What is the best material and procedure to shim-up the floor joists and rim joist over an existing first floor plate that “dips” about 5/8 inch in the middle of a 42 foot span. I am converting a single story ranch to a cape with 12/12 pitch roof. The plans call for the roof rafters to “sit” on the second level subfloor. I am planning on using CDX plywood ripped to the required thickness under each floor joist that requires shimming. I would cut each shim to about 3 1/2 X 3 1/2 to catch the rim joist and allow the floor joist (TGI’s) to bear fully on this shim. This will allow me to level out the floor and make the roof framing uniform so that the rafters can all be pre-cut before we tear off the old hip roof. Will the CDX plywood be suitable and minimize shrinkage so that there are not structural problems later. Some have said to use dimensional lumber for the shims, but I think this will be more of a problem with shrinkage. What do you guy suggest?
Darin
Chester, NJ
Replies
I would use something hard, heavy, and rot resistant. Black locust, ipe, galvanized steel, Hardiplank, etc.
Install a 2x4 on perimeter walls , shim the 2x4 . Then cut your rafters with a birdsmouth to sit on the 2x4.Use anything that fits for shims, I use underlayment shingles. Plywood works fine, so does solid lumber. If the thickest shim is 5/8" you wouldn't get enough shrinkage to be able to measure anyhow.
mike
You can use about anything for the shims themselves. I would probably use cedars.
But I think something isn't quite clear here to me. I would not perch the roof rafters ON the subfloor. You need something a bit more substantial. Unless of course, each and every rafter is placed directly above each floor joist to avoid deflection. There are times when Mikey is right and you would be better off placing a 2x4 or 2x6 plate down and shimming it straight first and other times when the individual shimming is easier.
Hooray on you for making it right
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Piffin,
Thanks for the post. I should have clarified the design. First, there will be a continuous 2 x 4 "shoe" all the way around the perimeter and the roof rafters will actually have the birdsmouth seat cut bearing on the 2 x 4 plate, not the subfloor. In addition, I have laid everything out so that the rafters do in fact sit directly above the floor joists. According the the architect, this is not necessary, but I've done quite a few structural enhancements that weren't necessary. It doesn't really make the task that much more difficult, just a little more advance planning!
A local building inspector told me that I should not use plywood to shim the floor and rim joist because they will shrink and will not be supporting the roof/floor loading and will eventually cause squeeks in the floor. I do not think plywood will shrink enough to be worried about and thought his suggestion that ripping green doug fir to size would actually result in more shrinkage. Looking for opinions from the highly experienced crew here on breaktime!
Darin
Use MDF..if any thing it'll swell. Dense as all get out..or old asbestos shingles..I know, I know..don't say the word. We USED to use them for lally colum bases..then pour the floor..long time ago.
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Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations.
You're right about the ply/fir. Still, how much would the fir shrink? Ply would be perfect as long as it's not soaking wet. Plus, you can probably find enough scraps of different thicknesses to make any height you want.
Jon Blakemore
I don't think yuou need to stack the rafers over the joists, as long as you use the rim board across the ends of the I-joists.
Most rim board is good for something like 2,000 PLF.Opportunity is when luck and preparation come together.
green anything will shrink more than plywood.
any wood can be compressed too with enough force but you aren't building a stadium, I don't think.
Plywood is more dense then cedars and less likely to split out into splinters than fir.
So you are adding a plate over the floor system, but you want the floor framing to be level so you want to shim under the I-joists, but you cannot add a plate there because the existing roof is being kept for awhile, right?
So you shim individually with whatever.
If you and the inspector are looking for something denser than wood, there are snapaway shims made of composite materials but I don't know how they handle the larger anils without exploding. Just thinking out loud here.
Really, any wood is OK, Think about it this way - if you shim a joist with something harder and denser than it is, what will sufer crushed fibres if it is overloaded to that extreme? Put a steel shim betyween a wood top plate and a plywood I-joist, and the softest of the three will be the one that suffers.
Now, I don't believe that anything will be hurt simply because the loads just aren't that great. If you shimed under a sill on foundation with cedars at four feet OC and built a two or three story house on it, you might see some of them crush a bit! But you are individually loading at 16" OC and only supporting a floor system and roof.
So you use your plywood. make it 2-1/2" wide maybe to spread load.
If Mr Inspector is really anal about denser material, try running masonite by him. Really dense. But then when he OKs it, ask him what will happen when it decomposses from water.
Me, I'll stick with cedars because they are easy to use, cheap, and have a track record. We've been shimming houses with them all my life.
and the scraps are good for kindling
;)
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