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atxjoe 02-15-2011 12:30 PM

Routing T&G subfloor
I'm installing 3/4" T&G plywood for a subfloor. How should I tackle the last row against the wall that requires a partial sheet? I've got strips from 16" to 30" wide. My scrap pieces have been cut so they have no groove left. I could buy 8 full sheets and trim to fit but that seems like both a waste of wood and $160. I've read that I can use blocking between the joists, and that's one option I'm considering.

Could I buy a router and tongue and groove bit and add grooves to my scrap ply pieces? I've never used a router so I'm unsure if that would work, but it seems pretty straightforward. The money saved on labor would almost pay for a decent router and T&G bit, plus it's a great excuse to buy a new tool. ... "Honey, I'm SAVING us money."

Link to photo

mrgins 02-15-2011 05:15 PM

2x4 flat blocking. glue and screw them under one layer of subfloor and then install the other piece. No need to even attach them to the joists if your joists are 16"oc or less

Gary in WA 02-15-2011 09:18 PM

Yep, though I would attach them to the joists as that edge without the t&g is now the weakest place in the whole assembly and you would get more deflection there without.


atxjoe 02-16-2011 07:30 AM

Okay, the joists are about 18.5" o.c. so I'll attach the blocking to the joists as well. Thank you.

elarreau 02-16-2011 01:40 PM

You will increase the chances of squeaks if you run the 2x4 blocking to the floor joists. If someone steps right in the middle of the block it will push down on the ends. If it moves even the slightest little bit it will squeak for sure. I would cut the 2x4 blocks short so you have a quarter inch gap at the ends. This way the ends can move a little without squeaking. I don't think strength is a concern because the plywood is designed to span between floor joists. 1/4 inch shouldn't be an issue. Once you attach both sides of the seam in the plywood to a block with glue and screws it will be pretty secure. If you do run the 2x4 blocks to the joists make sure you glue them and nail them really well because you will have a squeak problem.

When I do blocks like this (and I have many times) I use glue on the block and screw the plywood to the block. The screw will suck the block up and set the glue between the block and the plywood really well. Make sure you have a screw as close to the end of the block as you can get it without splitting the block.

You might try using a router first just to buy a new tool. I completely agree with that. I wouldn't actually try to tongue & groove your plywood but any excuse to buy a new tool, right?

Gary in WA 02-17-2011 10:18 PM

If the plywood was not t&g to begin with, either way would work, somewhat. With the t&g edge, the shear flow is transmitted through the sheathing as well as the additional strength gained at the edge. Notice step #1 here;

This from APA, their definition of “blocking” at edges; “Blocking; Light lumber strips nailed between major framing members to support edges of structural panels where they meet.” Bold is mine.
I’ve spent many hours nailing blocking for cut t&g edges on spec. houses since early ’80, so the deck would pass inspection in our seismic area here. Would be a lot quicker not to nail them… other than a reduction in strength and shear flow.

rditz 02-18-2011 01:27 PM

put in extra blocking if you wish, but wherever it is going to come in contact with the flooring material put on some PL adhesive.

then cut your plywood pieces to fit the gap and and on joists/blocking (leave enough space for next sheet), glue and screw.. repeat until floor is filled.

no need to try to replicate the T&G.

what material will the finished floor be?? hardwood, laminate, tile, carpet??

What do you envision will be in that part of the room?? dresser, bed, etc... it may be a non-issue once furnished.


Dusty1 03-19-2011 01:18 AM

I have done both. The router trick can work although I have noticed that some subfloor material now comes with tapered T&G edges. Use screws and glue. Keep the blocks back from the joists a wee bit to avooid squeeking.

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