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slate roofing

Started by Aeneas61, August 26, 2016, 02:13:40 PM

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Aeneas61

Hello all,

We are putting up a small timber frame cabin soon and I am considering a slate roof.

I am unsure of the weight load and if my style roof can handle it.

We have a frame about 16X21 (inside of timbers) frame with 4x6 common step lap rafters pitched at 12 in 12 on 24" centers.

Has anyone had experience with slate on time frames and any thoughts on this for our cabin?

Brian_Weekley

With a 16 foot wide building and 12/12 pitch, the length of the rafters is about 11.3 feet.  Using 2 foot spacing, each rafter supports about 22.6 square feet.  It looks like slate is about 8-10 lbs per square foot (226 lbs/rafter) compared to asphalt shingles that are about 2-3.5 lbs per square foot (35 lbs/rafter).  Although slate is heavier, it might be a relatively small contribution in weight compared to all the other roof components you need to consider to calculate the total dead load of the roof (rafters, sheathing, insulation, plywood, etc.).  More importantly, if you live in a cold climate, the snow load may actually be a greater contributor compared to the roof components.  For example, if you have a potential ground snow load of 70 psf in your area, that would be 2x8x70=1120 additional pounds each rafter must support.  The bottom line is that you really need to calculate the total roof weight and also consider the snow load before you can determine if a 4x6 rafter is acceptable (or what size rafter is actually needed to support the total weight).
e aho laula

Jim_Rogers

My materials weight chart says slate is 10 lbs per sqft

As mentioned, you need to accurately figure your loads.

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

ChugiakTinkerer

Don't forget to account for roof pitch in calculating your dead load.  For snow load you just use the horizontal dimensions for the roof, but the steeper the pitch the more roofing material gets used.  For my cabin design I am estimating the entire roof weight and dividing that by the horizontal square footage.  A simpler method would be to divide the weight per square foot by the cosine of the pitch angle.
Woodland Mills HM130

Aeneas61

Thanks guys,

can you also help with a few questions to help me calculate my roof loads?

1. Is there some map or website I can get my local snow load from?

2. I was planning to use 1" T&G over the rafters, then 2x8s or similar to run 6" of rigid foam between, then horizontal nailers, then slate or metal roofing to the nailers, how do i calculate the loads of all these materials?

3. Im not sure I understand the pitch and cosign ideas either, or how this effects the roof loads. I thought I read a steeper pitched roof was also stronger and less strain on rafters and plates but maybe snow load is different.


Jim_Rogers

1) There are many sites where you can get your snow load weights from, here is one:

http://snowload.atcouncil.org/

2) You're going to need to figure the weight of each piece based on it's actual length and width and add it all up to get a grand total and then divide by the number of square feet and get a per sqft load.

3) To figure the live load (snow load) you use the run of the rafter. So if you have a 16' wide building the run is 8'.
If you have a 16' wide building and the roof pitch is 12/12 then the actual dead load is higher as the roof slope is longer than 8'  see below drawing showing length of roof slope in my example:



 

To get the rafter length you multiply the run by 1.4140625

These types of calculations are what we hire engineers to do in order to be sure we have the right size rafters and correct spacing to support the load.

Here is a chart of known weights of materials:


 

I got this from a very nice lady engineer, just recently, when she was helping me with a roof load calculation.

Jim Rogers

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

ChugiakTinkerer

Aeneas61,

If you're building somewhere where a building inspector or other government official must approve the design then they can answer your questions about snow load.  If you're like me and can build without any official involvement whatsoever then it's up to you to make sure what you build will withstand the snow and wind expected in your area.  Many jurisdictions have adopted the International Residential Code, which is now available for free online viewing at http://codes.iccsafe.org/app/book/toc/2015/I-Codes/2015%20IRC%20HTML/index.html

Even though I don't have to build to code, I am using it as my design standard for ensuring my investment in time and resources will last longer than the time it takes me to build.  If you're not technically inclined the IRC can be very intimidating.  But with a little patience and plenty of morning coffee you can work your way through a given chapter and get a good sense of what residential construction should look like.  The problem with timber frame construction though is it doesn't fit the cookie-cutter mold of the typically framed house.  For roof rafters, for example, there is no table for using 4x6 rafters.  A good starting point though is to use 2x6 rafters at half the spacing you plan to use.

In the IRC link above, if you click on Chapter 8 and scroll down a ways you will find Table R802.5.1(4) which has a table for ground snow load of 50 pounds per square foot (psf) and ceiling not attached to rafters.  If you look at the right side of that table there is a heading for a dead load of 20 psf.  This may be close to your actual roof dead load although I suspect yours may be a little higher.  For 2x6 on 12" spacing, the lowest grade rafter shown can span 7'-5".  For this table the span is the horizontal distance of unsupported rafter.  If your walls are 16' inside edge between the top plates then you should use 8' for your span.  If it is 16' outside to outside then you should use 8' minus the plate width.

