Hello,
I am finishing my attic. It is made with storage trusses to create a room that is 12×40. The trusses have 2×6 rafters and 2×8 joists to make the attic floor, spaces 24″ on center. (I have also built 14 cabinets and mounted them in between the studs of the knee wall.)
The floor flexes a little when walking on it, especially along the edges of the kneewall. I am interested in putting down another layer of subfloor. Would this make it sufficiently stiff and would it be too much weight?
Thanks, Eric
Replies
BUMP!!
I would like to hear this answered
One of the best fellows around here that can talk about your question is BossHog. He'll probably happen by sooner or later. Good luck.
Don K.
EJG Homes Renovations - New Construction - Rentals
If just the subfloor is flexing and the trusses are designed for storage, I can't imagine that a second layer of subfloor would be too heavy.
You don't tell us what the span of the truss is or how thick the existing subfloor is.
Rich Beckman
Another day, another tool.
Thanks for pointing out that I forgot to mention the span and the material. The span of the truss is 25ft and the subfloor is 3/4" Advantech, which is an osb that I believe is especially weather-resistant.
This will be a "Quilting Haven" for my wife and have the occasional overnight guest with a futon.
Thanks, Eric
Edit:
After reading a little closer to actually understand the original question I'll simply say the trusses were designed for a certain load in the "attic" area and additional decking won't increase that, only better distribute weight between supporting members.
If the free span of the lower 2x8 portion of the truss is anywhere near 25' the space wasn't designed for much other than holding up sheetrock.
Second edit:
If there is a halfway support so the upper area spans something like 12' you'll still need additional reinforcment for use as a livable area and still maintain enough stiffness to keep the sheetrock below from developing cracks.
If it weren't a truss, but separate joists supported at the midspan, it would require adding additional 2x8s inbetween what's there now to bring the load carrying capacity up to a reasonable level.
Knowing the shape of the trusses is important, including the location and span of the "livable attic" space. At least our truss designer would also insist on a site visit to see for himself how the trusses are constructed and any other details that might impact the load capacity of the truss design before saying definitively what the setup would hold.
Happy holidays,
Don
Edited 12/17/2005 8:16 pm ET by IdahoDon
Edited 12/17/2005 8:26 pm ET by IdahoDon
Hope this helps clarify: The house is 25 ft wide by 40ft long. The "room" created by the storage trusses is 12 ft wide by 40ft long.
Eric
Just to clarify... the flex you are talking about is the plywood only, and especially near the edges because it's not blocked?
Blocking along the edges is a good idea, and you should check the engineering that came with your trusses to see what loads they were anticipating.
Hello,
Correct, just the subfloor is flexing, not the joists as far as I can see. There is no blocking along the edges. I will at least do that if I don't put down the 2nd layer.
Unfortunately, I don't have any engineering documentation for the trusses.
Thanks, Eric
Engineering may be on file with the building permit for the job, at the department office. Sometimes the inspectors just look at it onsite and don't take a copy. Probably doesn't matter as long as you don't add a lot of weight. The edge blocking will help the flexing immensely.
One of the techniques used to stiffen a floor is to add an additional layer. It's recommended and done all the time for tile underlayment.
Advantech is T&G and the interlocking enhances the stiffness of the boards but if your boards end at you knee wall and blocking between the joists is not installed at that point you will experience flex at that point, especially at 24" oc.
Apparently you've already nailed/screwed down the floor and it would be a real pain to take it up to provide cross-blocking, unless you can get to it from below. If you can, go for it. Or, at least, block the edge of the boards where they end at the knee wall, which you should be able to reach from above.