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adding another layer of subfloor


hammerash's Avatar
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05-15-15, 02:01 PM   #1  
adding another layer of subfloor

so I had to use floor leveler in one room before laying hardwoods. So now the adjoining room needs to have subfloor raised before laying hardwoods. the original subfloor is advantech 3/4 tongue and groove and I am putting another layer on top of it. Now the room is just under 16' so two sheets would cover it, but when original subfloor was laid, before walls, the room was obviously larger than 16' so the end of one row has a piece that is about 3" (with another 4 1/2" under the wall) if I lay 2nd layer over 1st, using opposite spacing of joints, I will come up a little short. hard to explain but typically on one row you have joint then next row you normally would place the joint at middle of a full sheet from previous row-staggering the joints by 4'. any issue with staggering this a few inches? that way I could use two sheets for the span. otherwise, if I center the joint in the center of the previous sheet, I will end up with 3 pieces total with one piece only being about 3 inches.

also, what is spacing to use to screw the 2nd layer to 1st? every 8"?

 
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czizzi's Avatar
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05-15-15, 02:14 PM   #2  
Split one board right in half so that you have a good stagger not only from the new boards you are laying, but also the ones that are underneath.

 
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05-15-15, 02:18 PM   #3  
Needs to be fastened every 4" on the edges and from 6 to 8 in the field, no glue.

 
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05-15-15, 04:39 PM   #4  
right, I am staggering seams from layer to layer. But can I simply move one seam about 3"from the 48" center of a board so that I only have one seam and don't have to cut 3" pieces to go on the end. so first row I had boards of 43", 96", 51". next row would be 91", 96", 3". Can I just make it 94", 96" by moving the seam 3".

 
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05-16-15, 09:14 AM   #5  
I have screws that are 1 5/8". So the two layers are each 3/4" for total of 1 1/2". any issue with the slightly longer screws which would enter into joists below? Didn't know if that was a no-no, like I know gluing to board below is.

 
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05-16-15, 09:49 AM   #6  
so here are some pictures. first shows the first row of new subfloor and the hardwoods in other room.



Second photo shows closeup of new layer. Note that the full sheet is centered above the seam from layer below-but I pushed the second layer toward the new flooring to span the joint between the rooms(one room in old house and other in addition). So I think staggering the seam in second row makes sense not only to reduce seams but if I put it in middle, it will be right over that small area of seam below since that first row was pushed toward the hardwoods.

 
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05-16-15, 11:23 AM   #7  
http://www.tecotested.com/techtips/p...plicationguide

Page 17 tells about spacing requirements of 2 layers of OSB.

Additionally, you usually lead with the tongue so that when you add the next row, you can use a beater bar and a block of wood to bring the tow panels together banging on the groove side. That way, you do not damage the tongue.

 
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05-16-15, 12:26 PM   #8  
they show putting the underlayment in opposite direction than subfloor. the long side of board (what advantech calls strength axis) is to be across joist spans per advantech. I actually found some instructions and huber (maker of advantech) says second layer should be run same as first (perpendicular to direction of floor joists) also say to put 15# felt paper between the layers. They also say stagger long axis joints at least 4" (which I did since I moved it closer to the finished floor) and stagger the short axis 1 floor joist from 1st floor. So I am good with the first row that is already cut and I will have to move the seam on the second row and end up with 3 pieces-but not a short 3" piece like was going to with usual center-staggering (a new term I just made up!) I think I have some felt paper so I should be good. Sorry, I should have search advantech before and I could have avoid this. no worries about damaging the tongues-I don't bang on them. just like hardwoods I laid the groove first and then use an old piece of groove, put it on tongue, and use as beater block. If you bang on on protected groove, you can damage that and then tongue won't go in.

 
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05-16-15, 01:24 PM   #9  
Sorry, should have read that page more closely, it is for adding 1/4" underlayment for glue or floating vinyl/linoleum flooring. I'll at least give myself credit for remembering that I had something showing overlaps. I never referenced as I always overlap by half a board both ways.

 
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