Question about load on floor joists/sub floor

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Old 01-11-07, 06:19 PM
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Question about load on floor joists/sub floor

Can someone tell me, how you determine the load you can put on an existing (old) sub floor/floor joists?

I'll be more specific.

Say I want to replace my kitchen floor from hardwood to ceramic tiles.
The floor is in the first kitchen of course and we have full basement.
Obviously I will need to tear down the hardwood floor and most likely put new plywood, or even cement board before I lay the tiles. I even thought about using self-leveling cement on the sub floor simply because it's an old house and the current floor is not leveled.

So, the question is, taking in account the weight of tiles and plywood or even few inches of cement (self leveling) - how can you tell if it's not too heavy for the floor joists? In all those TV shows I don't see that they "consult" with an engineer whether they can do it or not. Is there a way to determine whether my floor joists will be able to carry this load (plus new/heavier cabinets, oven, fridge, etc)?

Thanks for any response.
 
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Old 01-11-07, 06:39 PM
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Whats the zize of your floor joists and what is the span between them?
 
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Old 01-11-07, 06:48 PM
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They are 2.5"X8" and the span is between 11 and 13 inches...the span is not the same between them.
 
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Old 01-11-07, 07:55 PM
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Your floor should handle the weight without any problems. I recommend removing any 1/4" underlayment you have to include particle board, or plywood. Then install 1/2" bc or exterior plywood.
 
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Old 01-11-07, 08:00 PM
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Remember, the plywood will spread the weight of any load across the joists so all the weight is not on one area.
 
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Old 01-12-07, 04:13 AM
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Thank you.
 
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Old 01-12-07, 07:08 AM
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Wait - not so fast. You need to make sure that you meet the minimum required deflection of L360 for ceramic tile. You gave us the size of your joists and the on center spacing of the joists but not the unsupported length of the joists. So what is the unsupported length of your joist from below. This can be observed in your basement by measuring the distance between support from below along the length of the joists. Read this link to understand and answer your question.

http://www.tileusa.com/Deflection_faq.htm

What do you currently have for a subfloor? Is it plywood, osb, planking, whats the thickness, is it tongue and groove or square edge?

You do not want to use plywood as the substrate for your tile, you'll want to use cement board or a membrane. You may need an additional layer of plywood, depending on what your report back to us.
 
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Old 01-12-07, 01:31 PM
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OK, I read the link and did not understand a word

I believe the sub floor is planking...that's what I see from the basement anyway, and I think there is nothing between the planking and the hardwood floor.
I know I need cement board or self leveling cement for tiles, but while I am pretty sure self leveling cement will be much better, it is also heavier than cement board, so I am not sure if the floor joises will support that and the tiles on top of that.
 
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Old 01-12-07, 01:45 PM
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Oh well.

From down in your basement, measure underneath the kitchen along the length of the floor joists from support to support. By support, I mean an outside wall, a load bearing wall, a center support beam etc. How long is the span. Give us this measurement.
 
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Old 01-12-07, 01:54 PM
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If the subfloor is plank this is called dimensional lumber,you must lay a layer of 3/4 plywood over it prior to installing cbu or an uncoupling membrane this will absorb the deflection rating that johnny is explaining but it will also add wieght to the are so you may still want to be sure your joist span is well adjusted.
 
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Old 01-14-07, 07:25 AM
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Similar situation

I have travertine tile 3/8 x 12x12" and am ready to get started on installing it in a 10'x8' laundry room. I have 3/4" plywood subfloor, on pairs (sistered) of 2x10's at 16" on center which span 12'. I planned to use 1/4" hardy board (as opposed to 1/2") to keep the floor as close in height as possible to the adjacent hardwood floor.

Is there a way to determine if this meets the load (e.g. L360? L720?) requirements for the floor? I haven't found any tables or methods to calculate this. Or do I need yet additional structural support?

Thanks
 
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Old 01-14-07, 08:30 AM
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You are ok for L720, but there is another criteria for natural stone. All natural stone needs two layers of plywood totalling 1 1/4" in thickness. L720 refers to deflection along the length of the joists, you are ok there. The floor also has to be stiff enough between the joists as well and thats the reason for the 2 layers of plywood requirement. You need to add a minimum of 1/2" bc or better exterior plywood before you can use a cement board or membrane. If height is an issue, using a membrane such as ditra or noble cis will help a little. You can make a custom wood saddle to transition the height difference. Its pretty much not possible to keep the height the same for natural stone so this is a very common scenario for natural stone.
 
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Old 01-15-07, 06:08 AM
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Thanks HeresJohnny for your prompt and complete response. For my understanding, does the additional plywood arrest bending between the beams by loads from above (people, appliances) or it is to prevent horizontal movement, eg. as the house settles? It seems I could address the former by stiffening from below, e.g. adding 2x4s in a truss pattern long the length of the joists or some similar approach, whereas the latter plywood sheets from above would be the only solution.

Once additional plywood is added, does it matter if I use 1/4 or 1/2" wonderboard?
 
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Old 01-15-07, 12:14 PM
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The second layer of plywood has to do with the movement between the joists, not "horizontal movement" as you describe it. Bridging between the joists from below will not help here.

1/4" wonderboard is all you need. As a general rule, 1/4" is for floors and 1/2" is for walls. You can use 1/2" on the floor as well but it does not provide any additional structural support.
 
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