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subfloor buildup

JohnCA07 | Posted in Construction Techniques on April 7, 2004 09:04am

I have a floor question for you pros.

 

I am planning to install a new plywood subfloor over an existing three quarter inch thick T&G subfloor. What I need to know is which direction the new plywood subfloor should be laid to give me the best strength and remove the maximum amount of flex from between beams. I have attached a drawing of the current structure.

 

Some important information is the new subfloor I am planning on is 1-1/8†plywood. But I wont be getting its full strength in some areas as we plan to route out the subfloor 5/8†at 12’ oc for laying radiant heating into the plywood.  That brings up another question of which direction is best to run the radiant heat in this situation, with or across the joists?

 

Thanks for your input. 

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  1. User avater
    johnnyd | Apr 07, 2004 09:59pm | #1

    What's the square footage?  If the main reason for the 1 1/8" thick ply is to allow for radiant heat,  why not use two layers, one 3/4" layer in 12" rips, lay aluminum plates with slots for the PEX, snap the PEX in, and cover with with the next appropriately sized sheet.  What kind of finished floor is planned?

    Imagine yourself on your hands and knees, routing ___lineal feet of grooves, cleaning up, and actually compromising the original strength of the 1 1/8".

    All this, of course, IMHO, and maybe you've thought of this already.

    1. JohnCA07 | Apr 07, 2004 11:54pm | #2

      Funny how I sometimes get a vision in my head and I’m just unable to take it to the next level. In this case I never even considered the option of using two sheets of plywood.  (Well, I did look at some manufactured options but had to balk at the cost of pre attached tin/plywood 4'x4' sheets at $80.00 each.) I think your suggestion is brilliant and yet so obvious. Wish I were there to buy a beer!Yes the main reason for using 1-1/8 ply was to include the radiant heat within the floor.  I’m obviously no expert on floors so I am also concerned with the flexing in the current ¾“ floor.  The plan is for mostly hard wood except for the kitchen and entry which are to have stone. Is there some type of flex test to help judge if there is too much?  Still looking for professional input on the proper layout direction for the plywood in relation to my floor beams? If I go with the suggestion of two layers would it be better to lay them perpendicular? 

      1. User avater
        johnnyd | Apr 08, 2004 03:36pm | #5

        Although I'm not a structural expert or engineer, I have done alot of research into the sleeper/sandwich/Al plate system for embedding radiant tubing in the floor, and plan to use this system in my house.  The guy I'm buying my tubing and manifold from has done several of these, and recommends 3/4" plywood or OSB ripped into 12" strips, layed with 3/4" spaces between sheets, screwed down.  Then staple one side of each Al plate with the groove over the space, lay your tubing into the groove, and step on the assembly to snap it all into the sleeper space. Then staple the other side of the plate.

        You need to cut semi-circles at the loop ends, and of course have to plan this out so the loops line up with the sleepers.  He says just scribe the semi-circle line and cut with a sabre saw.

        As far as the direction of the plywood goes, the direction of the final flooring, if hardwood, has more to do with it than stucture, as you need to SEE the tubing when you nail the hardwood down.  Hardwood can be laid directly over the sleeper/plate sandwich,  I plan to screw down another layer of pegboard under carpeted areas, and cement board under tiled areas. 

        I'm guessing that the principle that comes into play when deciding which way to put your sleepers down is the same as that of plywood...with the grain direction alternating, and make sure the seams are always staggered.

        I hope other guy that said "been there done that" can post more details about his experiences.

        1. GCourter | Apr 08, 2004 08:11pm | #7

          I ran the plywood strips the same direction as the existing floor joists,  then the subfloor end up running the same way as the existing subfloor.  The secret is in the layout; you don't want to have a tube at the seam for you new subfloor,  you don't want to have a seam over the bends at the loop ends. 

  2. GCourter | Apr 08, 2004 12:49am | #3

    Just a quick note.  If you are using 1/2" tubing you will need to use strips of 3/4" plywood to create the "racetrack" for the tubing. Been there, done that!

    1. JohnCA07 | Apr 08, 2004 01:08am | #4

      GCOURTER  

      Thanks for the tip but why 3/4" track when most manuals state 5/8" track for 1/2" PEX. Did you experience a problem fitting the PEX into 5/8 or was the extra space for expansion?  A this point it looks like I’ll be using ¾” ply anyway but I was just wondering.

      -John

      1. GCourter | Apr 08, 2004 08:05pm | #6

        I did 3000 ft in Virginia last year, you need the 3/4".  The specs may say 5/8" but by the time that you put the alum fin in and a spec of sawdust you have a problem.  I ripped the plywood in 6" strips for the first six runs around the exterior and then went to 8" strips for the balance.

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