This is a ballpark estimate that suggests if your dead load or snow load is at all greater than the numbers used in the table then you should probably consult an engineer.  If you're in a 20 psf snow area you can use Table R802.5.1(2) which indicates a rafter span of 10'-2" for Southern pine #3.  In this circumstance 4x6 rafters on 24" spacing would have no problems supporting the roof load.

Were I in your shoes I would work through the weight of everything being supported by a rafter, including the rafter.  Add that all together to get the material weight in psf.  Then multiply that by 1.415 as Jim shows above to get the dead load for the 12:12 pitch.  Then find the table in IRC Chapter 8 that best matches your snow load and dead load.  If there is a match then you can use the 2x6 on 12" value for the kind of wood you have.  If your snow or dead load is beyond what is in the tables then you'll need to do some beam calculations to see how your rafters hold up.

Actually with a fresh cup of coffee I'll walk myself through the process.  I'm looking at a very similar roof for my cabin so the exercise will help me too.  First up is the rafter itself.  Assuming it will be built green, you're looking at up to 64 lbs per cubic foot depending on species.  A linear foot of rafter will be 4x6x12 inches, or 0.1667 cubic feet, resulting in a rafter weight of 10.67 lbs per linear foot.  With 24" spacing, that makes your rafter weight 5.33 lbs per square foot.  Let's assume your 1" T&G is a dried softwood like cedar.  With a density of about 30 lbs per cubic foot, your T&G will weigh 2.5 psf.  Six inches of rigid foam board will be .8 psf (Foamular 250 weighs 1.55 lbs per cubic foot) and 30 lb felt will weigh 0.3 psf.  Slate will add 10 psf and let's throw in some nails at a pound per square foot.

So the roof materials will up to the following:
  Rafter  5.33
  T&G   2.5
  Foam 0.8
  Felt  0.3
  Slate 10.0
  Nails etc  1.0

  Total 19.93

Now we need to account for the roof pitch.  Multiplying 19.93 by 1.415 we get 28.2 lbs.  This is the dead load you should use in any rafter table or beam calculation.  The IRC tables for rafter spans do not offer a 30 psf dead load heading, so you may need to do a little more calculating to determine the suitability of your rafters.  If you are using dry softwood for rafters you will lose a few pounds in the rafter weight, but I don't see your roof getting below 25 psf for slate on a 12:12 pitch.
Woodland Mills HM130

Aeneas61

Thanks for the information Jim, that will be helpful.

ChugiakTinkerer,

wow, thanks that really gives me an idea of what goes into a calculation of roof dead load. So by your figuring I might be looking at about 25-30 lbs sq foot for my load, and then I need to add snow load for my area? Or is that not right?

This may sound overly naive or foolish, but most of my plans were just copied off what others have shown doing online, including my rafter size. Its now that we will raise soon Im thinking of roofing materials, and wandering if I should splurge on slate or clay tile, or just stick to the norm and boring barn metal roofs so ubiquitous to our area...

I have to say, in a way its great we have these detailed calculating procedures to help us make a safe roof, but on the other hand, I wander how it was done in the past, Im trying to visualize this stereotypical illiterate carpenter, apprentice trained and skilled hands gripping a broad axe...how would he go about designing a house or barn, I doubt he would be calculating in this way, perhaps it would be a simple matter of copy catting a successful building he knows did not collapse? Or maybe each framing crew had one educated oxford type man who drew up their blueprints, elegantly scrolling trigonometry equations along a rough sawn board to the chorus of many crosscut saws...maybe we won't know, maybe those buildings that lasted were just lucky, and the ones which were not fell apart...but I wander...

ChugiakTinkerer

Quote from: Aeneas61 on August 27, 2016, 02:20:57 PM
...
wow, thanks that really gives me an idea of what goes into a calculation of roof dead load. So by your figuring I might be looking at about 25-30 lbs sq foot for my load, and then I need to add snow load for my area? Or is that not right?


Yep, you've got it right.  You might think that load is load, but the difference between dead load and live load is that the dead load is there 100% of the time.  The live load, in this case snow, will be part-time, ranging from a few days to many months.

Nothing foolish about copying what you see other people do, I started out there myself just a few months ago.  The only foolish part is thinking it applies all the time and not making allowances for different build conditions.  I don't doubt that there was a lot of wisdom in olden days carpentry, but there would have been failures and screw-ups too.  Those buildings didn't survive long enough to be reminders to us in the modern day, so we might assume all builders of old knew their craft perfectly.  I doubt that was the case.  But I do credit folks a few hundred years ago, heck even a few thousand, with being just as smart as we are today.  The clever solutions to various problems in carpentry, ship-building, etc are proof enough that when faced with a problem an observant individual could devise a practical solution.

My build is in a remote part of Alaska so I'll be doing the unoriginal metal roofing thing.  I think so, at least.  If I let my wife see this thread she may decide we need slate too! :D

By the way, if you update your profile to show your geographic location, it will really help folks target their advice for your local conditions.
Woodland Mills HM130

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