What exactly is cold cut asphalt mastic?
What does it do for the install ?
Does it level as well as provide a vapor retarder?
Can you glue to it?
Can you poke concrete fastiners through it or will it crumble?
Whats the scoop?

1. its sticky black tar.
2.used in conjuction of asphalt felt and or polyethelene to provide a moisture barrier
3.not really and yes
4.no way
5.no need to
6.what you trying to do?

One of my installers wants to use cold cut asphalt mastic on top of a slab- then put 6 mm poly on top- and then put ply on top as a new subfloor for my solid.

He has to attach the ply to the concrete. So there will be nails/fasteners going through the ply, the poly and the mastic into the slab. I really wanted to make sure it would not get all crumbly under foot.

You guys recommended glueing down the ply but I don't think one can glue ply to a plastic sheet or can they as it is firmly imbeded into the mastic?? hmm

I never understand the thinking of all that work to provide a seamless waterproofing membrane, and then you go and poke 32 holes per sheet of plywood, through it.

I will do a floating subfloor, but will get a waiver if they want the ply fastened down, that I will not be held responsible for concrete moisture issues later.

[quote="Floorguy"]I never understand the thinking of all that work to provide a seamless waterproofing membrane, and then you go and poke 32 holes per sheet of plywood, through it.

yes I see you write this often and I get your point-However the hwfa and nofwa do it this way so I think it is safe.

I was simply inquiring what asphalt was and if it would crumble and if it could be glued to if it was placed on top of another retarder say felt.

When we poke holes in the 6-mil poly we then fill those holes with the fasteners. The poly stays tight around the fastener and the mastic also helps to seal around the fastener.

Here is a crude example I made of a flooring cleat, a split pin fastener and a staple. The water was there for two days and not a drop made it through. The mastic on the slab helps even more to keep out the moisture.

Doesn't the NWFA also state what the moisture vapor emissions have to be tested and read below a certain level... Pretty much the same level, as it would be OK, to go directly over the concrete with the wood, without all that mess. Now think about that!!!




Buddy, there is a big difference in liquid and vapors.

Also the fact that metal fasteners can actually wick moisture. and form condensation.

Yep I have had 3 calcium clorides in the room which all read about 2. But we all know slabs can change year round though so he is being protective-maybe we can blow off the Mastic.

That was another reason I was inquiring. There are so many ways do do things and I can never get agreement.

Thanks so much for your photo!! I asked allready about the holes as I wanted to know their method.

This is from NOFMA's Hardwood Installation Manual:

"Polyethylene method. When slabs are well above
grade and the expected annual rainfall is light to moderate,
cover the entire slab with 4- to 6-mil polyethylene
film, overlapping edges 4-6" and allowing enough
to extend under the baseboard on all sides.
Where moisture conditions are considered more severe,
prime and apply* cold-type cut-back asphalt mastic with
a straight-edge or fine tooth trowel over the entire slab
surface (100 sq. ft. per gal.). Allow to dry about 1 hour.
Lay the 4-6 mil polyethylene film over the slab, covering
the entire area and lapping edges 4-6". “Walk in” or roll
in the film, stepping on every square inch of the floor to
insure proper adhesion. Small bubbles are of no concern,
and may be punctured to allow captive air to escape."

"PLYWOOD-ON-SLAB SYSTEM.
This system uses 3⁄4" or thicker sheathing grade exterior
plywood as the subfloor over the appropriate vapor retarder.
Loose lay 3⁄4" plywood panels over entire floor. Laying plywood
on a diagonal to the direction of the finished floor will
help prevent cracks associated with panel edges.
Stagger plywood and joints every 4' by cutting the first
sheet of every other run in half. Leave 3⁄4" space at all wall
lines and 1⁄4" to 1⁄2" between panels. Cut plywood to fit within
1⁄8" near and around door jambs and other obstructions
where finish trim will not be used.
Fasten the plywood with a powder-actuated concrete
nailer or hammer-driven concrete nails. To be sure to
flatten out the plywood, start at the center of the panel
and work toward the edges. Use at least nine nails per
panel or more to fasten securely."

http://www.nofma.org/Portals/0/Publicat ... Floors.pdf

I honestly do not know how much, if any, moisture (vapor or liquid) a steel fastener can transmit. I will say that when I have attached plywood to slabs with masonry nails and glue (along with a vapor barrier), the plywood moisture content tested as acceptable (10% MC) and I have never had moisture problems with those floors. Maybe I was lucky or maybe Buddy is right that the moisture barrier seals around the fasteners well enough to prevent any moisture transmission. In NOFMA's manual, they don't seem to say one cannot drive fasteners through the moisture barrier membrane or that if one does, it would cause a problem. I would agree a floating subfloor would be safer and less likely to be effected by moisture problems from concrete slab emissions.

yes I know that. Read that 20 times at least. I am very anal )

But it still never explains what mastic is which is why I wrote this.

Thanks all for responding. I am trying not to insult my floor guy as I know his way is acceptable but I am curious as to why he is doing things this way.

I prefer sealer and glue and ply rather than plastic and mastic and fasteners.

I have seen 20 floor peeps each with a diff way.