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Unread 01-18-2021, 09:09 PM   #1
go_habs_go
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Bathroom and Laundry room reno

Hello, my first post, really enjoy reading this forum.

There was emergency repair work done on my 11x11 crawl space due to a plumbing leak. The floor was opened up down to the joists. The contractor cut back the original 1x6 diagonal plank subfloor to about 3/4" along all 4 walls. He only put some 2x4 blocking on 2 of the walls (the ones that run parallel to the joists). He did not put blocking on the 2 walls that are perpendicular to the joists saying its not necessary as the joists are supporting the remaining planks and wall.

The contractor will install 3/4" plywood then another 3/8" plywood then ditra 1/4" heated membrane and then porcelain tile.

The crawl space is on 2x8 joists 16" OC. There is a beam underneath the joists that is supported by posts on the floor of the crawl space.

Is the blocking on the 2 walls enough? Should I push for blocking on the other walls? Is there any structural danger to my wall or is it just a question of reducing flex and possible squeaking of the floor?
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Unread 01-18-2021, 09:54 PM   #2
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Hi Dave,

The subfloor will require all edge to be supported so you should block around all edges. Doesn’t need to be 2x4; ply wood glued and screwed works just fine.

Since you are doing tow layers I’d recommend going with 1/2” for second layer. Much more likely to get a flat sheet. You can always use 5/8” for base layer if you are worried about total floor height. Both layers need to be minimum of B/C rated with no voids.

How is your contractor planning to attach that second layer of subfloor?
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Unread 01-18-2021, 09:56 PM   #3
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Welcome, Dave.

Yes, there should be solid blocking around the entire perimeter of your new plywood subfloor. There is no danger to the walls, but your other concerns are valid.

My opinion; worth price charged.
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Unread 01-18-2021, 10:30 PM   #4
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Thanks to both of you for your replies PC0760 and CX.

The contractor said he would glue and screw the 1st plywood then screw the 2nd one on top (I believe in perpendicular direction to 1st). I can ask him to clarify but what is the recommended way?

Thanks for confirming there is no structural danger to the walls. I will push my contractor to do the blocking, however they just sprayed urethane insulation today and now the joists are pretty much filled up. I guess they could chip some of it out? Or use the plywood method as it would be thinner than 2x4?
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Unread 01-18-2021, 10:43 PM   #5
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There's an article in the 'liberry' that discusses how to add subflooring. The second layer does need to be installed across the joists, not aligned with them (IOW, the same direction as the first layer, but offset so the seams don't line up).

The whole goal with any subfloor and blocking is to ensure there's no deflection. If the plywood is cantilevered over the joist into the joist bay, and the end is not supported, then it needs blocking.
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Unread 01-18-2021, 10:44 PM   #6
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Quote:
Originally Posted by Dave
however they just sprayed urethane insulation today and now the joists are pretty much filled up.
Not your problem, Dave. There must be blocking for the edges of your subfloor.

Here is a good article from our Liberry on what I think is the very best method of installing the second layer of subflooring. All layers of structural subflooring must be oriented with the strength axis perpendicular to the joist structure. And that would be especially true with that nominal 3/8ths" plywood, which would amount to almost nothing structural were it oriented in the direction of the joists. I personally recommend nothing thinner than nominal 1/2" plywood for that second layer, but I know Schluter says you can use that thinner stuff.

My opinion; worth price charged.
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Unread 01-19-2021, 07:15 AM   #7
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Excellent, thank-you all for your replies. I will read the articles and confirm with the contractor on his approach for the 2nd plywood. I did initially ask for 1/2" on the 2nd plywood but he said the 3/8" was more than enough especially with the ditra membrane on top of that. I think he is honestly trying to save me some money but it would be easier if he just does as I ask, especially for requests like this that don't change anything for him.

I'm obviously a bit frustrated that I have to push him on this and the blocking but that is the nature of renovation work I suppose!

I'll keep you posted on progress.
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Unread 01-19-2021, 09:21 AM   #8
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One other question I hope you can help me with before I speak with my contractor.

