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Discussion Starter #1
I had to remove a rectangular section of my 5/8" plywood sub floor (see photo) between the joists to do a repair below sub floor. (You can see one of the joists). I have now made a new piece of plywood to fit over this hole.

First I need to install new pieces of wood (~36" long) the same size as joists (~7 1/8" high X 1.5") below the sub floor butted against the existing joists. This will constitute a support for the new piece of plywood.

The new pieces of wood are almost exactly the same height as the existing joists. It is going to be difficult to fit them in, since some parts of the new wood are just a little higher (~7 3/16") than existing joists. Now one could plane the new wood a little and use a rubber hammer to force it into place up against the existing joists and below the sub floor?

Could I be advised of the best way to deal with this situation;

Thanks

EMIL
 

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Scotty D
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You don't need to put full 2x8's in.

2x4 glued and screwed to the 8's will be suficient.

You will also need to block across the perpendicular, if it is 16" or less 2x4 there also.

If wider than 16" I would use 2x6.

Glue and screw everything. :smile:

Also because the sheet will only be supported on the edges, a center perpendicular nailer will eliminate center sag.
 

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OLD DUDE AT WORK
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Yeah, I had to do something similar in the last house. What mdntrdr said.
 

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where's my table saw?
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somethin's goofy here

If the new wood is 36" long, the opening must be almost square..?
That will require a "sister" on either side of the opening and a "center" to support the middle. I don't see where you cut away the center joists. The span between joists must be 16" on center or 14 1/2" between faces.
Is the space accessible from below?
What size is the opening?
Did you cut away any floor joists? how many?
A better picture would be helpful showing the edge of the opening.
;) bill
 

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Discussion Starter #5
Thank you for your comments;
Here are answers to your questions:

"the new wood is 36" long, the opening must be almost square..?"

The new wood is 5/8" and is cut to fit into the hole which is 24 1/2" (from right to left) and 19 1/4" wide between the joists (this is the actual gap between the edges of the adjacent joists . The new piece will sit on top of the new wood blocks added.

"That will require a "sister" on either side of the opening and a "center" to support the middle. I don't see where you cut away the center joists. The span between joists must be 16" on center or 14 1/2" between faces."

I pan to insert a 2 blocks between the joists perpendicular to the new blocks screwed to the original joists.

"Is the space accessible from below?" No
"What size is the opening?" see above
"Did you cut away any floor joists? how many?"
No
"A better picture would be helpful showing the edge of the opening."

Please can you describe better what you want the picture to show.

EMIL
 

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where's my table saw?
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no picture needed now

If my math is correct.... 19 1/4" minus 3" equals 16 1/4" of unsupported span. A little beyond code by 2".
You could drop down a size and use 2 x 6" and use a thicker patch piece say 3/4". But 5/8 should work depending on the floor load. For extra rigidity double your subfloor thickness between the sisters, and that will do it.

I would attach cleates, 3/4" x 3" x 19 1/4" pieces, to support the long ends under the existing subfloor, just 3/4 ply and some constuction adhesive, Liquid Nails, that way you are supported on all 4 sides.

If you never need to get back into that space use Liquid Nails on all the wood attaching surfaces and joints for a squeak free job.
I personally would use 2 1/2" deck screws on the sisters, rather than nails, but that's just me. ;) bill
 

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Old School
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You don't need to put full 2x8's in.

2x4 glued and screwed to the 8's will be suficient.

You will also need to block across the perpendicular, if it is 16" or less 2x4 there also.

If wider than 16" I would use 2x6.

Glue and screw everything. :smile:

Also because the sheet will only be supported on the edges, a center perpendicular nailer will eliminate center sag.
+1. This would be a simple fix.








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Discussion Starter #8
Woodnthings:

Your math is correct; 16 1/4" of unsupported width. Yet I plan to add other blocks (see drawing). The size of the wood I have is 7 1/4 X 1 1/2". The 7 1/4" (same height as existing joists )puts the height of the added wood blocks at the level of the sub floor: hence it will provide needed support to the new piece of plywood when it is placed into the hole.
The new pieces of wood attached to the original joists provide a landing of 1 1/2" for the new plywood.
The new pieces of wood perpendicular (left and right in drawing) to the above new piece will be placed such that 3/4" is below the existing sub floor and then 3/4" will be available as a landing for the new piece of plywood when it is placed in the hole. (do you follow?).

