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Could someone help me? I am in the process of remodeling a Front Gable New England Colonial house. The homeowner wishes to re-roof the house in slate.
The first thing I did was to verify that the roof trusses would be able to take the roof load (~950 lb/sq). Being an ex-engineer, doing this was no problem, or so I thought. The house has basically a T-shape. The roof plan can be considered a top heavy T, consisting of a major axis-the top part of the T, and a minor axis-the front gable-the stem of the T. Both sections of roof are framed using trusses. The room underneath where to two truss-axes meet is open and the owner does not wish to change that. The trusses along the major axis are Fink shaped trusses spanning 27 feet, 24″ O.C., with lumber rated at 2100 psi and 1.8 modulus of elasticity (2100f-1.8E). The major axis trusses are in good condition and will be able to carry the load. The only minor axis truss that is visible is the one that is at the intersection of the top of the T and the stem. This truss spans only 13 feet; however, this truss is also being used as a girder. As a girder, this truss supports the ends of the major axis trusses along its 13-foot span. The girder truss is sheathed with plywood making it possible to see only one face of the truss; unfortunately, there is no visible grade stamp on the unsheathed side. The bottom cord of the girder truss is a 2 X 8; the remaining cords are 2 X 4’s.
After calculating the loading, one of the top cord members must carry 10,358 lbs in compression under snow load conditions (40 lbs/sq ft), which means that this member must resist 1973 psi of compression. Taking 15% credit for snow load conditions the lumber would have to be rated for approximately 1720 psi in compression. This would be fine, if the lumber were rated for 2100f-1.8E, but I am not sure. Do you think that I should assume since the rest of the lumber is rated 2100f-1.8E that this girder-truss lumber is rated the same? Apparently, the original builder wanted extra stiffness by nailing and gluing sheathing to the girder-truss; however, this makes it impossible to see the rating stamp
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If you can't visually verify data you NEED to know, why even consider going ahead on an assumption?
The truss engineering drawings should be available with the prints filed for the permitting process. If you know who the builder was you could check with him/her. The truss fabricator might still have a record of their work. The owner might be able to come up with a copy of the prints and specs since he should have had a copy.
Or, failing that, plan to tear in there and make it right.
*The builder and architect have failed to return my calls. The permits and drawings just say 'built-up girder'.
*Martin -As the unofficial resident "truss guy", I'll take a stab at this. You've definitely done your homework. I get a lot of questions about old trusses, but rarely does anyone take any time to investigate lumber grades, etc. But there are a few things that concern me here. First, why did the guy sheath the girder truss with plywood ? That makes me suspicious - I wonder if he was covering something up. Maybe part of the girder was broken or something. The second thing that concerns me is the added load on the main trusses. You mention a 40# load, but don't say whether that's the live load or total load. I don't know, since you don't mention what part of the country you're from. But trusses are typically designed for a 10PSF dead load on the top chord. This includes the weight of shingles, plywood, and the truss itself. If the new roof is 950# per square, that's 9.5 PSF. That basically wipes out almost all of your allowable dead load. Even if the lumber in the truss works, have you considered the plates ? They may be undersized with the added load. Particular attention should be paid to the heel plates and the bottom chord splice plate. What about additional deflection of the trusses ? That may or may not cause problems depending on how the house is constructed. You ask about assuming what the bottom chord lumber is on the girder truss. Typically, a truss company is going to carry high grades of lumber in the 2X8 size, since lumber that large is generally used in girders and pole barn trusses. But every company is different, and you never know. You mention a lot of lumber values, but not the species. Have you checked that out? How about the approximate age of the trusses? The pitch?You mention the homeowner wants to keep the area under the girder truss open. Would they maybe go for a beam underneath it ? That way you wouldn't have to worry about beefing up the girder truss.
*BossThank you for the thorough reply. Yes, the 40 lbs/sqft I'm speaking of is live load. The house is located in a suburb of Syracuse, NY (snow country). And, I used a dead load of 12 lbs/sqft.From my inspection of the girder truss, I did not see any visible signs of checking or cracking. The sheathing appears to be the work of an ultra-conservative builder, and having remodeled other houses built by him, generally, I've observed good workmanship and attention to detail. But, I am somewhat concerned by the covered grade stamp, along with the architect not returning my calls. The builder has retired and moved to another area, I believe that eventually he will return my calls but I need to get the work done.I agree-the type of lumber is critical. But that is the problem, I do not know. The other trusses are constructed of machine graded (2100f-1.8E) SYP 2 X 4's. This lumber has a compressive rating of 1875. And, I probably was not clear, the bottom cord (2 X 8) of the girder truss carries the load of the intersecting trusses. The girder truss is resting on a double top plate, which is directly supported on a built-up post of 3-2 X 4's studs on each side. The girder truss would place a max load of 5900 lbs under snow load conditions, which would compress the plate, but the built-up post should carry the load. I do share some of your concern about the ability of this arrangement to support the truss, but the design seems reasonable for construction grade lumber. Since the house has been standing since 1987 and survived several winters, and the live load is the major component of the load, not seeing any signs of compressive failure has given me some assurance.I have also suggested a steel beam boxed by finished lumber, but the owner does not like that idea (I also suggested using a chamfered ceiling to disguise the beam, but the owner bulks at the added cost).
