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Old 10-04-2004, 04:44 PM   #1
snake
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Floor Reinforcing

Hello all.
I have a newer house and would like to put 12"x 12" granite tiles on the front entrance,hallway,mud room and kitchen floors.The floor joists run front to back of the house and are 11 7/8" enginnered I joists spaced 19" o.c. supported by a 4 ply 2x12" main beam on teleposts in the basement.The stamp on the joists is LP MILL 437 LP1-20 PFS NER#QA-251 NER-612 CCMC 12724-R1CBO PFC-4787 LA CITY RR 25099 and a date stamp.

The span of the joists vary as the front entrance,which is in the middle of the house, comes back into the house approximately 3' so that the span from the front entrance basement wall to the main beam is 13' and then 16' from the main beam to the back wall.

The attached garage,which is on the right hand side as you enter the house has its back wall come back another 3' into the house so that the span for the hallway, mudroom off the garage,and most of the kitchen is 10' to the main beam and once again 16' to the back wall.The kitchen does extend over the main beam aproximately 5.5' towards the back of the house,on the 16' span side.(Hope this makes sense to you guys,lol)

The subfloor is 5/8 " t&g fir plywood,glued and screwed.

What floor reinforcing would I require and could it be done from the basement(which is wide open),such as screwing 19"x1/2" plywood strips between the joists up into the 5/8" above and using ditra right on the 5/8"?

Thanks,Phil
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Old 10-04-2004, 06:26 PM   #2
jadnashua
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You can reinforce the floor from the basement. I'll leave the details to those more qualified. Basically, though, you would rip plywood so that it is running with the top grain from joist to joist (the harder way because you have only 4' wide pieces rather than 8' or longer). The plywood has the most strength in deflection running that way. You'd need to probably block it in place. It probably would also be best to glue it in addition to screwing it in place.

If you can afford the height, though, it is easier to add stuff to the top.
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Old 10-04-2004, 07:54 PM   #3
bbcamp
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Looking at the longest spans, your I-joists are a little shy of being strong (stiff) enough for natural stone. You could fix this by 1) shortening the spans with a new beam and supports, or 2) adding 3/4" plywood along one side of the I-joists (or 1/2" along both sides of the I-joists. More on this later.) The plywood would be glued and screwed into the flanges making a boxed beam.

The subfloor is marginal for vinyl tiles as it is, so for stone, it needs major improvement. I'd like to see 3/4" plywood added to the underside of the floor, using the 1/2" plywood "sisters" we discussed earlier for cleats. This will give you about 1-3/8" of subfloor. This new plywood must be glued and screwed to the underside of the existing subfloor and fully supported by the 1/2" sisters. Also, as previously pointed out, the plywood face grain must be perpendicular to the I-joists. If you don't think there's enough room on the I-joists flanges to attach the plywood sisters after the subfloor, then add the sisters before the subfloor re-inforcement, then add 1x2 cleats to support the subfloor.
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Old 10-04-2004, 10:35 PM   #4
snake
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Thanks guys for the replies.Great site by the way!

Bob I just want to be sure I'm understanding what you're saying.The longest span I'm dealing with would be the 16' span from the main beam to the back wall ,as about 5.5' of the kitchen crosses over the main beam to this side.Do I box and reinforce the full 16' length of the joists for this area or just the 5.5' portion for the tile for the kitchen and as well do I do the same for the full length of the 10' and 13' joist spans or just the area I'm planning on tiling.

Thanks again,
Phil
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Old 10-04-2004, 10:58 PM   #5
cx
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Welcome, Phil.

It's the unsupported span of the joists that must meet the deflection criteria, not just the portion to be tiled. The theory being that if a live load is applied to the center of the span, even if that's not a tiled area, the movement affects the entire span, some of which is tiled.

So, for purposes of stiffening the joists, you must consider stiffening the entire span without regard for the location of the tiled area. Injineer bob will usually let you get away with sistering as little a 2/3ds of the span, so long as it's the middle two thirds. The more the better, of course, and he may not allow only the center 2/3rds in this case. And, if I understand your situation correctly, at least one of the tiled areas is near one end of the joists and will require sisters to support the additional subfloor pieces, so you'd be needing to do nearly all the length of those joists and I'd certainly sister the rest of it unless there is a substantial reason why you can't.

My opinion; worth price charged.
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Old 10-05-2004, 12:11 AM   #6
snake
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Thanks,appreciate the help
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Old 10-05-2004, 06:27 AM   #7
bbcamp
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You need to reinforce the subfloor only in the areas to be tiled, but sister the joists to the full length practical.
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