Hardwood over conrete questions from newbie


Old 01-15-09, 07:34 AM
Thread Starter
Join Date: Jan 2009
Location: us
Posts: 6
Hardwood over conrete questions from newbie

I want to do hardwood floors in my family and living room where currently carpet is layed over concrete slab.
Here is the basic plan I have so far.
1. Cleaning and leveling concrete slab and checking for moisture.
2. Putting moisture barrier
3. Nailing 3/4" 4x8 OSB 45 degrees to future hardwood strips
4. Leyer of construction paper
5. Nailing hardwood strips with 1-1/4 staples
N1: No questions here. I need no leveling and moisture 2x2 poly sheet test produced excellent results.
N2: I have no clue what to do. BORG folks advise 6mil film, but as I understand it will become useless if some nails from hardwood strips nailing will penetrate it. Good idea I found online in those official recomendations is to use couple layers of asphalt mastic with asphalt felt or construction paper. It is supposed to seal any holes that stamples/nails will produce. But I cannot find anything like "asphalt mastic" in my local BORG. They have some expensive vapor barrier paint for concrete but it hardens so there will be no auto-sealing. Can I use something from roofing dept? Or maybe dryway sealer??
N3: Do I really need this "45 degree" thingy? Would make is so much easier to just lay full 4x8 sheets without cutting them so much. Also what is the best way to nail them? Shoud I just use powerdrill with screws for concrete or some fun Remington gadgets?
N4: Do I really need this? I already have some moisture barrier why is it no sufficient?
N5: 1-1/4 vs 1-1/2 staples? (subfloor will be 3/4 and hardwood strips are 3/4 too). Shoult it completely penetrate OSB subfloor? I heard some opinions that full penetration is not as strong.

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Old 01-15-09, 08:37 AM
Join Date: Aug 2000
Location: USA
Posts: 15,834
Go to www.nofma.org for the National Oak Flooring Manufacturer Association's Technical manual on wood floor installation. Click Publications. Click How To Manuals. Click Installing Hardwood Floors. On the right click PDF to download for free the technical manual.

As you can see, a 15# roofing felt is recommended. This is available at hardware stores and home centers. It is installed with cut back asphalt mastic which is also in the roofing department. Some folks use the 6 mil polyethylene plastic. Both are described in the instructions.

Install plywood, preferred, or OSB with power-actuated concrete nailer or hammer driven concrete nails. Start at center of panel and work toward edges, making sure panels are flat. Use nine or more nails per panel. Countersink any protruding nail heads.

Installing the OSB or plywood on the diagonal helps prevent gaps in flooring associated with movement of the seams of the underlayment boards.

Then on top of the wood subfloor another moisture retarder is installed. This is an extra precaution against moisture emissions from concrete and also helps with squeaky boards.

Use 1 1/2" fasteners with plywood subfloor on concrete. Space about every 8 inches and nail no closer to ends of boards than 2-3".

Don't forget that concrete should be clean, flat, and dry. Flooring should acclimate in room where it is to be installed.

A moisture test should be taken of wood subfloor and flooring to determine if flooring has acclimated enough. No greater than 4% difference is installing strip flooring. No greater than 2% difference if installing plank 4" or wider.

Dont' forget to leave the expansion gap around the perimeter of the room. Wood expands and contracts as temperature and humidity fluctuate. Monitor humidity with hygrometer (found where they sell thermometers). Maintain 35-55% year round and temp about 70. If too dry, humidify. If too humid, dehumidify.
Old 01-15-09, 10:36 AM
Thread Starter
Join Date: Jan 2009
Location: us
Posts: 6
Thank you twelvepole!

Just finished this NOHMA howto. They also have as alternative method glueing subfloor with mastic. This sounds easier for me as I have no experience in nailing anything to concrete and it requires using smaller pieces of plywood like 4x4 or 1x8 which seems to be more managable.

This HowTo also states "Donít use tongue and groove boards". My local HD has only this kind of boards at this moment and I am wondering why canot I use it as long as I leave 1/4-1/2" of space between sheets?

Also when I was looking for this magic "asphalt mastic" I also went to roofing department and failed to identify anything close to that. Can you please point me on products that would work for me? Orange linky and Blue linky

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