*Boss,Sorry, I forgot to mention the pitch is 5/12.
*SYP lumber in New York ??? I didn't know that it would be practical to ship it that far. Does the Bottom chord of the girder appear to also be SYP? If it is, the lowest grade it could possible be is #2, and I doubt it would realistically be less than #1. (I think TPI specifies that chord lumber not be less than #2) You could calc it out using #2 lumber and see what you got. You should be able to tell just by looking at it if it's SYP or SPF. A 5,900# reaction on a 2X4 SPF plate isn't out of line. I doubt that would be a problem. I'm still a bit concerned about the truss plates and deflection from the added load. You don't say whether you've addressed those things or not. Dealing with old trusses gets a bit dicey, as NDS and ASNI have changed since the time this house was built. Plate values and designs have also changed. I doubt the truss manufacturer would keep any paperwork from that far back - Most places keep their stuff for 7 years.
*Martin, you're on the right path, however, is there any evidence the roof ever had slate to begin with? Many homeowners like to think their house was the cream of the crop back when, but a colonial could just as likely had cedar shakes, or even (gasp) asphalt shingles (not all new england homes are that old). Just a thought to chew on, even shakes are cheaper than slate and may even look more appropriate.
*Jeff, this Antique has been standing since 1987.Martin, has the customer seen the new fake-slate lightweight composites? One major builder supply had them on display (ABC),and from 20 feet away, they looked reasonable.I'll assume they're unwilling to tear it all off and timberframe?Rats. Didn't think so.
*I had a look my self at the new rubber slate and was impressed even at two foot away. It sounds like a great option in your case. Its a whole lot quicker to install too. Good Luck.
*Martin, While your figureing, and if you decide to go there , don't forget to look at header sizes, and if, there is any vaulted ceiling's or "buried point's of attachement".I just got thru throwing away a 3 year old concrete tile roof, 11 lbs p.s.f.- it was overloading this home.Hopefully the roofer's will finish installing the new metal roof at the end of this week.Best of Research to You. Jim
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Could someone help me? I am in the process of remodeling a Front Gable New England Colonial house. The homeowner wishes to re-roof the house in slate.
The first thing I did was to verify that the roof trusses would be able to take the roof load (~950 lb/sq). Being an ex-engineer, doing this was no problem, or so I thought. The house has basically a T-shape. The roof plan can be considered a top heavy T, consisting of a major axis-the top part of the T, and a minor axis-the front gable-the stem of the T. Both sections of roof are framed using trusses. The room underneath where to two truss-axes meet is open and the owner does not wish to change that. The trusses along the major axis are Fink shaped trusses spanning 27 feet, 24" O.C., with lumber rated at 2100 psi and 1.8 modulus of elasticity (2100f-1.8E). The major axis trusses are in good condition and will be able to carry the load. The only minor axis truss that is visible is the one that is at the intersection of the top of the T and the stem. This truss spans only 13 feet; however, this truss is also being used as a girder. As a girder, this truss supports the ends of the major axis trusses along its 13-foot span. The girder truss is sheathed with plywood making it possible to see only one face of the truss; unfortunately, there is no visible grade stamp on the unsheathed side. The bottom cord of the girder truss is a 2 X 8; the remaining cords are 2 X 4's.
After calculating the loading, one of the top cord members must carry 10,358 lbs in compression under snow load conditions (40 lbs/sq ft), which means that this member must resist 1973 psi of compression. Taking 15% credit for snow load conditions the lumber would have to be rated for approximately 1720 psi in compression. This would be fine, if the lumber were rated for 2100f-1.8E, but I am not sure. Do you think that I should assume since the rest of the lumber is rated 2100f-1.8E that this girder-truss lumber is rated the same? Apparently, the original builder wanted extra stiffness by nailing and gluing sheathing to the girder-truss; however, this makes it impossible to see the rating stamp