# blocking below sub floor



## Emil VolK (Jun 4, 2011)

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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## mdntrdr (Dec 22, 2009)

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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## H. A. S. (Sep 23, 2010)

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


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## woodnthings (Jan 24, 2009)

*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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## Emil VolK (Jun 4, 2011)

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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## woodnthings (Jan 24, 2009)

*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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## cabinetman (Jul 5, 2007)

mdntrdr said:


> You don't need to put full 2x8's in.
> 
> 2x4 glued and screwed to the 8's will be suficient.
> 
> ...


+1. This would be a simple fix.












 







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## Emil VolK (Jun 4, 2011)

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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## cabinetman (Jul 5, 2007)

Emil VolK said:


> 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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## woodnthings (Jan 24, 2009)

*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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## gus (Oct 31, 2010)

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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## SteveEl (Sep 7, 2010)

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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## angusmdmclean (Jun 28, 2010)

errors


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## Emil VolK (Jun 4, 2011)

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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## woodnthings (Jan 24, 2009)

*Here's my suggestion*

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


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## Emil VolK (Jun 4, 2011)

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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## cabinetman (Jul 5, 2007)

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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## Emil VolK (Jun 4, 2011)

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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## Emil VolK (Jun 4, 2011)

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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## woodnthings (Jan 24, 2009)

*OK, last proposal..*



Emil VolK said:


> 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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## angusmdmclean (Jun 28, 2010)

I think I follow what you are describing. You are going with what you originally said the cleat size should be and providing a uniform platform around. The support sister joists are included only from left to right attached to the original joists (lengthwise) but not included across the width. The platform is bigger than your previous thought (1 7/8")

I think your description: "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."

means the following to me

First prepare the cleats as follows: Take a 1" x 4" (3/4" x 3 1/2") 10 feet long.

Cut 2 pieces 20 3/4" (long cleats) and 2 pieces 19 1/4" for the wide cleats.............

Am I following your thoughts properly?

EMIL


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## woodnthings (Jan 24, 2009)

*Yes, so far*

Screw the wide ones on first. *Then put your support 2 x 4 or 2 x underneath.* attach them up snug to the bottom of the cleats.
Measure for the long cleats. It may be a tad off from the dimension so work from the actual distance. then fit the long cleats on top screw them down.
Now you should have a "platform" under the opening to support the new subfloor "patch". When you screw down the patch use screws long enough to penetrate the cleats all the way through and into the 2 x 4.  bill


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## Emil VolK (Jun 4, 2011)

I understand your new elegant design, a modification of the earlier one. One reservation that I have is the elimination of the 2 x 4 at the LH and RHS. I wonder if it would give more strength if they were included.

Now I had better explain the project I am doing below the sub floor. I have not done as yet, since it would complicate matters too much.
It may necessitate somewhat a rework of above wood structure design. The project is not to repair rotting joists.

I have found out that this shower had no liner so I had to remove the existing drain. The project is to fit new plumbing below the sub floor of a shower : to be fitted is a P trap plus connective piping downstream. Upstream to be fitted to the P trap is a rising pipe and just above the rising pipe there is to be a 4 1/2" diameter hole in the new plywood sub floor section. Hence the plywood sub floor will have have the 4 1/2 hole in it.

I attach photos of the (a) sub floor cavity, (b) plus previous drawing with 4 1/2" hole included in the new sub floor section (c) drawing of p trap to be included.

Now first the p trap will be attached to the existing drain pipe and then the P trap will be clamped to the exiting base just below the sub floor. The potion of the base of the p trap is shown marked in pencil in the photo. Then the rising pipe will be glued into place. I need to immobilize the rising pipe and p trap below the sub floor so that I can fit the shower drain above the new sub floor through the 4 1/2" hole in the new sub floor into the rising pipe from the p trap. I need to have the rising pipe (and P trap) well-immobilized. Now I need some suggestions on how best to do this. I am thinking of how to clamp it to the wood structure. I have a metal strap coil and clips. I may have to add another perpendicular piece of wood from back to front and close to the rising pipe; and anchor rising pipe to it.

Now on top of the shower floor there will be some roofing felt and there will be cement mud added and layered with a pitch to constitute a shower prepan. The prepan will surround the shower drain, but will not support it. Previously when I did this before in a basement project the rising pipe was already embedded in concrete. There was no plywood sub floor per se.

The question arises as to whether the proposed wood structure is OK to accommodate the hole in the sub floor and this project. Or do we need more support

Also how to immobilize the P trap and rising pipe to wood structure

Any more details needed please ask;

EMIL


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## cabinetman (Jul 5, 2007)

Emil VolK said:


> *Now I had better explain the project I am doing below the sub floor. I have not done as yet, since it would complicate matters too much.*
> It may necessitate somewhat a rework of above wood structure design. The project is not to repair rotting joists.
> EMIL


From this thread:
http://www.woodworkingtalk.com/f16/posting-question-4-nubies-others-9786/


woodnthings said:


> *POSTERS, Give all pertinent details, woods used, tools available, dimensions, spans, thickness, tool model and brands etc. when asking a Question*.....You may want to Search this site before posting to get an idea of what others have encountered. There is a wealth of information here and folks are willing to spend their valuable time answering a well thought out question.The answer will be only as good as the question. TO AVOID ANY CONFUSION, POST A SKETCH OR PHOTO. The toolbar has FAQ's to answer how to post "attachments" just follow the instructions. This will help everyone to fully understand your question and you will insure getting an answer that is useful. Digital cameras are cheaper than goats milk these days and would be a valuable addition to record your success :yes: or failures! :no:..bill













 







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## angusmdmclean (Jun 28, 2010)

Here is a better description of this project: I have an upstairs shower. It has been necessary to remove a rectangular section of the sub floor and to remove the shower P trap. Below the sub floor there is a sheet of thin plywood. Below is the ceiling of the living room. So the only way to access the plumbing from below is to remove some dry wall and that piece of thin plywood.

Advice has been received from woodnthings regarding building a support platform all the way around the rectangular hole in the sub floor. {This will support the new wood sub floor rectangle when it is slotted into place (see drawing). This will be implemented.

Now a new P trap is to be installed (drawing has one) and a riser pipe put in place (drawing has one). At this point the new plywood with a 4 1/2" hole is to be put in place on the hole. The base of the P trap can be clamped to the thin plywood. I am thinking how best to immobilize the P trap and the riser pipe. I am concerned that after the new plywood rectangle with the 4 1/2" hole is put in place that I have to secure the shower drain in place to the riser pipe with a clamp or straps. I am thinking to add a block of wood between the 2' x 4" (perpendicular to the joists) close to the riser pipe and clamp the riser pipe to it. The riser pipe will be 5 1/2" from the 2" x 4 " new wood (at rear drawing) and 9 1/2" from the front 2" x 4".

Any thoughts on this predicament


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## Emil VolK (Jun 4, 2011)

I have implemented the excellent plan provided by Woodnthings; looks very solid platform. Now I have to deal with the plumbing.


EMIL


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