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Unread 12-03-2019, 11:02 AM   #1
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Location: Sault Sainte Marie, MI
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Joist Deflection

I have a deflection issue in a tile project I want to do. The span of the room is 15' 3". The joists are 12" on center. The original joists are 2x8's of unknown type but good condition. Previously I had sistered some 2x8's under the room to remove some of the bounce. I did it every 3rd joist. Now, several years later I am wanting to tile the floor. I sistered 2x10's to all the joists that were still single. So the room now has a doubled 2x8 joist then two doubled joists made up of a 2x8 and 2x10. (The 2x10's are notched at the ends to sit on the sill and the ends have a piece of plywood glued and screwed to keep the 2x10's from splitting at the sill).
Using the deflecto calculator, with measurements of standard 2x8 at 3" wide with 12" spacing and 15' span (3" short of actual span), the calculator gives a measurement of L/339. When I run the calculator to account for the 2x10's (kind of split the difference in heigth), I used a measurement of 8" high, 3" wide, spacing 12" and span 15'. The calculator gave an L/442.
My question is whether the joists with a L/442 on either side of a joist with a L/339 would kind of offset the deflection difference. Any experience or advice would be appreciated.
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Unread 12-03-2019, 12:12 PM   #2
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keep in mind the "span" measurement for the deflecto calculator is NOT the span of the room. (width or length of the room). It can be much much larger, or much much smaller. The deflecto calculator wants the "unsupported span".

You need to look at the structure UNDERNEATH the room. For example, if this is a second story room, look at the first story, and see what rooms/walls are under it. The spacing of the walls or beams under the room is the unsupported span. If this is a first story room, look at the basement, or look at the posts/beams under he room if you have a crawlspace.
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Unread 12-03-2019, 03:09 PM   #3
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I haven't thought about the implications of having one out of three joists different from the others, nor about the detail you've described for notching the 2x10s over the sills. But I can tell you how to use the Deflecto to model your composite 2x8 + 2x10 joists:

First model each one separately, I'll assume Unknown Wood in Good Condition and modern dimensions, and I'll round the span up to 15'6" to be conservative. For 2x8 at 12" o.c. I get L/155. For 2x10 at 12" o.c. I get L/298.

For the composite, you just add the denominators, so you're at L/453 (at least for those 2 out of 3 joists). The reason this works is as follows: L/155 means that, say 155/1000 times the standard load would deflect the joist L/1000. And L/298 means that 298/1000 times the standard load would deflect that joist L/1000.

If you are going to deflect both joists simultaneously to L/1000, you'd need to add those loads, in other words 453/1000 times the standard load. Which means the standard load deflects the composite joist L/453. [The use of the factor 1/1000 was arbitrary, it's just to illustrate the point and to keep the loads involved in the linear region.]

Cheers, Wayne
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