Replies
-
andy_engel | | #1Is this an existing structure?
-
nomorecoffee | | #2Yes, this is an existing structure. The house is a 1950s cape. The attic was finished later. All douglas fir construction as far as I can tell but the roof rafters are 2x6, actual dimension of some of them closer to 5". The insulation is haphazard at best. Plan is to tear out all the walls/ceilings, build vent baffles, insulate, and put up new drywall. Since there is so little rafter bay space, I'm planning to build out the structure a bit but I wanted to make sure the additional material won't over-stress the roof structure since the roof was just redone with thicker shingles and they threw another 1/2" of plywood over the old 1x board roof sheathing.
-
MJDuval | | #3Good morning. I'm a practicing Engineer in NY. I don't believe that your using the table correctly. The ground snow load is 50 psf. With a 7 on 12 pitch roof, just above freezing, partially protected from wind gives a sloped roof snow load of 38.5 psf. The code min. might be 30 psf, but you wouldn't ordinarily design for the minimum.
-
nomorecoffee | | #4
Thank you for answering. I'm not surprised I'm not using the table correctly. However, I'm not designing the structure, I was checking if it's safe to add dead load to the rafters and now I'm trying to figure out how my existing 2x6 rafters are meeting spec for snow load.
I found the AWC max span calculator here:
https://awc.org/codes-standards/calculators-software/spancalc
According to the calculator, for No. 2 douglas fir 2x6 rafters 16" spacing for deflection limit L/240 or L/180, max span is 12' 1" under 10 psf dead load + 30 psf snow load so I pass. Anything more challenging than that, it drops below 11' max span so I fail. Roof looks just fine to me. Is this just a new code vs old sconstruction issue? The roof has been standing for 70 years and I'm guessing if I ended up with 12 psf of deadload it would keep standing just fine but this has me jammed up. Might be time to hire a structural engineer.
-
MJDuval | | #5
Good morning. One thing that isn't always considered with an older existing structure with a "substandard" roof structure is that the original structure may not have been insulated, or insulated well. Adding attic insulation actually causes snow to remain on the roof for extended periods of time rather than melting off. It's as if your adding a new snow load to the roof. There have been many roof failures due to this. You didn't state the rafter span, but for a typical house width the rafters would be considered substandard structurally.
-
nomorecoffee | | #6
That's what I was afraid of. The attic was unfinished originally with only the 1/2" thick layered paper insulation. The horizontal span of the rafters is just under 12 ft. I don't have exact measurements because it's finished. The 1st floor joists are ~11' 4" span sill to beam. Is there something I can do other than rebuild the roof with 2x10 rafters, like glew and screw 2x4 to the bottoms of the existing rafters to reinforce them?
-
andy_engel | | #7
Possibly making up for the increased snow loads is that Doug fir from the 1960s was able to span more than today's charts will show you. Because it was old growth, it had greater strength. The AWC has derated Doug fir at least once and maybe twice in my memory.
-
-
andy_engel | | #9
Collar ties don't actually do much to help with gravity loads. Their purpose is to hold the top of the roof together to better resist wind loads. What might help is purlins or strongbacks across the rafters mid-span, supported by struts running back to a center bearing wall. But this is getting into engineering, and it's mainly a detail I remember from the old CABO code book.
-
nomorecoffee | | #10
There are collar ties and knee walls for most of it. Also I measured wrong, the roof slope is 9/12, not 7/12.
I had an engineer take a look at it. He said that rafters and joists are undersized but, with everything, the structure should be strong enough to support snow and dead loads if I add brackets to hold the bottoms of the rafters on the top plate and bolt the collar ties to the rafters. Something like solar panels would require reinforcing the structure.
-
mike_augustine | | #11Quickly running the details you've given thru the Forte design software, (12' horizontal span, 9/12 pitch, 12lb dead load) a single 2x6 DF#2 @ 16" O.C doesn't pass 50 psf or 30psf snow load, and it's not even close. But a doubled 2x6 passes both. But Forte assumes a structural ridge, a more demanding setup than you have. Even if we don't know the exact extent, your collar ties and knee walls are doing something just by being there (especially the knee walls).
Planning to add insulation between rafters and wanted to make sure the result meets code. Could use some help, please.
The roof is 7/12 pitch, horizontal span just under 12′, 2×6 douglas fir rafters a bit over 16″ O.C. I looked up the snow load for my town in MA and it’s 50 psf for ground snow load and 30 psf for min flat roof snow load. The estimated dead load from the insulation, roof shingles and sheathing, drywall ceiling, etc is 10-11 psf.
I’m assuming no.2 douglas fir 2×6 which gives me Fb=1345 for normal duration and 1545 snow loading from the tables. Based on the span tables on the AWC web site, I’m just short of 10 psf dead and 30 psf live load requirement, which is 1500 Fb for 11-11 2×6 @16′ O.C. Does this mean my rafters aren’t up to code for the snow load because they’re 2×6?
Am I using this right? Structure is dead load on rafters, snow is live load. I’d like to meet the conservative ground snow load but it is what it is. Compare the Fb for snow loading in the span tables. Weight of the rafters themselves doesn’t count as load.