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Old 03-05-2010, 10:44 AM   #1
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Floor Joist Spacing


Hi Y'all!

First post, long time reader!

We are having a house built and I have been diligently keeping up on the construction of the house. However, yesterday was the first time I was able to see the first floor joists. The first floor deck was laid a month or so ago, and I never got a chance to see the joists before the deck went on.

The basement slab was just poured, and I took a peek in through the egress window and snapped this shot. What caught my eye on the photo is the floor joist spacing. They look to be varying around the 16"OC mark, but one set looks to be around 24" OC. I am only guessing here and using the 24" form lines on the concrete as a guessing guide. The decking is 3/4, non-T&G OSB.

They builder is "looking into it for me," but what are the thoughts of the folks here on the Chatroom?

Directly above the joist space is the kitchen and breakfast area which will be tiled. My concern really is floor flex which would quickly lead to cracked tile and grout.



Thanks!
Morgan

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Old 03-05-2010, 10:56 AM   #2
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Floor Joist Spacing


Depends upon the I-joist & what they are rated for

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Old 03-05-2010, 11:08 AM   #3
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Floor Joist Spacing


Do you have a set of your plans? Check them to be certain of what is supposed to be there.
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Old 03-05-2010, 11:10 AM   #4
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Floor Joist Spacing


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Originally Posted by Scuba_Dave View Post
Depends upon the I-joist & what they are rated for

Is the rating something that is stamped on the I-joist?
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Old 03-05-2010, 11:11 AM   #5
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Floor Joist Spacing


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Do you have a set of your plans? Check them to be certain of what is supposed to be there.

I do not. The builder does not release the plans until the development is done. However, they will let me look at them. I will look into them on Monday.

Thanks for the suggestion!
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Old 03-05-2010, 11:46 AM   #6
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Floor Joist Spacing


My I-joists are stamped - but that is only the size
Loads & span dictate what size joists & how far apart
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Old 03-05-2010, 12:39 PM   #7
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Floor Joist Spacing


It looks like the joist to the right of it is spaced a bit smaller than the others. Could it be spaced that way to accomodate HVAC trunks?
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Old 03-05-2010, 03:43 PM   #8
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Floor Joist Spacing


The key number in describing flex of a joist is the d/L ratio, which is the ratio of the center deflection under load to the span length of the joist. Typically this number is computed based upon the code mandated floor loading in psf, which is generally a function of room type. The d/L ratio depends on the span length of the joist, the moment of ineria of the joist, and the modulus of elasticity.

Most folks are not structural engineers, so they have the designer of the house take care of sizing the structural elements. For large format floor tile, you want a ratio like 1/720 (very stiff), versus for hardwood floor you can live with 1/240. Your plans are not likely to list the d/L ratio, but you should be able to ask the builder to get the number from the engineer of record. If the floor is not going to be stiff enough for your liking, there are techniques to improve the stiffness, of course at extra cost.
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Old 03-05-2010, 05:04 PM   #9
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Floor Joist Spacing


Quote:
Originally Posted by Clutchcargo View Post
It looks like the joist to the right of it is spaced a bit smaller than the others. Could it be spaced that way to accomodate HVAC trunks?
I had noticed the same thing. Is it possible that there is a wall directly over the joist on the right side of the 24" spacing? Or is the spacing open on the floor above?
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Old 03-05-2010, 07:10 PM   #10
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Floor Joist Spacing


Daniel- Thanks for the education in I-beams! I really appreciate it!

There is a chance that the space is for the HVAC return trunk to pass through the floor in a mechanical chase closet above the floor on that fall wall in the picture. HVAC started going in the house today. I was out of town, so hopefully I will get up there in the morning to check it out.

Thanks again everyone for the input!

Morgan

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