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Snow load calculations

Started by Ljohnsaw, November 22, 2012, 02:47:35 AM

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Ljohnsaw

I'm sure Jim will respond as he shares his knowledge and hard work freely - I really appreciate the help and advice I've received so far.

I'm beginning to plan my little cabin in the woods and the first consideration (after the floor plan) is snow load.  I just learned today that there are two different numbers.  The county gives me a GSL (Ground Snow Load) and then I need to convert to a RSL (Roof Snow Load).  I always assumed that the GSL you just stuck up on the roof!  I found the GSL for my area (varies widely in the Sierras from 40 to 500 PSF) and an on-line calculator for the conversion.  I made the assumptions that my space is unheated (since I will not be living there full time), the roof slope will be 12/12 and the roof material will be metal.  The building will be in an open area so I'm not quite sure about the unequal snow loading.  The GSL I start with is 303 or 341 PSF (depending on the source, the lower is the building department) and it calcs out to a RSL of 115.69 PSF (I used two different web sites and came up with the exact same answer).

Some of you Easterners may think that the GSL is high, but the average snow fall at 6,000' is 12 feet (144 inches!).  We also have a phenomena known as Sierra Cement.  We can get some nasty warm snow that has an amazing water content and it sticks to everything.  With my metal roof and steep slope, I'm comfortable with the apparent drop in loading from ground to roof.

So, my question is basic.  When calculating the area that the purlins are supporting, is that a plan view or a perpendicular view of the roof surface?  With a 12/12, that is a 40% difference in the weight value!

Also, I'm debating over whether or not to have eaves.  With eaves, you get some shade in the summer but potential ice dams in the winter.  Also, if I do the few principal rafters and purlins, it seems to me that eaves are a bit harder to create.  My cabin will be a story and a half, 20'W x 30'L with the 20' long tie beam at 8' and the knee wall probably 4' so there is room for the brace.  The rafters will have a tie at least 8' up from the floor of the loft area so it ill be approximately 12' long.

Thanks for your comments.
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

Jim_Rogers

Quote from: ljohnsaw on November 22, 2012, 02:47:35 AM
So, my question is basic.  When calculating the area that the purlins are supporting, is that a plan view or a perpendicular view of the roof surface?  With a 12/12, that is a 40% difference in the weight value!

It is a plan view for snow-load and which is the "live load" for figuring loads. But it in the roof slope for figuring "dead load".
When you have both loads figured then they are combined to use as a "total combined load".

The dead load it the weight of the actual roofing materials, including the metal roofing, roof decking (if any), and supporting timbers, including rafters and purlins.

When using metal roofing the "load duration" is usually less because when the sun warms the metal up the snow slides off the roof sooner then other types of roofing. And you need to plan on what's going to be under the eave as it may be in the area where the snow will land when it slides off the roof. Doorways on the eave wall side will/should have snow breakers on the roofing so the snow gets busted it up as/when it slides off. I'm sure your metal roofing supplier can advise you what to use with their roofing materials.

With that high snow load you may have to have some external support of the overhanging eaves depending on exactly how much they will overhang.

Your complete frame design should be reviewed by an experienced timber framing engineer licensed in your area. If you need help finding one I can point you to the Timber Framers Guild Engineering Council (TFEC) website where there is a list of engineers. Although I am not an engineer, I am a member of the council to support their efforts in getting more data into the NDS (National Design Specification) book so that we can have accurate information to design by.

You will need to adjust your snow load based on your site's actual elevation. Which it appears you have taken into consideration.

Exposure is a factor as well, which will tend to be used when your engineer figures wind loads on your roof framing.

I hope my comments haven't muddied the waters.

Good luck with your project and keep asking questions.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Ljohnsaw

Quote from: Jim_Rogers on November 22, 2012, 10:13:12 AM
<<snip>>

I hope my comments haven't muddied the waters.

Good luck with your project and keep asking questions.

Jim Rogers

Jim,
Far from it, Thanks! 