I like the idea of using adhesive plus screws.

I do not follow the use of the word "sister". Is my plan consistent with "sister".

Also other blocks could be added as shown.

To cabinetman: I do not follow your recommendation of 2 x 4" wood....not at all.
I added the drawing to clarify what I am proposing
 

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Old School
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To cabinetman: I do not follow your recommendation of 2 x 4" wood....not at all.
I added the drawing to clarify what I am proposing
If you are making a cleat to hold the edge of the plywood, gluing and screwing a 2x4 to the joists would be sufficient support, they don't have to be full height. Adding a center support was also recommended, that I agree with.








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where's my table saw?
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Sistered joists

A sistered joist is simply another joist attached side by side to an existing one either for support as in your case or for added strength for a weak or rotted one. Your sketch shows additional blocking lengthwise. In my view that's a bit of "overkill". If there is any doubt as to structural strength, just double the plywood which rests on your "platform" edges and use a 2" x 6" for the "new" wood as sisters. Locate the 2 x 6 below the existing subfloor the thickness of your second plywood. So if using 5/8" plywood, the platform edge will be 1 1/4" below the top of the existing sub floor. If possible make the bottom piece longer by a few inches to support the edge cross wise. The your top piece is cut to the opening size.
Keep in mind this is all free advice and you only get as good as what you pay for .....:laughing:
 

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Emil VolK, the suggestion is to use 2x4's as the 7-1/8" pieces in your drawing. you will just be using the top edge of the 2x4 to act as the top edge as the 7-1/8" pieces. if you glue and screw the 2x4 to the existing joist they will be plenty strong to support the sub-floor. i would also recommend the center support. hope this helps.
 

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Do the new 36" lengths have any job besides holding up the new subfloor? It sort of sounds like you might be repairing some cracks in the original joists with those sticks, too.
 

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Discussion Starter #14
Thanks for the explanation. Now I understand the strategy of using 2” x 4” instead of 8” x 2”. The wood I have is the latter ( 7 ¼ x 1 ½”), but I can get 2 x 4 if it would simplify implementing project.

Last night I tried to work with one piece of 7 ¼” x 1 ½”. I rounded the edges by sanding with my belt sander and sanded the top surface somewhat. The reason I did this was to try and make it easier to slide between the sub floor. I put it into place below the sub floor by beating with a rubber mallet. When I looked at the top surface of the wood it was not as flat as it originally was. I have removed this piece of wood.

Woodnthings
“I would attach cleates, 3/4" x 3" x 19 1/4" pieces, to support the long ends under the existing subfloor, just 3/4 ply and some construction adhesive, Liquid Nails, that way you are supported on all 4 sides.”

Here are 2 new drawings. The first one offers more of a true size perspective than previous. The second one illustrates the proposed use of “cleats” as I understand it.

Commentary on Procedures
As I see it the cleats will be ¾” x 3” x 16 ¼” (not 19 ¼”). Do you agree? The cleats will be placed below the existing sub floor and glued (any screws?) to the sub floor. Half (1 ½”)of the width of the cleat (3”) will be below the existing sub floor; the other half will be a landing.

Now will the cleats be attached to the joists by glue? and attached by right L bracket to joists?

Below the cleats there will be a new piece of wood (4” x 2”). Now this new piece will be glued to the joists? And attached by right L brackets to joists? This strategy would provide a landing of 1 ½ on the cleats for the new piece of top sub floor. The strategy I put forward using the 7 ¼” x 1 ½”wood provided only a ¾” landing for the new piece of sub floor. (The reason for this is that half of the 1 1/2'” would be below the existing sub floor). No cleats were envisaged to be necessary for this approach.

Now the question arises which of the 2 approaches ( nominal 8 x 2 v nominal 2 x 4) is the easiest for a novice like me to put into effect?

