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Asphalt Shims

J

Jim Katen

Member
Joined
Oct 25, 2009
Messages
11
Location
Gaston, Oregon
R502.6 states: Bearing. The ends of each joist, beam or girder shallhave not less than 1.5 inches (38 mm) of bearing on wood ormetal and not less than 3 inches (76 mm) on masonry or concreteexcept where supported on a 1-inch-by-4-inch (25.4 mmby 102 mm) ribbon strip and nailed to the adjacent stud or bythe use of approved joist hangers.Quaere: In the attached picture, does the beam have adequate bearing per 502.6?

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/monthly_2011_11/572953e43a0ee_AsphaltShim6.JPG.f235d36ff60f30426566aa2d553c5fe6.JPG
 
Compressible material, need to head to work and can't recall the section off the top.
 
Jim,

Welcome to the BCF. Shims must be wood or metal and protected from decay. Additionally material must be approved for the intended end use i.e. cannot use osb and sheathing.
 
Jim Katen said:
Quaere: In the attached picture, does the beam have adequate bearing per 502.6?
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If it has 3", then yes it has adequate bearing. I wouldn't allow the shingles for no particular reason other than the fix is easy.
 
Shims are allowed but they cannot be compressible. I would contend that shingles are compressible and would not allow it. Shims for girders and such are usually metal (sheet steel) or concrete board (hardyplank etc). I would argue that cut wood shims are compressible also.......ducking now. Let er rip......

And of course....welcome.
 
DRP said:
Compressible material, need to head to work and can't recall the section off the top.
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Jim Katen said:
What reference would support that contention?
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My bank statement would support the contention that I needed to go to work :D

My memory was imperfect, R601.2.1... but I think the intent goes further than just on top of the floor

Francis, can you explain/support the prohibition on osb or sheathing? They have Fc values at least as high as wood IIRC. Daddy-O-, I think there is/was a requirement for hardwood or steel but I didn't come across it.
 
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I'm still not buying it. 601.2.1 is from chapter 6, which is about walls. It's usually a mistake to take a specific requirement from one chapter and apply it to an item that isn't governed by that chapter. In other words, I don't think it's reasonable to divine intent from one chapter and apply it to another.

Over the years, I've heard all sorts of "rules" about what is and isn't allowed for shimming under structural members, but I've never seen any of these rules in print. Shimming is a fact of life in residential construction, so it's odd that the IRC doesn't address it. I think that, if I were to propose a change to address it, I'd specify that the shim material would have to have an Fc at least as great as the thing that it's holding up. Of course, if the IRC were to address shims, it would then have to address the uncomfortable aspect of lateral support when shims are present. I'd think that there would also have to be some kind of limitation on the height of a shim and, if wood, the direction of the grain to take splitting into account.

However, right now, it seems that I could shim a girder with marshmallow peeps and the IRC wouldn't have a thing to say about it. (Maybe PETA would get upset, though.)
 
All materials must be used within the allowances of the manufacturers specifications. Will the shingle manufacturer specify their product to support a structural load? Not likely. Will the beam manufacturer allow the (highly) compressible material to support their product? Again, not likely. Lastly, would the Design Professional of Record allow it? (I know, Oregon + Single Family Dwelling = NO DPR...).

Asphalt shingles aren't covered in Chapter 3, 4 or 5, so their use in this application would not be prescriptive. If an element of the construction falls outside the scope of the prescriptive methods, it shall be 'designed'. As a part of the structural system, the shim must be capable of supporting all imposed loads - dead, live and environmental. The shims should be no softer than the beam they support.

BTW, I'm pretty sure ICE's question was asked in jest...
 
Any structural member must be "sufficient to support imposed loads." A shim under a girder must be capable of supporting the entire load that the girder can handle. Similarly, when posting point loads of for instance a PSL post, you must use PSL blocking below. Compressive strength of shims or point loads or posts or whatever must be the same as the designed load above.

If you shim one floor joist with a wood tapered shim cut properly with the grain, no big deal. Wood shim can handle the load of one joist. Wood shims cannot handle PSL or LVL point loads or most loads imposed at the end of a beam in a beam pocket. It is simple engineering. You won't find a code section prohibiting wood shims but I know there are several for loading. My 2€.
 
JBI said:
All materials must be used within the allowances of the manufacturers specifications.
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It would be nice if that were true. Many materials have no "manufacturer" (milled lumber, for instance) and some other materials are used outside the allowances of their specs all the time -- and blessed by the AHJ. For instance, asphalt shingles can be found between (almost) every post and the footing that it bears upon in every crawlspace in Oregon for well over 50 years. It's also universally permitted to use a single asphalt shingle in a beam pocket as a moisture break. In fact, if you omit it, the municipal inspector is likely to require you to put it there. This is not within the manufacturer's specs.

Will the shingle manufacturer specify their product to support a structural load? Not likely.
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I hope not. However, I have had roofing shingle manufacturers say just about anything under the sun in order to remain in the good graces of the builders who buy their products. If I have to call this installation wrong, I'll need something better than the manufacturer isn't likely to approve it. Been stabbed in the back by too many manufacturers.

Will the beam manufacturer allow the (highly) compressible material to support their product? Again, not likely. Lastly, would the Design Professional of Record allow it? (I know, Oregon + Single Family Dwelling = NO DPR...).
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There is no beam manufacturer, it's milled lumber. Last I checked, it doesn't come with specifications, just a grade stamp. There's no DPR, not because it's Oregon, but because the builder is, supposedly, using a prescriptive code. And, under that code, the asphalt shingle thing has been universally accepted for as long as he's been building and no one before me has ever said squat about it.