Quote from: Jim_Rogers on November 22, 2012, 10:13:12 AM
And you need to plan on what's going to be under the eave as it may be in the area where the snow will land when it slides off the roof.
I've looked around at cabins in the area and plan NOT to have access points where the snow falls!
Quote from: Jim_Rogers on November 22, 2012, 10:13:12 AM
Your complete frame design should be reviewed by an experienced timber framing engineer licensed in your area.
Absolutely!  I can figure out the sizes, but another set of well-trained eyes would be critical.
Quote from: Jim_Rogers on November 22, 2012, 10:13:12 AM
You will need to adjust your snow load based on your site's actual elevation. Which it appears you have taken into consideration.
I found an engineering firm that has a site that will give the GSL based on you pointing to your area in a Google Map-like environment.  However, it did not cover exactly my spot (different county), but it did in an area about 1/4 mile away.  I'm right over the county line!  This corresponded with the building departments GIS information on my parcel - so I'm confident with the number.  The firm built this site because the town of Truckee, CA has a vast number of micro-climates and the snow load has some dramatic changes in a very small area.  I will err on the side of caution - I tend to over-build a little bit.
Quote from: Jim_Rogers on November 22, 2012, 10:13:12 AM
If you need help finding one I can point you to the Timber Framers Guild Engineering Council (TFEC) website where there is a list of engineers.
I'd like to talk with one early on for some guidance, especially for dealing with the planning department/permits.  I'm building in Nevada County.  As stated before - my timeline is to put in well, septic and power summer 2013 and build a mill shed/lumber storage.  Possibly build the foundation (block wall, poured floor) for the cabin during that time as well.  Then in 2014, build the cabin.  It looks like ground work only has about 4 months but lumber harvest/framing could be a month or two earlier and a month later.  To save a bit of time and make use of boards FOHC (since it will only be visible from the cellar), I'm planning on doing a standard floor joist system of 2x or 4x joists (one of your earlier comments/recommendations) with a center support beam.  That will limit the joists to a 10' span and provide support for two interior posts.
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

Stephen1

Quote from: ljohnsaw on November 22, 2012, 02:47:35 AM

Also, I'm debating over whether or not to have eaves.  With eaves, you get some shade in the summer but potential ice dams in the winter.  Also, if I do the few principal rafters and purlins, it seems to me that eaves are a bit harder to create.  My cabin will be a story and a half, 20'W x 30'L with the 20' long tie beam at 8' and the knee wall probably 4' so there is room for the brace.  The rafters will have a tie at least 8' up from the floor of the loft area so it ill be approximately 12' long.

Thanks for your comments.
If you are not using in the winter I would not be concerned with icedams forming at the eaves, also with steel roofing it will always slide with your pitch. I would think the shade and protection for the siding from the eaves will be a great benefit. My logs on my cabin only get wet on a real bad nor-easter rain. I think now they are a little long but the protection is great. I heat my cabin in the winter and never have a icedam as the snow slides after 6-8" buildup.
We hope to see some pics of the build.
have fun with the project
IDRY Vacum Kiln, LT40HDWide, BMS250 sharpener/setter 742b Bobcat, TCM forklift, Sthil 026,038, 461. 1952 TEA Fergusan Tractor

Ljohnsaw

Quote from: Stephen1 on November 22, 2012, 02:41:02 PM
<<snip>>
I would think the shade and protection for the siding from the eaves will be a great benefit.
I forgot to mention.  For the walls, I'm planning on 2x cedar tongue and groove planking (visible inside and some thermal mass), 3 or 4" of rigid foam (wrap and strap) and then wood-grained (embossed) concrete lap siding.  I want this building to be near-fireproof and absolutely woodpecker proof ;D  Other structures in the area are damaged from the little pests.  I won't necessarily need the weather protection, per se.  Also, I will be using it in the winter for some play in the snow get-aways, so that is why I'm concerned about ice dams.  Additionally, if I have eaves, I'll need to protect all that exposed area as well from the woodpeckers.
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

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