Underlayment for attic remode - size and screws

Vermont1 · 01-31-2009, 02:03 PM

Please help.
Remodeling an attic space in a Cape Cod home. My joists are 2X8 and there is currently 3/4" tounge & groove as my subfloor. Should I put 1/2", 5/8" or 3/4" plywood as my underlayment? I am not sure of what flooring (hardwood, carpet), but heading towards carpet. Also, what size screw should I use and should I glue besides screwing. Do I need to leave any space for the underlayment for expansion? I am laying the underlayment perp to the floor joists. I take this is correct?

JazMan · 01-31-2009, 06:24 PM

Hi,

I take it the T&G subfloor you have are planks and not plywood? You didn't mention the spacing of the joists nor their unsupported span. How does the floor feel now? It's not as critical to have a solid floor if you go with carpet, but it has to be safe and stiff enough so that the floor doesn't flex too much.

Measure the joists' on center spacing and the unsupported span and report back for advice on underlayment spacing. Generally the more plywood the better.

Plywood goes across the joists as you stated. Always gap the plywood 1/8" between sheets and at least 1/4" at perimeter. No glue, screws only long enough to penetrate through the subfloor not the joists.

Jaz

Vermont1 · 02-01-2009, 08:53 AM

Thanks Jaz.
THe joists are 16'' oc and the 3/4" subfloor are planks. Room size is 30' x 20'. This does not include the knee wall space which is approx 3' on each side. There is a supporting wall on the 1st floor that runs 10' from the knee wall above at one end for approx 12' and there is another supporting wall on the 1st floor in the center of the house that runs the rest of the length. Questions:
-The subfloor is solid and that is why I am thinking 1/2" would suffice, but just don't know?
- I am putting a bathroom up there as well. In the bath I will be putting cement board on top of the 1/2". Does that makes sense? THe floor will be tiled.
-Also, why not screw into the joists and only into the subfloor? Also how far apart should I screw?
-The plywood, should I stagger the seams?
- What type of plywood should I use?
Sorry for so many questions, just want to make sure I do it the correct way.Thanks for all your help.

JazMan · 02-01-2009, 11:50 PM

I'm not sure what your unsupported span is and we don't know the species and grade of the joists. I'm not sure if you can find that info. Generally I would say if your span is about 12'6" or is OK for standard floor covering like carpet. For ceramic tile however, that would be a little risky, likely to result with grout cracks down the road.

Usually the best species for joists is Southern Pine, typically I believe #2 grade is used? If that was what you have that max recommended span for the bath areas is about 11' 5" max. As I said it would be nice to know species and grade.

So your sub is 3/4" planks, go ahead and fasten them well to the joists, 2" floor screws. Install 1/2" underlayment grade plywood, it'll be marked exposure 1 or exterior grade you want C/C or better. B/C is common. Do not buy CDX. Screw ply using 1 1/4- 1 1/2 screws, (no drywall screws), 6" in the field, 4" on edges.

Install 1/4" cement board into fresh thinset mortar and fasten and tape seams per directions for the bath floor.

Jaz

Vermont1 · 02-02-2009, 09:40 AM

Thanks for the great feedback. I will let you know how it turns out. Very much appreciated.

Vermont1 · 02-03-2009, 05:49 PM

Forgot to ask if I should stagger the underlayment?

JazMan · 02-03-2009, 09:43 PM

Yes, always stagger plywood.

You want the joint to make a "T", no where should 4 corners meet.

Jaz

01-31-2009, 02:03 PM	#1
Vermont1 Member Join Date: Jan 2009 Posts: 30	Underlayment for attic remode - size and screws Please help. Remodeling an attic space in a Cape Cod home. My joists are 2X8 and there is currently 3/4" tounge & groove as my subfloor. Should I put 1/2", 5/8" or 3/4" plywood as my underlayment? I am not sure of what flooring (hardwood, carpet), but heading towards carpet. Also, what size screw should I use and should I glue besides screwing. Do I need to leave any space for the underlayment for expansion? I am laying the underlayment perp to the floor joists. I take this is correct?

02-01-2009, 08:53 AM	#3
Vermont1 Member Join Date: Jan 2009 Posts: 30	Underlayment for attic remode - size and screws Thanks Jaz. THe joists are 16'' oc and the 3/4" subfloor are planks. Room size is 30' x 20'. This does not include the knee wall space which is approx 3' on each side. There is a supporting wall on the 1st floor that runs 10' from the knee wall above at one end for approx 12' and there is another supporting wall on the 1st floor in the center of the house that runs the rest of the length. Questions: -The subfloor is solid and that is why I am thinking 1/2" would suffice, but just don't know? - I am putting a bathroom up there as well. In the bath I will be putting cement board on top of the 1/2". Does that makes sense? THe floor will be tiled. -Also, why not screw into the joists and only into the subfloor? Also how far apart should I screw? -The plywood, should I stagger the seams? - What type of plywood should I use? Sorry for so many questions, just want to make sure I do it the correct way.Thanks for all your help. Last edited by Vermont1; 02-01-2009 at 10:51 AM.

02-01-2009, 11:50 PM	#4
JazMan Tileguy Join Date: Feb 2007 Location: Troy, Michigan Posts: 3,287	Underlayment for attic remode - size and screws I'm not sure what your unsupported span is and we don't know the species and grade of the joists. I'm not sure if you can find that info. Generally I would say if your span is about 12'6" or is OK for standard floor covering like carpet. For ceramic tile however, that would be a little risky, likely to result with grout cracks down the road. Usually the best species for joists is Southern Pine, typically I believe #2 grade is used? If that was what you have that max recommended span for the bath areas is about 11' 5" max. As I said it would be nice to know species and grade. So your sub is 3/4" planks, go ahead and fasten them well to the joists, 2" floor screws. Install 1/2" underlayment grade plywood, it'll be marked exposure 1 or exterior grade you want C/C or better. B/C is common. Do not buy CDX. Screw ply using 1 1/4- 1 1/2 screws, (no drywall screws), 6" in the field, 4" on edges. Install 1/4" cement board into fresh thinset mortar and fasten and tape seams per directions for the bath floor. Jaz __________________ Tile 4 You LLC Troy, MI KERDI Shower Specialist ... DITRA Installs ... Product-Method suitability consultation. I have NEVER made a mistake, I thought I did once...........but I was wrong! - PRODUCTS ADVERTISED WITHIN MY POST ARE NOT ENDORSED BY ME!

02-02-2009, 09:40 AM	#5
Vermont1 Member Join Date: Jan 2009 Posts: 30	Underlayment for attic remode - size and screws Thanks for the great feedback. I will let you know how it turns out. Very much appreciated.

02-03-2009, 05:49 PM	#6
Vermont1 Member Join Date: Jan 2009 Posts: 30	Underlayment for attic remode - size and screws Forgot to ask if I should stagger the underlayment?

Underlayment for attic remode - size and screws