermal and air leak point guaranteed to cause you ice damming issues when it snows, and higher moisture levels in the roof deck whether it snows or not.

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