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Old 08-30-2010, 04:16 PM   #1
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Basement subfloor questions


I am getting ready for the next step in finishing our basement and have a few questions. All of the walls are framed at this point

1. Does the subfloor go down before or after the drywall is installed?

2. Can I put a high density foam directly on the concrete floor and then cover with t&g plywood? or do I have to strap the floor first?

Any advice is greatly appreciated

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Old 08-30-2010, 07:09 PM   #2
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Basement subfloor questions


Hi Firefighter,

If you're a firefighter you know that water always wins in the end.

Installing wood framing and a subfloor over a concrete slab either on or below grade then covering with floor covering is almost always a bad idea. You should never install a subfloor direct or over sleepers on a slab. Even when the slab appears to be dry, you can be assured it is not. The small amount of moisture will cause problems years later in the best of circumstances and perhaps in a short time in some cases.

Where will the moisture go if trapped under the plywood and floor coverings?

Jaz

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Old 08-31-2010, 07:33 AM   #3
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Basement subfloor questions


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Installing wood framing and a subfloor over a concrete slab either on or below grade then covering with floor covering is almost always a bad idea. You should never install a subfloor direct or over sleepers on a slab. Even when the slab appears to be dry, you can be assured it is not. The small amount of moisture will cause problems years later in the best of circumstances and perhaps in a short time in some cases.

Where will the moisture go if trapped under the plywood and floor coverings?

Jaz
Just out of curiosity.... how would you recommend he install a sub floor floor in his basement ?...... (other than tile)

I dont see an issue with rigid foam board on the floor (completely sealed, taped along all joints and corners) then 5/8" - 3/4" T&G ply on top via tapcons etc etc.

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Old 08-31-2010, 07:52 AM   #4
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Basement subfloor questions


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Originally Posted by FirefighterGW View Post
I am getting ready for the next step in finishing our basement and have a few questions. All of the walls are framed at this point

1. Does the subfloor go down before or after the drywall is installed?

2. Can I put a high density foam directly on the concrete floor and then cover with t&g plywood? or do I have to strap the floor first?

Any advice is greatly appreciated
1. Subfloor on BEFORE you drywall..... (actually I would've had it down before the framing of the walls.....but thats me)

2. As for the straps, Are u trying to level off the floor? or is that the method ur thinking of going with to build the actual subfloor?......

I've seen it tons of times and my honest opinion.... I prefer a 1"- 2" rigid foam glued down (PL Premium etc) tuck tape all joints in between foam boards, corners etc etc than lay the 5/8" - 3/4" T & G board over with Tapcons straight thru into concrete slab. It is best to make sure the floor is as true as can be.

uneven subfloor = uneven finished floor.....I've seen some basement floors dip up and down like waves....JUST HORRIBLE!

***just remember, any strapping, wood etc etc touching concrete should have some type of moisture barrier in between to assist in preventing any water from seeping into wood from concrete. hope you did that to the bottoms of your framed basement walls.***

Last edited by 57_Hemi; 08-31-2010 at 07:56 AM.
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Old 08-31-2010, 08:05 AM   #5
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Basement subfloor questions


Most basement flooring materials should have a subfloor that provides moisture protection. Ceramic and vinyl tile have other requirements, such as surface flatness and strength that are more properly addressed with specialized underlayments like cement backer board.

The subfloor structure takes the form of a grid of 2 by 3’s (called sleepers) attached to the concrete slab and covered with sheets of 5/8’’ plywood. Sleepers are usually separated from the bare concrete by some type of vapor barrier; either thick felt paper or 6-mil plastic sheeting. Here’s how it’s done:

Sketch out the floor area on a sheet of paper. Figure out how many 2’’ Χ 3’’ lumber boards you need for the perimeter sleepers and the length each needs to be. Then, establish how many 2’’ Χ 3’’ lumber boards you need for the lengthwise sleepers; these should be spaced 16’’ center on center. Last, determine the number of 4’ Χ 8’ sheets of plywood you need for the subfloor.

