Attic Flooring

DnetMHZ

Diamond Member
Apr 10, 2001
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This weekend I want to install a plywood floor in our attic. My only question is, what is my best bet as far as thickness?

If my joists are 16" on center should 3/8" be fine for walking on and for light storage, or should I go with 1/2"?


thanks
DnetMHZ
 

Kelemvor

Lifer
May 23, 2002
16,930
7
81
If you will be walking on it at all, don't go with 1/2". Definitely go with at least 5/8 if not 3/4. We just did this last summer. Just for storing stuff thinner is fine but if you have to get up there to walk around to put stuff in place, I wouldn't trust it.

I had mine cut in half at the store so it was 2' x 8' so I coudl get it up the openig and then just laid it down and screwed it into place.
 

KMc

Golden Member
Jan 26, 2007
1,153
0
76
Uhhh, what kind of insulation do you have up there and how thick is it? Keep in mind, if you end up compressing the insulation down into a smaller space, the R-value will be reduced significantly.
 

MichaelD

Lifer
Jan 16, 2001
31,529
3
76
The cost of doing the floor correctly is nothing compared w/the cost of you falling thru the floor...and the ceiling below the floor and breaking your leg, hip or back.

Of course, that would only happen if you are carrying a 75lb. box of glass Christmas ornaments, and guaranteed that you fall backwards, you land on your ass and the box hits you square in the chest.

Yes; 1" tongue and groove is way expensive, overkill and practically indestructable. You'll be FINE with 3/4" plywood and LOTS of screws. The attic floor doesn't have to be flat or even (unless you're making living space up there). So, get the semi-cheapest 3/4" plywood Home Depot has. You'll have to pick and choose the sheets (bring gloves with you!) to ensure the ones you get aren't totally wavy like a roller coaster track. But in the end you'll be fine.

Can you FIT full 4x8' sheets up into your attic? You might want to measure the opening that you'll use to get them up there. Let HD cut them w/the giant panel saw accordingly. Will save you much grief.
 

waffleironhead

Diamond Member
Aug 10, 2005
6,764
278
136
3/8" isnt rated for flooring applications iirc. But a good way to know would be to look at the plywood sheet and look at the span rating. for instance the 3/8" will say 16/0 or the 1/2 will say 24/16 where the top number is the rated span for roofing application and the bottom number is for flooring application. Those numbers I wrote are most likely not accurate, but you get the idea.
just pick up cdx or osb either will work fine for your application and usually have the same span ratings. just get wichever is cheaper.
 

Lizardman

Golden Member
Jul 23, 2001
1,990
0
0
Originally posted by: eldorado99
go with 5/8"
This thickness has my vote. Put a nail in every corner and it will be more than fine. Go for 1" thick if you plan to do jumping jacks up there.
 

Superself

Senior member
Jun 7, 2001
688
0
76
Originally posted by: KMc
Uhhh, what kind of insulation do you have up there and how thick is it? Keep in mind, if you end up compressing the insulation down into a smaller space, the R-value will be reduced significantly.
How else do you deal with this? I'm doing the same kind of renovation to my attic this spring.

tks
 

DnetMHZ

Diamond Member
Apr 10, 2001
9,827
1
81
Originally posted by: MichaelD
The cost of doing the floor correctly is nothing compared w/the cost of you falling thru the floor...and the ceiling below the floor and breaking your leg, hip or back.

Of course, that would only happen if you are carrying a 75lb. box of glass Christmas ornaments, and guaranteed that you fall backwards, you land on your ass and the box hits you square in the chest.

Yes; 1" tongue and groove is way expensive, overkill and practically indestructable. You'll be FINE with 3/4" plywood and LOTS of screws. The attic floor doesn't have to be flat or even (unless you're making living space up there). So, get the semi-cheapest 3/4" plywood Home Depot has. You'll have to pick and choose the sheets (bring gloves with you!) to ensure the ones you get aren't totally wavy like a roller coaster track. But in the end you'll be fine.

Can you FIT full 4x8' sheets up into your attic? You might want to measure the opening that you'll use to get them up there. Let HD cut them w/the giant panel saw accordingly. Will save you much grief.
I think I will go with 3/4" and even if I could fit full sheets up there I was planning on having them cut anyway just for ease, since I will be doing this by my self.
 

waffleironhead

Diamond Member
Aug 10, 2005
6,764
278
136
Originally posted by: Superself
Originally posted by: KMc
Uhhh, what kind of insulation do you have up there and how thick is it? Keep in mind, if you end up compressing the insulation down into a smaller space, the R-value will be reduced significantly.
How else do you deal with this? I'm doing the same kind of renovation to my attic this spring.

tks
If you have fiberglass insulation, you are better off removing the excess insulation than stuffing it all below the plywood floor. insulation loses it r-value when compressed.
I LOL at people who buy r-19 insulation and try to stuff it into a 2x4 stud wall. they are worse off than having just r-11.

