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Home » Installation » subfloor over 24" oc truss framing?

Unfinished, prefinished, subfloor prep, etc.
11/3/2012 9:09:39 PM

A_the_P
A_the_P
Posts: 44
Hi folks,

I gave an estimate on a job today where the subfloor was laid over 24" on-center trusses. However, when you walk across the carpet in the dining room, the china cabinet trembles. I assume this is because the subfloor is sagging between the joists. The customer wants me to install hardwood running parallel to the joists, so I told them I would first need to lay a second layer of plywood running crosswise over the first layer before I could install hardwood. First time I've run into this kind of subfloor/framing situation: do you all see any cause for concern, or will the extra plywood take care of the problem?

Thanks for your help!
edited by A_the_P on 11/3/2012
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11/4/2012 1:12:06 PM

petesfloors
petesfloors
Posts: 99
The "sag" between the joists is really just deflection. The floor will have a little movement with a load. This is what gives the wood floors more comfort than a tile floor where the whole system is like a platform so there is not much deflection if at all.
There should be blocking in the framing system which allows the load to be spread over more than one joist.
Another layer of plywood would need to be glued to the existing sub-floor to make any difference structurally.
If the furniture is rattling then put some floor protectors under it to provide a cushion so the weight is more evenly spread out.
You may be able to go under the house and add some blocking between the joists where there is too much deflection under a load.
This can be lots of work, go for it. Try the peel and stick floor protectors first though, one under each corner and along the longer span.
Pete
edited by petesfloors on 11/4/2012
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11/4/2012 4:59:46 PM

Stephen Perrera
Stephen Perrera
Posts: 1010
Wow, 24 inches....when was that built, back in the 1820's. lol I wouldn't touch it with a ten foot pole. Unless someone went under there and worked on it. I ain't no framer.
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11/5/2012 1:49:18 PM

A_the_P
A_the_P
Posts: 44
Pete,
Thanks for the info on deflection; I searched it up and did some reading on deflection, and, as you implied, it doesn't seem to be a cause for concern. I can't reach the framing without removing the ceiling covering in the basement, so my main concern at this point is whether the extra layer of plywood will sufficiently stiffen the flooring so that I can install oak parallel to the joists. I will probably just go with 3/4" glued and screwed, and leave it at that.

grooving,

Actually it's only a few years old. Apparently with truss framing you can go with wider spaces...go figure with all that deflection!
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11/6/2012 4:59:41 PM

Jeff
Jeff
Posts: 314
I laided a nail down bamboo floor over a 24"OC joist floor and it had a huge span over the basement, maybe 30' or more. The home owner call after they moved in and said the floor was causing the framing to fail! "Ok...I will come right over" I said. When I got there she said "watch this..." and she had the kids run across the floor which caused her china cabinet to wiggle and the dishes to clink together. No matter what I said it was the bamboo floors fault becuase none of the other rooms which had carpet and tile didn't do this...(plus they had load bearing walls under them). Still one of my favorite call backs...I drive by once in awhile to see if the house is still standing.
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11/6/2012 10:34:22 PM

TDMAC
TDMAC
Posts: 1348
Wow that sounds like the worst!!! Like the election span...24" OC and the house is still standing>???

A the P...my comment to you would be OK if the plywood is 1-1/8" thick... OMG Joists at 24" O.c must have that ply to run parallel ...OR I would run like hell!
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11/7/2012 1:49:17 AM

petesfloors
petesfloors
Posts: 99
You know the framing system was engineered to meet todays specifications, plywood glued to trusses, saving nails,trusses instead of heavier joists saving lumber. It's faster to build, saving time.
An engineer will tell you that adding another layer won't stiffen the floor unless you glue the ply to each other actually making one thick layer instead of one layer on top of the other. This extra layer will change the load by adding more weight to the floor.
The engineer went through a lot of schooling to design this project with these spec's. You shouldn't need to modify things.
Pete
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11/7/2012 7:25:22 AM

johannes
johannes
Posts: 417
A,
You don't have a choice but to lay another layer of plywood (with adhesive between and using screws to draw it tight!), think about trying to nail over such span. The bouncing will affect fastener penetration (not getting drawn tight).
With such span, even with the additional layer, sagging is more than likely to occur between such wide span if you nail parallel if you can not install blocking from below.

ps; use plywood and not osb for the additional layer as osb still deflects too much over such span. you need at least 1 1/8" thick as Tom suggested. I have screwed/glued 2x4's up against a subfloor to add stiffnes and to reduce deflection where the osb still deflected when walked over it. Don't worry about the weight as the engineered joists are designed for added load, for tile installation it is also recommended to have at least 1 1/8 inch thick subfloor over 24 oc.

Installing perpendicular or under a 45 I would not worry as much.

Johannes
edited by johannes on 11/7/2012
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11/10/2012 2:29:27 PM

A_the_P
A_the_P
Posts: 44
Thanks again for the info, this is good to know. When you say 1-1/8 thick, are you talking about the combined thickness of both layers of plywood? Or do I actually need to get 1-1/8" sheets to glue and screw on top of the original layer, making 1-7/8" total? That seems excessive.
edited by A_the_P on 11/10/2012
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11/10/2012 7:12:09 PM

johannes
johannes
Posts: 417
Subfloor at least 1 1/8" total thickness.

Johannes
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11/10/2012 8:06:26 PM

TDMAC
TDMAC
Posts: 1348
1-1/8" thick plywood, tongue and grooved has been available since the 60's It is strong enough to span 36" spans and be very sturdy.
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