Now check your local building code for requirements on vapor barrier materials. Lay 15 lb. felt paper or 6-mil polyethylene sheeting on the concrete slab. The goal is to prevent any ground moisture infiltrating up to the underneath of the plywood, which can cause mold, fungus, rotting and bad smells, among other things. Allow approximately 4’’ of vapor barrier laying up against each basement wall as well.

When this is done you can begin with the perimeter, laying down 2 in. Χ3 in. pieces flat along the walls, immediately on the vapor barrier. Then place the 2 in. Χ 3 in. sleepers inside this frame. Position the first sleeper 16’’ from the wall and the rest 16’’ apart center to center. These sleepers should be installed parallel to the shortest length of the room.

Now take your level and level the 2’’ x 3’’ sleepers, placing shims underneath as needed to ensure the plywood sheets will sit flush. After all sleepers are leveled, fasten them to your concrete slab with ramset fasteners. Use these to drive 2 ½’’ concrete nails at a spacing of 24’’ apart. Trim off the excess vapor barrier along the walls with a utility knife and pencil mark the centers of each sleeper on the basement wall as guidelines for securing the plywood to the sleepers.

When this is done apply a thick bead of construction adhesive on top of the sleeper’s then lay and fit 5/8’’ thick Tongue and Groove plywood sheets over the sleepers, parallel to the shorter wall. Using as a guide the pencil marks made on the wall, screw the plywood sheets into the sleepers with 1 ½’’ ring shank or screw shank nail or flooring screws every 12 inches from one row to the next, stagger the joints of the plywood sheets.

Creating a wood basement subfloor on a concrete slab is a DIY project that can be done by most homeowners comfortable with a mid-level difficulty project. If you are going to be installing ceramic tiles on the subfloor, be aware that you will additionally need to install ½’’ thick plywood sheets running in the opposite direction over your subfloor, or cement backer board underlayment.
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Old 08-31-2010, 09:09 PM   #6
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Basement subfloor questions


Hemi,

You didn't get my point. A wooden subfloor is NOT recommended and is often a sure failure over a slab. Especially on a slab below grade.

Thru the years people have come up with ways that they believe might work, rigid foam boards being just one. I'm saying it will not work if there is too much moisture under the slab and it is impossible to know at what % is the threshold of too much. BTW, you can NOT completely seal it, plus even if you could, you would NOT want to since any moisture will accumulate below and cause mold. Moisture needs to be allowed to come thru the flooring and evaporate naturally.

You can not tell how much moisture a slab emits just by looking at it and the fact that it may have never flooded is immaterial. In addition things are always changing in a basement.

There is no approved method that uses the method you describe.

Jaz
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Old 08-31-2010, 09:26 PM   #7
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Epson,

Please refer us to the method & specs you just described as being approved by any reputable association of organization that specializes is flooring or even general construction anywhere on earth. Handymen maybe, but not flooring professionals.

You won't be able to do that. It may work for some time in some situations, but it's more likely to fail long term. BTW, sometimes things fail to a degree, but the homeowner may not realize it or complain about it.

HUH? CBU on a slab? And you suggest running the second sheet of ply underlayment parallel to the joists?

Jaz
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Old 09-01-2010, 07:45 AM   #8
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Epson,

Please refer us to the method & specs you just described as being approved by any reputable association of organization that specializes is flooring or even general construction anywhere on earth. Handymen maybe, but not flooring professionals.

You won't be able to do that. It may work for some time in some situations, but it's more likely to fail long term. BTW, sometimes things fail to a degree, but the homeowner may not realize it or complain about it.

HUH? CBU on a slab? And you suggest running the second sheet of ply underlayment parallel to the joists?

Jaz
I have used this method many times and had it inspected by the city with no issues. I just finished this type of set up two weeks ago with the same results.

