01:00PM | 08/20/03
Member Since: 08/19/03
2 lifetime posts
Hi everyone, newbie here, both to board and diy stuff...

I am installing 3/4" solid prefinished maple through my townhouse and the first floor is a concrete slab. The slab is exactly 30 years old this year.

So, last night we used a Hilti low-velocity gun (rented from the depot) to shoot the plywood down into the slab. On the highest velocity with the strongest powder shots (red "5"), most of them went in completely(I'd say around 85%). So, we've got around 20 or so that are sticking up anywhere from like 1/16" to 1/4". (with one board being particularly crabby, I think it's a certain area of the slab that is harder than the rest)

So, my question(s)

#1: how to get out the ones that are sticking up (I have been told grind them off, but grind with what... a grinding wheel attached to drill? or some other tool? any idea how much effort this would take?)

#2: Could we try to hammer what is left of the plywood fastener into the slab? My instinct says no, but I figured I would ask.

#3: If it turns out that we can't get enough fasteners into this one cranky board for it to hold appropriately, should we try to drill with a masonry bit and then screw the board down into the slab?

Thanks for reading and any information you can provide! Any help is greatly appreciated )


07:21PM | 08/22/03
Member Since: 01/26/03
542 lifetime posts
i was hoping one of our hardwood pros would answer this,but here go's.i would hammer down the nails in to the plywood,(add more as needed) normal would be one fastner every six inchs in every direction....good luck

KD Fisher

06:11AM | 08/25/03
Member Since: 03/17/03
49 lifetime posts
1- Just pound them into place with a 3# mini sledge or similiar.
2- Very hard to do. Try "tapcons". Tapcons are concrete screws that need to be pre-drilled first.
3- Refer to #2

More on installing plywood to concrete slabs for hardwood floors....

Solid 3/4" Hardwood On Concrete


08:58PM | 08/25/03
Member Since: 07/24/03
80 lifetime posts
One of the things I first noticed about your post is that you neglected to say the thickness of the plywood you used. This could be a factor when you start nailing to the plywood if the cleats hit the concrete. They do make shorter cleats but you still need at least an inch of plywood to nail to.

If you are using 3/4" plywood or less I can tell you form personal experience that you will hit the concrete. If you are using 3/4" plywood you could add a layer of 1/4 inch plywood over the 3/4" plywood and cut out for the high fasteners.

Most gym floors are exactly what you are trying to do...3/4" or more often 33/32" nail down maple over a concrete slab. Usually the plywood is not fastened to the concrete at all. Often it is two layers of 1/2 inch plywood stapled together with the first layer laid parallel to the walls and the top layer laid at 45 degrees to the wals so no seams match up.

Pounding the fasteners could work but more often than not you shatter the concrete under the plywood when you do this leaving a floor with a gritty sound of the loose concrete under the floor when you walk on it.



08:24AM | 08/26/03
Member Since: 08/19/03
2 lifetime posts
Thanks for the replies - but they were a little late ) It's ok, we figured it out.

WoodFloorist guy, I put the thickness of the plywood in my subject line of the post - so, yeah, 3/4". That is basically the least thickness you can get away with if you're using 3/4" solid plank.

Anyway, so turns out, that the gun was a little off, so when we rented another gun from HD to just shoot a few more in the bad board, it was shooting it down without any trouble. I ground off the ones that were sticking up with an angle grinder (which was very cool - I had alot of fun with that, and it was only 30 bucks to buy a grinder).

Also, the ones that were a little sticking up (like 1/8"), you can actually shoot the gun on those again though Hilti does not recommend you do that (according the the HD rental guy). My contractor neighbor came over to see what we were doing and he took the gun, put it over the fastener and shot it again, so we just did that for the ones that we wanted to keep in the board.

As for hammering it in, I am not sure that would have worked, at least on an older slab. I think hammering it in with a baby sledge might cause some shattering since the concrete (at 30 yrs old) is fairly old. Newer concrete is a different story.

Anyway, thanks for your help and replies!



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