When you say 3/4" plywood and 1/2" plywood is this the "nominal" thickness or the real actual thickness? If my contractor brings a 3/4" plywood to the job and I measure it, should it I expect it to be a full 3/4" or will it be a bit thinner? If thinner, how much is acceptable? The contract says 3/4" but I want to be prepared with the right info in case he says that 3/4" is the nominal and not the real thickness.

So confused by this real vs nominal, thanks for your help!!
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Unread 01-19-2021, 09:51 AM   #9
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Dave,

No, nominal 1/2 and 3/4 inch ply wood doesn't measure to those thicknesses. 1/2" ply will actually be 15/32" and 3/4" will be 23/32".

Referring to them 1/2" and 3/4" is common.
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Unread 01-19-2021, 09:56 AM   #10
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You'll not find actual 3/4" exterior glue plywood these days, Dave, it's all "allowed" nominal thicknesses. The place from which he'll purchase will likely even advertise it a 23/32nds" plywood as many retailers or even manufacturers have started to list their sizes. I've not seen it in writing, but I suspect that more recent thickness identification now gives them the option of making the panels even thinner due to the 1/32nds" "allowance" on the final product. I haven't put a mic on any of it lately.

So, don't be alarmed if the material he brings is not the size commonly used in the vernacular of on-site discussions. And his nominal 3/8ths" plywood is likely to be closer to 5/16ths" than to a full 3/8ths" and not particularly flat, which is one reason I don't like to use it.

My opinion; worth price charged.
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Unread 01-19-2021, 11:31 AM   #11
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Awesome, thanks guys. Calling my contractor shortly, feel much better now that I armed with this info. Cheers!!
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Unread 01-19-2021, 08:32 PM   #12
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Update: contractor said no problem for 1/2" for the 2nd plywood. He's coming tomorrow morning and I'll discuss the installation of plywood layers with him then.

My question now is on the missing blocking. As per pics attached, you can see how the insulation has filled where the blocking should go.

1. I don't want to ruin the insulation by removing large chunks so what if some of the blocking is installed a few inches away from the edge of the old sub-flooring? Is it still effective?

2. What about where I have water lines and the heating vents running, how do I block those gaps?

3. For the one with the standing drain pipe, do I still get some impact by installing a block in front of the drain? Does the blocking ONLY help if it's at the exact edge of the plywood or does it still help somewhat if it's installed 2-3 inches back from the leading edge?

4. How do you block with plywood instead of 2x4? Someone mentioned it above but I'm not sure how it would work with plywood for blocking (glue and screw where?)
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Unread 01-19-2021, 09:05 PM   #13
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A good bread knife would probably slice that foam easily and let you cut a slot to put some blocking in. A hot wire cutting tool would, too, but those are harder to come by.
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Unread 01-19-2021, 09:24 PM   #14
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The method of using plywood for your between-joist blocking involves 5 or 6-inch rips of 3/4" plywood that you would glue and screw under the stubs of subflooring you see sticking out from the wall. You'd pre-drill holes in the boards, put construction adhesive on one side of your plywood strip, put the strip under the board ends and screw the plywood in from the top of the boards. Works better with plywood to plywood subflooring, but if you counter-sink the holes in the boards and use some care not to split them with the screws, it should work OK. Then, of course, you put some construction on the exposed edge of your blocking and fasten the sheet of new subflooring to it the same way.

I'm always careful to cut those blocks a bit shorter than the joist spacing so they don't touch the joist on either end so's not to create any possible squeaks.

Places where plumbing or electric are interfering a bit you just do the best you can with what you've got, but you need those edges supported.

The foam is gonna make that a PITA, but, again, that ain't your problem.

My opinion; worth price charged.
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Unread 01-19-2021, 10:19 PM   #15
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Thank-you for the replies. I'll try to get them to put in 2x4 blocking but for the areas that are tight I'll suggest the plywood as that seems easier to slip under (will require less cutting of the foam).

When installing 2x4 blocking do you screw from the top then down at an angle to the joist?
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