As per running the second underlayment I said run it in the opposite direction to stager the joints.
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Old 09-01-2010, 09:45 AM   #9
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Basement subfloor questions


I too have seen many home improvement shows on television that show a plywood floor put over sleepers which in turn are placed over a vapor barrier. I think that the sleepers were pressure treated, if I recall correctly.
Now, I do not wish to add fuel to the fire but I have two questions. Many concrete floors expel a certain amount of moisture and there seems to be no dispute over this. Therefore, that leaves us with the question as to what happens to the moisture trapped between the vapor barrier and the concrete floor. The second issue is whether the holes made in the vapor barrier by nails through the sleepers that attach the sleepers to the concrete floor will have any effect on the ability of the vapor barrier to block moisture.
Many years ago I put up furing stips on cinder block walls in order to finish the basement with wall paneling. I used masonary nails made for the job. Little did I know that masonary nails are prone to rusting so that 19 years later the totally rusted nails (not all but plenty) left perfect holes for water to slip through after a heavy rain. I bring this up because if the nails used to atttach the sleeper rust though, the sleepers will not be securely attched and also because with any home project you should always consider whether the project holds up not just tomorrow but over the long haul.
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Old 09-01-2010, 11:43 AM   #10
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The concrete in your basement is the subfloor......You may need to skim coat it for various products....but you need to know the annual humidity variations to know what type of products are suitable... fastening a studded wooden subfloor below grade is not a smart idea....better to work directly over the slab......if you pass certain moisture tests you can glue an engineered floor directly to the slab....if you have flood moisture issues it will be easier to repair than a rotten studded subfloor and damaged flooring material...If you really insist on instaaling like a plywood subfloor...


use pressure treated 2 x 2 and fasten it with masonary screws.


BTW I read Epsons and it sounds good...dummy heat vents and gapping of sleepers will help protect you from moisture building up underneath the plywood and causing delamination
use a good 5/8" plywood... and put in some "Dummy" 4x10 floor vents so that air can travel below the plywood. stagger gaps in your studding so that air travels freely underneath the wood....And buy a good quality dehumidifier to control the moisture percentage in your basement
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Last edited by Aaroncarpet; 09-01-2010 at 12:05 PM. Reason: other thoughts
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Old 09-01-2010, 11:56 AM   #11
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I used masonary nails made for the job. Little did I know that masonary nails are prone to rusting so that 19 years later the totally rusted nails (not all but plenty) left perfect holes for water to slip through after a heavy rain. I bring this up because if the nails used to atttach the sleeper rust though, the sleepers will not be securely attched and also because with any home project you should always consider whether the project holds up not just tomorrow but over the long haul.

Totally true.....thats why I asked............

"How would you recommend he install a sub floor floor in his basement ?"

Wait until these big names & TV shows find out they are teaching people all the wrong methods.....haha

Or better yet maybe they know already and have researched that the methods they use are probably the "best solutions" for these projects..............because everyone knows nothing lasts forever, but by using thier methods Im sure it may last a damn loooong time.
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Old 09-01-2010, 12:11 PM   #12
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I bring this up because if the nails used to atttach the sleeper rust though, the sleepers will not be securely attched and also because with any home project you should always consider whether the project holds up not just tomorrow but over the long haul.
They make dual threaded masonary screws witch you drill a 5/32 carbide tip masonary bit with a hammer drill..In block one should use a lead anchorsystem and not masonary nails....lead anchors work on slab too...but you better have a 300$ drill to cut the holes....
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Old 09-01-2010, 12:29 PM   #13
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Figures #2 and 3: http://www.buildingscience.com/docum...ms?full_view=1

P.t. wood is not waterproofed, unless you buy it specifically added: http://www.greenbuildingadvisor.com/...-building-code

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Old 09-01-2010, 01:13 PM   #14
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Many stores have signs on P.T. wood that says "not to be used indoors" in fact it violates code here.
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Old 09-01-2010, 01:41 PM   #15
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I personally think studding and sub flooring below grade slab is a big mistake when there are plenty of materials that can go directly over the slab....plus we have codes about gnomes out here ....I am not disagreeing with you rusty...if you have to use another wood make sure you prime all the lumber with oil/petroleum base primer...It will protect the wood incase you have more moisture than expected......besides, everything is outdoors, we just have a few walls and doors and a roof to give us the facade of protection. In my opinion, everything is outdooors....

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Last edited by Aaroncarpet; 09-01-2010 at 01:51 PM.
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