Other good questions have been raised and I will answer them. First I would like to clarify above issues.
 

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where's my table saw?
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Here's my suggestion

A drawing showing the existing 2 x 8's and the cleats.
 

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Discussion Starter #16
Furthermore to sum up as I see we have the following options (a) and (b) for building support below the sub floor;

(a) Stick with my original plan of using 7 1/4" X 1 1/2" wood blocks for both back to front and left to right. The disadvantages are
(1) For front and back it may be a little difficult hammering the wood blocks into place parallel and up against the joists. {The new block will be screwed to the joist will be screwed and adhesive will be used}.

(2) for LHS and RHS blocks will be secured perpendicular to the joist using brackets and adhesive will be used. There will only be 3/4" landing for new piece of sub floor as opposed to 1 1/2" for front and back blocks above.

(b) Use 4" x 2" parallel wood blocks in place of 7 1/4" X 1 1/2" for front and back secured with glue and screws.
For LHS and RHS use perpendicular cleats ¾” x 3” x 16 ¼” under the sub floor and stuck to it with adhesive. The cleats will be secured to the joists with glue and brackets. Below the cleats there will be 2 x 6 blocks (or 2 x 4"?) of wood to provide support. Now which? The wood blocks will be secured to the joist with glue and brackets. The cleats will provide a a 1 1/2" landing for the new sub floor, as opposed to 3/4" for option (a), but it seems there is more work involved.

There will be other blocks of wood added to address concerns of additional support and sagging.

I am trying to summarize and condense the posts. Any comment on above? You see inexperienced folks like me are prevaricators!

Thank you

EMIL
 

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Old School
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You might be over thinking this fix. What you could do is the 2x4's that you will glue and screw to the existing joists, glue and screw a block, that will be a stop block for cross 2x4's to go across the unsupported floor.

Cut two cross 2x4's for the unsupported edges, and connect them with a center 2x4 for support. This will look like an "I". Then just install the "I" in the opening with the top edges lined up with the other 2x4's. Screw the ends of the "I" to the stop blocks that are at the 4 corners of the initial add on 2x4's.








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Discussion Starter #18
the drawing is brilliant! you suggest that we do not need wood blocks to support the cleats. Now the cleats will be glued to the 2" x 4" and I am thinking that a right L bracket should be used to secure the cleats to the 2" x 4". Do you agree or how best to secure the cleats?

I agree with you design, since there will be other support wood added.
 

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Discussion Starter #19
Cabinetman: I have zero first hand experience and you need that to envisage what do. Can sketch what you have in mind?

Emil
 

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OK, last proposal..

the drawing is brilliant! you suggest that we do not need wood blocks to support the cleats. Now the cleats will be glued to the 2" x 4" and I am thinking that a right L bracket should be used to secure the cleats to the 2" x 4". Do you agree or how best to secure the cleats?

I agree with you design, since there will be other support wood added.
Now I realize you are getting 2 different opinions on how to do this. This is the way I would do it.
Take a 1" x 4" (3/4" x 3 1/2") 10 feet long

Cut 2 pieces 20 3/4" long and 2 pieces 19 1/4" for the width.
Install the cleats across the width first.
Then install the 2 x 4's about 30" long under them. Glue and screw.
Then install the long cleats in between the width ones and resting on the 2 x 4's.
Frame the opening by screwing the cleates from the top. The screws will countersink themselves under the pressure. Use the glue (liquid nails) in between all the new wood, joists and original subfloor.

Once the frame is complete you will have a border of approximately 1 3/4" all around the top.
Nothing is needed under the short/width just a glued and screwed cleat since the ends are resting on the 2 x 4.
By supporting the new subfloor patch with the cleats there is no need for additional wood underneath. It will be better than the original since the cleates extend inward by 1 3/4".

The end view drawing is what it will look like except the cleates will run all the way across the width.

Once all the glue is dry, overnight, cut your new subfloor piece 24 1/4" x 19 1/4" and glue and screw it on to the platform you have created. If you want to try this fine, if not, I will not be offended. ;) bill
 
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