Roof/Rafter

[Polaris Junky]

I am trying to figure out all of the roofing load factors. My area requires a 60/70 ground snow load. What determines the values for dead loads and deflection? How do I determine my live load? I am trying to build a 24x40 Cathedral ceiling with 14ft walls and a 12/12 pitch with a 1 ft over hang. Everything I see shows using a 2x12 syp#1. Some of the builds I see show what appears to be 2x8/ Any help would be appreciated. It may be easier to drop to a 20x40. Opinions please?

Thanks PJ

[Don_P]

The 60/70 for live load (snow) is unclear to me, I'd use 70lbs/sq ft unless that can be clarified. Dead load (the self weight of the building materials) on a 12/12 is often assumed to be 15 psf unless something unusually heavy is being used, such as slate or clay tile roof. So total load of 85 psf. Live load deflection on a roof is code limited to L/240, you can always go stiffer.

for that load and span it looks like a #1syp can span 12'10" @ 24" oc... at that load you are not in syp country... a #2 spf 2x12 @24" oc is good for 12'0".

A 14' tall sidewall is outside of the prescriptive 10' limit.

[Polaris Junky]

What are my options for the 17 ft span I need for the 12/12 roof from the top of the wall to the peak?

[Don_P]

for that load and span it looks like a #1syp can span 12'10" @ 24" oc.

Oops, for that load and span a #1syp 2x12 can span 12'10" @2' oc
Rafter span is given as the horizontal measurement, with a 24' typical gable roof the rafter span is going to be a few inches under 12'.

[Polaris Junky]

Can longer lengths be obtained by adding collar ties?

[Polaris Junky]

Sand, Silt or Clay Sand Soil Types, Ground Snow Load = 60/70 psf From Figure R301.2 (5) MCR or MRC table R301.2(5)

[Don_P]

Can longer lengths be obtained by adding collar ties?

no, collar ties in the upper third of roof height or straps over the top connecting together rafter pairs over the ridge are required every 4'.

Where are you located?

Edit:
You can usually obtain longer spans by using a different species or grade or tighter oc spacing. Beyond that engineered materials can usually get you into longer spans for a price.  For instance select structural Dougfir-Larch 2x12's@24"oc can span 15'8"... a 32' wide building. The same rafters at 12" spacing can span 22'1", you could span your building the long way, if you can find rafters 30' long.

[Polaris Junky]

The property is located in Kalkaska County, Michigan. I may go with an engineered truss system. Also, I was going to run 16" OC with dimensional lumber.

Thx for the help

[UK4X4]

I'm at 90# snow load, I looked at every way for dimensional lumber, but at the end of the day the trusses were engineered and built for not much more than the timber and labor costs of a carpenter.

I got peace of mind and a huge space, we just went with a platform build and 6ft sidewalls upstairs the vault is so large we added an attic too at one end