 

MichaelD

Lifer
Jan 16, 2001
31,529
3
76
Originally posted by: DnetMHZ
Originally posted by: MichaelD
The cost of doing the floor correctly is nothing compared w/the cost of you falling thru the floor...and the ceiling below the floor and breaking your leg, hip or back.

Of course, that would only happen if you are carrying a 75lb. box of glass Christmas ornaments, and guaranteed that you fall backwards, you land on your ass and the box hits you square in the chest.

Yes; 1" tongue and groove is way expensive, overkill and practically indestructable. You'll be FINE with 3/4" plywood and LOTS of screws. The attic floor doesn't have to be flat or even (unless you're making living space up there). So, get the semi-cheapest 3/4" plywood Home Depot has. You'll have to pick and choose the sheets (bring gloves with you!) to ensure the ones you get aren't totally wavy like a roller coaster track. But in the end you'll be fine.

Can you FIT full 4x8' sheets up into your attic? You might want to measure the opening that you'll use to get them up there. Let HD cut them w/the giant panel saw accordingly. Will save you much grief.
I think I will go with 3/4" and even if I could fit full sheets up there I was planning on having them cut anyway just for ease, since I will be doing this by my self.
:thumbsup: 3/4" plywood will be more than enough, especially with 16" joist centers. Better too much than not enough. Use plenty of screws and you'll be able to jump up and down on it w/o a problem.

 

KungFused

Member
Mar 9, 2007
85
0
0
If Norm Abrams is to be trusted, plywood is stable enough that expansion/contraction isn't an issue.
 

alkemyst

No Lifer
Feb 13, 2001
83,967
18
81
Originally posted by: KungFused
If Norm Abrams is to be trusted, plywood is stable enough that expansion/contraction isn't an issue.
This is one of the properties to why laminated wood came to be.
 

AlienCraft

Lifer
Nov 23, 2002
10,539
0
0
Originally posted by: KungFused
If Norm Abrams is to be trusted, plywood is stable enough that expansion/contraction isn't an issue.
If you don't get exterior grade, it might be, depending on your location. It gets hot and depending on locale, humid up there. I use the old roofers trick, a nail in each corner, between the sheet and whatever as a shim, then pull it out after installing the adjacent sheet. bada bing, a nice gap along the whole length.
 

AlienCraft

Lifer
Nov 23, 2002
10,539
0
0
Originally posted by: Superself
Originally posted by: KMc
Uhhh, what kind of insulation do you have up there and how thick is it? Keep in mind, if you end up compressing the insulation down into a smaller space, the R-value will be reduced significantly.
How else do you deal with this? I'm doing the same kind of renovation to my attic this spring.

tks
Depends on how thorough you want to get, but I like the new spray foams, gets into corners out along the edges like nothing else, fills small voids. The high density foams along with a vapor barrier, beat fibreglass because of that "fill it up" characteristic.

 

Greenman

Lifer
Oct 15, 1999
19,056
3,964
136
The stupid question that hasn't been asked yet is, what size are the joist and how big is the span? You should know that before you put any plywood on them.

If the ceiling system will support the load, 1/2" plywood or OSB will be adequate, as that assembly is good for over 20 pounds per square foot.
 

AlienCraft

Lifer
Nov 23, 2002
10,539
0
0
Originally posted by: Superself
Originally posted by: KMc
Uhhh, what kind of insulation do you have up there and how thick is it? Keep in mind, if you end up compressing the insulation down into a smaller space, the R-value will be reduced significantly.
How else do you deal with this? I'm doing the same kind of renovation to my attic this spring.

tks
All you need to know.

 

Safeway

Lifer
Jun 22, 2004
12,081
9
81
Umm, shouldn't you be worried about the extra weight? If my Architectural Engineering B.S. taught me anything, it is that span + load = deflection.

Chances are, your ceiling joists were not designed to handle the extra loads from your plywood and miscellaneous stored goods. I really don't want your gypsum board (sheet rock) to crack.

SafeEngineer
 

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