Floor Deflection
Hello, I am building in climate zone 4A. I just got back the shop drawings for my trusses. The designer wants to know if I want them 2′ O.C. or 16″ O.C. the 16″ O.C. is about 25% more money. The problem is I do not know how to read the deflection charts (see attached). I have read some posts mentioning frequency and I do not know what any of that means but if some of you do and it is relevant these are gambrel roof trusses that span 30′. So my question is two fold. First, do these deflection numbers look like they will produce a stiff floor suitable for tile and be comfortable to the occupants? Second, I am using 3/4 T&G OSB for the floor sheathing. Would two layers do much to stiffen the floor or is it a waste of time?
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Replies
Generally you don't want floor deflections above 0.5" no matter the span. So even the 16" is a bit on the high side for bouncyness. If these are bedrooms, I would not worry about it, bouncy floors are more of an issue for larger open spaces. The deflection is low enough that tile won't be an issue with either spacing.
Attic rooms are very hard to insulate and air seal properly. Make sure you have a slid plan on how to get a good air barrier as well as air barrier continuity down to the walls supporting the trusses.
Akos.
Thank you for your reply. There will be 3 bedrooms and 2 baths in this loft (master suite and kids rooms). Do you have a guess on how much "stiffness" a second layer of 3/4 T&G subfloor would add, worth it or it would not do much? Would it need to be nailed to the trusses or just to the first layer like an underlayment? My preferred method is to do the 24" spacing with 2 layers or subfloor.
Thank you for the warning but I think I have that covered. These trusses are over a garage space so the living space inside will be completely sealed off from the space bellow and will have and interior and exterior air barrier.
I would ask your truss designer. They can run the numbers with the extra height added to the flooring.
A way to look at is if you take say a 2x8 and add two layers of subfloor to it, the extra height makes it into a 2x10. So definitely stiffer and can span longer but not a huge change.
With a structure like a roof truss, lot of the load is carried by the upper part so adding this extra plywood to the floor area might not make as much difference.
Akos,
I had heard the example before of the 2x8 and 2x10 but did not consider the fact that trusses support from above. Thanks for your insight.
It's a bit of a combination effort in the case of the truss. The top chord runs in compression, the bottom chord runs in tension. The web, or lattice, keeps the spacing constant between the two. An easy way to stiffen up floor joists in some cases is to glue and nail/screw 2x4s "sideways" along the bottom edge of the joist, making a sort of DIY I-beam. The extra depth gives more stiffness, the width of the 2x4 helps prevent the joist from twisting/bowing side to side.
Just try bending a piece of paper or card stock held with the long sides parallel to the floor, so the paper appears to be standing on edge between your hands. If you try to make the paper curve downwards, the way it would with a "load" on top, you can see the top edge buckle (a ripple will form), while the lower edge will try to pull upwards from the tension. Trusses an I beams are just working to counter those forces to keep the shape of the beam.
Bill
There should be a note somewhere indicating the floor loading for the room part of the truss. The code requirement is that general living spaces should be at least 40 psf, while bedrooms can be 30 psf. If the trusses were designed for 40 psf and you are just putting bedrooms up there, the deflection will be less.
Will there be any walls below? A somewhat large deflection isn't a big issue as long as you don't create a bump in the floor by the transition from 0.75" deflection to 0" deflection (wall below). You have to leave a gap at the top of the wall to avoid that.
Frank,
Thanks for your insight. That is a good point. I revised the original post and attached the entire truss document so you can see the loads and notes. It looks like to me it was designed at 40psf based on note number 12. Am I understanding this correctly?
Yes, that's it.
Note that I just edited my previous comment, in case you didn't see the second part.
Frank,
There is no rooms bellow. This is a 30x40 completely open garage with living space above. Thank you for your help.
Mick,
I'd use 1 1/8" AdvanTech, rather than two layers of 3/4" plywood for the sub-floor.
Malcolm,
What I was proposing was 2 layers of 3/4" Edge Gold subfloor (much cheaper for me to get in my market with the relationships I have with suppliers). You think one layer of 1 1/8" edge gold is stronger than 2 3/4" layers?
Mick,
Probably not, but 1 1/8" is plenty stiff at 2 ft oc. I don't think you need to go to two layers of 3/4" unless there isn't much difference in price.
Malcom,
That is what I was clarifying. Thanks.
Be aware that if you want to use stone tiles or large-format ceramic tiles, you need to design for L/720 deflection, both along the joist and across the joist. Many engineers use 30 psf live load for the entire bedroom level, but bathrooms should be designed for at least 40 psf live load and 10 psf dead load. With bigger, tiled bathrooms with large bathtubs and showers, they should do the actual calculations, which will show dead loads up to 20 psf, or sometimes higher.
3/4" Advantech can span 24" but if you plan to use ceramic tile, it needs to be thicker: https://www.huberwood.com/uploads/documents/technical/Ceramic-Tile-at-24-on-center-over-AdvanTech-subflooring-Technical-Tip-Subflooring-AdvanTech.pdf.
L/360 is the code-minimum deflection. I almost always use at least L/480 deflection, and never more than 0.6" regardless of the span.
I find it interesting that you don't want to spend 25% more on trusses but are thinking about doubling the subfloor, which in my market is $32 per sheet for LP350 or $62 per sheet for Advantech. I notice in your third attachment the trusses are made of 2x4s. I would get a quote on the same trusses but with a 2x6 bottom chord. That upgrade usually isn't too much more expensive and makes a better product. We had all of our trusses made with 2x6 bottom AND top chords. We have 40' trusses so not that much bigger than yours