# Taping together underlayment



## Brad Nailer (Apr 13, 2013)

I've laid a fair amount of hardwood in our upstairs. I've always taped the top side of the underlayment when piecing it together.

I had a guy at the floor store tell me I should tape both sides.

Is it necessary? If so, why?


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## joecaption (Nov 30, 2011)

I've never seen anyone use any form of tape on the seams when laying hardwood.
What's there to gain?
Should be laying Kraft paper or tar paper then the flooring.


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## Brad Nailer (Apr 13, 2013)

I'm guessing that regardless of what you use under the hardwood, it's not going to be one big piece to fit the room. There will be seams. So how do you keep the pieces from slipping apart while you're installing the hardwood?

The underlayment I've used is felt with a plastic film on one side. The film is on the up side, in contact with the hardwood. It serves as a moisture barrier. It comes in 4 ft wide rolls. It is supposed to be laid perpendicular to the flooring. As such, there's no way you can put one section of underlayment down, then some flooring, then more underlayment, and so on. I typically lay down all the underlayment and use duct tape at the seams to keep it together.

So, is it necessary to tape the backside of the underlayment and if so, why?


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## mako1 (Jan 7, 2014)

I don't have clue what taping both sides would do to benefit the floor. I always overlay by a couple inches and tape or hammertack the overlay.It's more of a way to keep the underlay from slipping while we are installing the floor than anything else .


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## chemman (Apr 2, 2012)

Per the National Wood Flooring Association underlayment should be overlapped by fours inches at all seams. I tapped my seams just because I kept sticking my fat foot in between the seams and tearing the paper. I wouldn't pay any mind to the guy at the flooring store. A lot of them are salesmen with no installation experience.


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## Brad Nailer (Apr 13, 2013)

An overlap of 4 inches doesn't seem right either. That would create an uneven surface to lay your floor on. 

I've always been told to run the underlayment perpendicular to the flooring so the floor goes across the seams. Whether you span the space between overlaps with one long piece or fill it with short pieces, even at barely an 1/8" think, there will be that dip that is not good for laying flooring material.

When you consider the room shape, do you really want to have that kind of overlap where you're piecing together bits of underlayment.


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## JustScrewIt (Mar 14, 2015)

If we're talking actual hardwood hardwood--like raw T&G that you sand and finish--the only "underlayment" you would put down is, like joecaption said, tar paper (wood sub). And tar paper is, well, paper thin. Not really going to affect the levelness of the floor. Not to any noticeable effect at least. Especially since it's going to be T&G'ed, glued, nailed down, and then sanded. I would be more worried about something seeping through an exposed seam than the 0.02" height difference. If it's a concrete sub, then the 6 mil poly (0.006") is used and overlaps should be about a foot and taped. Then plywood over that. Then tar paper. Overlapped 4".

If we're talking engineered or laminate click together floating floors. The underlayment typically comes with a sticky strip along the seams that you peel off and butt the edges together with, right? So why would you need tape? Isn't two layers of tape basically the same as overlapped layers of paper?...

The real question though is, if the flooring guy at the store says to tape both sides of the seam, does he tape all the seams on one side first, and then flip the whole pieced-together sheet over and do the other side? Can you really imagine doing that for every room? And what about cutting around vents and closets and things? Seems to me there'd be a lot of wasted material and time. It's underlayment, not crown molding...


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## Brad Nailer (Apr 13, 2013)

I'm talking 3/4" hardwood on the second floor, over an OSB subfloor.

I butt the underlayment and tape the seams just to keep it together. I don't even completely tape the seam. Just enough tape to keep everything together so it doesn't shift on me while walking all over it and laying the floor.

Here's a shot of the room almost finished. You can see the duct tape I put down on the seams.


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## JustScrewIt (Mar 14, 2015)

DEAR LIZA that's a clean room!! Did you say OSB?? Or OCD....... :biggrin2:

Jk, looks good man.

