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Replacing subfloor questions


spta97's Avatar
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Join Date: Apr 2007
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NY

08-21-07, 06:41 AM   #1  
Replacing subfloor questions

I am remodeling my kitchen and have 3/4" wood planks (about 3" wide) over the existing 6" joists.

Since I will be installing tile and a granite counter top (ie: a ton of weight), my plan is to remove the planks and glue and screw 3/4" plywood in it's place.

The floor as is is pretty stable, but I wonder if it is worth sistering the joists or even putting blocking inbetween them with hangers to really stiffen the floor.

I plan to put down 1/4" hardiboard as my tile base on top of the plywood.

Thanks!

 
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Tilebri's Avatar
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08-21-07, 07:13 AM   #2  
6" joists? What is the actual size and how long is the unsupported span?, The spacing on center? Another option to removing the planking and replacing with 3/4 ply would be to go over it with 1/2" bc ply and then use a membrane instead of cement board. Removing the planking and replacing with plywood will require blocking to support the perimeter panel edges as well as to support any non bearing walls that may fall between two joists. The plywood would also need to be t-g plywood.

 
spta97's Avatar
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08-22-07, 05:24 AM   #3  
I will have to measure for the actual size of the joists but I beleive it is approximately 6 3/4" (high). The span is approximately 10 feet and space on center is approximately 16" (there seems to be no exact spacing in my house).

I was orginally looking at T&G plywood but it seemed that the only difference (other than the T&G) was that it had a wax coating and I was concerned that might have issue bonding with my morter bed for the Hardibacker.

I was planning on either blocking to support the panel edges or just pry off the wood planks so the plywood rests on 1/2 the joists as the planks do. I would not disturb any of the walls but rather allow them to sit on the wood planks and just replace the middle sections for rigidity.

This is just the "cooking" section of the kitchen where the tile, appliances, cabinets, and counter top will be - the other portion has a finished wood floor that I will not be touching.

 
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08-23-07, 04:45 AM   #4  
Your joists as stated are not strong enough for ceramic. Sister each one and you'd be fine. Non t-g single layer subfloors also require blocking between the joists to support all panel edges as they cross the joists. If you use square edged panels, they need to be bc exterior or better. Never use anything marked as sheathing or CDX as there are too many voids in the layers to ofer the required support. You want the long edge of the plywood to cross the joists always. The thinset is not for bonding hardi to the plywood, it's only there for bedding the cement board to ensure you have no voids between the two. For that reason, a decent unmodified thinset can be used under the hardi. Hardi is also notoriously thirsty and will suck the water right out of your thinset before it cures properly if you don't first sponge it down before setting tile on it.

 
spta97's Avatar
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08-23-07, 10:49 AM   #5  
Thanks for the info. The contractor who removed the wall said that he has done many houses like mine with ceramic and didn't even replace the subfloor and did not have any problems as long as he used laytex fortified morter. Do you have any references that I can do more research on?

Regarding the sistering - I am a little confused about this. Do I have to butt them right on the sill? I'm thinking that if I do that (ie: wedge them in) it will transfer the load of the floor and wall above it to the new joist. Is sistering just meant to stop the boards from flexing? Also, do I have to allow the wood to fully dry before I install it? What direction should the crown be facing?

I will check my plywood to make sure it is BC or better. I am pretty sure I made a point to get the exterior type.

Great tip on the spongeing - I never heard of that.

 
spta97's Avatar
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09-04-07, 07:17 AM   #6  
Quote: "Non t-g single layer subfloors also require blocking between the joists to support all panel edges as they cross the joists."


Tilebri - Thanks for your help! I was able to finally find the BC Exterior plywood for the flooring but there was no T&G that met tile underlayment requirements.

I also sistered the 12 foot span with 10 foots joists glued and screwed together. I was able to find a specific answer by using this great Deflecto Calculator here:

http://www.johnbridge.com/vbulletin/deflecto.pl

Regarding the blocking, is that really required for only a 16" span? I will do it if it is but I do not want to create more work than neccessary.

 
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