Asphalt shingles aren't covered in Chapter 3, 4 or 5, so their use in this application would not be prescriptive. If an element of the construction falls outside the scope of the prescriptive methods, it shall be 'designed'. As a part of the structural system, the shim must be capable of supporting all imposed loads - dead, live and environmental.
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I agree with all of that. However, the builder would argue that the use of shingles is allowed by convention. He'd also point out that they must be capable of supporting all loads because none of the houses that he's built have ever fallen down.

The shims should be no softer than the beam they support.
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Great idea. Where does it say that again?
 
Daddy-0- said:
Any structural member must be "sufficient to support imposed loads." A shim under a girder must be capable of supporting the entire load that the girder can handle.
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Those two statements contradict each other. I agree that the structural members must be sufficient to support all imposed loads (301.1) but I disagree that a shim has to be capable of supporting the entire load that the girder can handle. That's quite a different thing. A girder might be capable of supporting thousands of pounds but only have a hundred pounds imposed on it.

Still, your overall point is not lost on me.

If you shim one floor joist with a wood tapered shim cut properly with the grain, no big deal. Wood shim can handle the load of one joist. Wood shims cannot handle PSL or LVL point loads or most loads imposed at the end of a beam in a beam pocket. It is simple engineering. You won't find a code section prohibiting wood shims but I know there are several for loading. My 2€.
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I disagree. It's all about the loads, not just the materials. I've seen wood shims crushed to nothing under some joists and hold up fine under others.

I still believe that the IRC should provide some prescriptive guidance on the use of shims.
 
Originally Posted by Daddy-0- Any structural member must be "sufficient to support imposed loads." A shim under a girder must be capable of supporting the entire load that the girder can handle.

Those two statements contradict each other. I agree that the structural members must be sufficient to support all imposed loads (301.1) but I disagree that a shim has to be capable of supporting the entire load that the girder can handle. That's quite a different thing. A girder might be capable of supporting thousands of pounds but only have a hundred pounds imposed on it.

Still, your overall point is not lost on me.

If you shim one floor joist with a wood tapered shim cut properly with the grain, no big deal. Wood shim can handle the load of one joist. Wood shims cannot handle PSL or LVL point loads or most loads imposed at the end of a beam in a beam pocket. It is simple engineering. You won't find a code section prohibiting wood shims but I know there are several for loading. My 2€.

I disagree. It's all about the loads, not just the materials. I've seen wood shims crushed to nothing under some joists and hold up fine under others.

I still believe that the IRC should provide some prescriptive guidance on the use of shims.
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This is interesting, I looked in some of my old books and didn't find anything.

The op does give a cite that this is in violation of. That looks like an engineered joist, if so the bearing is probably not there and not that.

I think the imposed loads are the design rather than the ultimate or actual loads.

A single shingle will probably not make the assembly fail in deflection where a stack of them probably would or would at least change the load path.

For solid sawn dimensional the "manufacturer specs" would be "according to the AF&PA's NDS". "For sawn lumber, the reference compression design values perpendicular to grain are based on a deformation limit that has been shown by experience to provide adequate performance in typical wood frame construction." The Fc perp values are for .04" deformation over a steel plate. For enhanced performance that value is multiplied by .73 to give a design deformation of .02". From that I think we are looking at the design intent of the bearing of solid sawn. I hope you guys don't show up on my job with plastiguage.

The crushed shim under a joist would probably mean that all the shims are of insufficient Fc or dimension, but it also means that shim was probably driven too tight and created a point load, the designed uniform load was shimmed to one joist and created a point load.

An aside, in scribed log construction the bottom edges are taken down to a sharp edge, a certain amount of crushing is anticipated and helps create the seal from log to log.

Just random stabs at it.
 
The shims would have to support the load at the end of that beam (reaction). Using a single shingle to ostensibly separate wood from concrete for decay purposes is one thing...I'd expect to see ice & water barrier instead. I can say I have never seen a stack of asphalt shingles used as a structural shim before, in this area. Hope I never do.
 
If you're worried about use of an inappropriate material, have them submit an ICC evaluation or engineered calculations.
 
DRP said:
Francis, can you explain/support the prohibition on osb or sheathing? They have Fc values at least as high as wood IIRC.
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DRP,APA promotes that plywood and OSB are suitable for shimming having a design capacity for loads perpendicular to the face at 360 psi. However APA cautions the possibility of wood that is not decay resistant and of exterior grade in crawl spaces is what may be swollen and delaminated.

Once again my answer was too short and presumed the picture is in a crawl space.

Good discussion; lots of different opinions.
 
"If you're worried about use of an inappropriate material, have them submit an ICC evaluation or engineered calculations."

Bingo, you get the prize.............
 
There is the true, code required answer from chapter 1. The BO has jurisdiction over this.

Welcome Mr. Katen. I know Jim and he will be a welcome member who will challenge the codes. This post had a great discussion and I am sure we will have more. Thank you Jim.
 
I'm in Canada so I don't use IRC. Our small scale construction (less than 600 square meters and three storeys and less) we use a prescriptive code which tells you what you CAN do along with any exceptions. If the IRC is a prescriptive code then if you question any practices they have to show where in the code it is permitted, or have an engineer sign off on the assembly. When I get into the argument with a contractor ("Tell me in the code where it tells me I can't do that") I remind them the code is a list of things they can do, not can't, otherwise it would start out as "you can't build your house out of gummy bears"
 
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