So since it's upstairs, I assume the underlayment is just to add an additional layer of sound control and cushioning..?

I just don't see why a moisture barrier would be needed for the second floor...

But I guess you're not using it for that since you're putting the plastic film side up rather than down? It probably does make the planks slide a little easier into place. Not sure if that's worth the additional cost of material for that benefit though, but hey to each their own! 

If it IS for additional sound protection on top of the OSB, I think you do want to overlap the seams so sound doesn't escape quite as easily through the gap. Tape would probably help in that respect, although I can't imagine it being necessary to tape both sides of the seam to get the same effect.

As far as the unevenness caused by overlapping the felt, I think once all the boards are clicked together and the underlayment and OSB compress with the weight of the floor itself and people walking on it, there shouldn't be any issue with humps or boards popping out. You might've noticed how you'll be working on a row and the boards in that row will have just slight incline to it from when you angled the boards to click in to the previous row. And it stays inclined until the next row of boards is tapped in, and then the next row after that, and pretty soon after 2 or 3 rows are done, the incline is gone. It settles and the felt beneath it compresses and it's even steven.

Now, if it was a pebble or something, then yeah that could pose a problem. But not some thin felt with moisture barrier attached. I mean really, how thick is it after stepping on it? a 10th of an inch at most? Shoot, splinters sit higher than that!

In any case, floor looks good man. I'm liking the color. Kudos on the cleanliness too. That's just plain ol' good practice right there!


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## Brad Nailer (Apr 13, 2013)

Thanks for the kind words. I must confess though, that picture is after I cleaned up for the day. That wasn't while I was working.

The underlayment is basically for sound. The floor is also nailed. It's 3/4" hardwood, T&G. 

I suppose the overlapping wouldn't be that big a deal, as far as unevenness, but I also don't see how an overlap is going to be that much more effective at blocking sound than a properly aligned butt-joint with some tape.


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## chemman (Apr 2, 2012)

As I was pulling up the particle board in my living room I found a pile of nails under one sheet.:surprise: It was as though some nails fell out of a workers tool belt and they just slapped the board down overtop and kept going. I never noticed the hump from that.


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## JustScrewIt (Mar 14, 2015)

Brad Nailer said:


> I suppose the overlapping wouldn't be that big a deal, as far as unevenness, but I also don't see how an overlap is going to be that much more effective at blocking sound than a properly aligned butt-joint with some tape.


There's a lot of rules and laws and regulations that don't make any sense, especially to the people who are expected to follow them. But that's the America we live in. Some guy in a lab coat determined in his controlled experiment that X = Z and -Y =/= N when Q and G are LMNOP. According to the product manufacturer who must be the the authority on the matter because the gov't is too busy with UFOs and other unidentified undocumented aliens to have its own uniform standards free from conflicts of interest. So some Bureaucrat decides that's how it is and that's how it's gonna be done and so here is the big book of _Procedures for How I Think You Should To Do Your Job_. That'll be $499.99 please. Due annually. Will that be cash or credit? Just kidding, we only accept cash. With your sanity for deposit. Which is non-refundable by the way. Thank you. Vote Hillary

Some companies say to overlap, some don't. Guess no one can agree on anything. Whichever method turns a 40 IIC into a 45 IIC rating I guess is the way to go. I guess you can ask either the IBCO or BOCA or NAIMA what they say. If they ever get back to you...

Here, have a look at this. About halfway down the page there's the Sound Abatement Do's and Don'ts: 

http://www.hardwoodfloorsmag.com/installation/the-lowdown-on-wood-flooring-underlayments.html


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## Brad Nailer (Apr 13, 2013)

Interesting article. 

The only thing I did wrong then, is I didn't totally tape the seams. I only taped them for the purposes of holding the sections together. 

Thanks for the info and I loved your commentary. 

Working in the computer industry, I've seen all kinds of standards for similar applications. It's a combination of lab testing and legal department approval. The ultimate in compromise.


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