Roof Slope Question

[Duffster]

I don't understand all the pitch/snow load comments.

Roof pitch has nothing to do with snow load.


Curly, I would not woory a bit more about a 4/12.

There are probably more 4/12 pitches around here than all other roof pitches combined.

 

[Hannibalbill]

Duffster,

Imagine a flat roof. Now add a huge snow load of, say, 50 lbs per square foot on a 2000 square foot roof. 100,000 lbs. or 50 tons. Like parking a couple of RV's on the roof. Lots of weight for a flat roof. But if the roof is steeply sloped, like those chalets in the mountains, the slope of the roof distributes the weight to the frame of the building lessening the weight per square foot dramatically. The steeper the roof the better. You also get the advantage of a certain amount of snow falling off. And, no one can park an RV on a steep roof

 

[Duffster]

Duffster,

Imagine a flat roof. Now add a huge snow load of, say, 50 lbs per square foot on a 2000 square foot roof. 100,000 lbs. or 50 tons. Like parking a couple of RV's on the roof. Lots of weight for a flat roof. But if the roof is steeply sloped, like those chalets in the mountains, the slope of the roof distributes the weight to the frame of the building lessening the weight per square foot dramatically. The steeper the roof the better. You also get the advantage of a certain amount of snow falling off. And, no one can park an RV on a steep roof

Sure, but any pitch can be built for any load you design it for.

Snow doesn't really slide off a 4 or 5/12 shingled roof.

 

[dave1949]

Sure, but any pitch can be built for any load you design it for.

Snow doesn't really slide off a 4 or 5/12 shingled roof.

That's true. The worst situation we see is 3-4 feet of snow on a roof, then a rainy storm comes through. Flat and low pitch roofs start going down - for real. You can say they are underbuilt and that would be a reasonable argument. You could also say it wouldn't have been a problem with a steeper pitch because the rain and melt water moves through the snow and down the roof.

There are usually a couple weekends sometime in late February, when roof shoveling becomes the state sport here.

But, it's not going to snow on CurlyDave's roof, we digress.
Dave.

 

[JB4310]

Just to add to the roofing discussion,
I know it's been established that Dave has no worry of snow, and I agree 4/12 should not be a problem. But around here where we do get a decent amount snow, Low pitch roofs do suffer, definitely more susceptible to leaks from slow shedding water through snow. Not even necessary to have ice dams, water just does not flow through snow and backs up. Kinda like breaking the law of gravity, going up hill defeating the overlapping from bottom to top shingle roof design.
May not always cause leaks inside, but does affect the plywood deck, very common to fell spongy, not wet but just soft, when walking on low slope shingle roofs between the rafters.

And let me tell you folks outside of the snow belt, that may own or maintain true flat (technically, referred to as low slope) commercial type roofs like found on schools, factories, plazas etc. That rain on snow combination is the absolute, end all acid test there is. Those roofs may not have a leak in a monsoon, but let it rain on a foot of snow and it's real hard to keep the water out. You can have 2-4 inches of water trapped over the entire roof even just a few feet from the drains.
Like I said water will not move through snow like most would think. When conditions are just right, it's not uncommon to see a bunch pails out catching all the drips, even in newer buildings.


JB.

 

[dave1949]

Just to add to the roofing discussion,
I know it's been established that Dave has no worry of snow, and I agree 4/12 should not be a problem. But around here where we do get a decent amount snow, Low pitch roofs do suffer, definitely more susceptible to leaks from slow shedding water through snow. Not even necessary to have ice dams, water just does not flow through snow and backs up. Kinda like breaking the law of gravity, going up hill defeating the overlapping from bottom to top shingle roof design.
May not always cause leaks inside, but does affect the plywood deck, very common to fell spongy, not wet but just soft, when walking on low slope shingle roofs between the rafters.

And let me tell you folks outside of the snow belt, that may own or maintain true flat (technically, referred to as low slope) commercial type roofs like found on schools, factories, plazas etc. That rain on snow combination is the absolute, end all acid test there is. Those roofs may not have a leak in a monsoon, but let it rain on a foot of snow and it's real hard to keep the water out. You can have 2-4 inches of water trapped over the entire roof even just a few feet from the drains.
Like I said water will not move through snow like most would think. When conditions are just right, it's not uncommon to see a bunch pails out catching all the drips, even in newer buildings.


JB.

That's for sure. Winter before last we have terrible sequence of snow,rain,snow .. Most people could count 7 or 8 icey layers on their roofs when they went to shoveling. It didn't look all that heavy, until you started clearing it and realized it was a lot of frozen slush ice and a little bit of snow.

That loading caused a laminated engineered beam to crack over the gymnasium of a school. Obviously the school wasn't all that old. It also took down a lot of weak cheap built garage roofs, damaged commercial roofs, collapsed a steel truss building too.
Dave.

 

[CurlyDave]

Well, I talked to my architect today and we are going to be OK with a 4.25/12 slope.

I like going just a hair above 4/12 because, as JB points out, 4/12 is the cusp between a low slope roof and a high slope roof. Looking into the future, I can see the inspector looking at a 4/12 roof and saying: " Here it says for 4/12 and over you need xxxxx, and for 4/12 and under it says you need yyyyyy. I am going to have to research which you need for exactly 4/12. Stop working on this for a few weeks while I decide."

With 4.25/12 I am definitely on one side of the line, not exactly straddling it.

 

[Pops15]

I'm not the world's best carpenter so I may be missing something. Why does the size of your rafters increase the height of the roof? A 5/12 pitch is a 5/12 pitch regardless of the rafter being a 2x4 or a 2x12.

5/12 means for every 12 inches of width the roof goes up 5 inches from the starting point. The starting point can be the same regardless of the size of the rafter. The bottom of the rafter would be lower but the top would not be higher.

In the case of replacing a 2x4 rafter with a 2x10, you just cut the bottom of the rafter where it meets the wall at an angle where the top of it is even with where the top of the 2x4 rafter was.

I can go in a house with 2x4 rafters and replace a rotten one with a 2x10 rafter without raising the roof height at all.

When you cut a rafter, the width of the bird mouth should be the same as the width of the top plate. So yes, a 2X10 rafter would be taller than a 2X6 rafter.
You can cut the bird mouth on a 2X10 so it will be the same height as a 2X6 but that's not the proper way to do it. You'd end up with a really big cut out on the bird mouth.

 

[PILOON]

I'll add a bit of confusion;
I built a log cabin 50 years ago with a 45 degree roof (12/12) as I wanted the snow to shed. It never did!
I later recovered with metal as I reckoned that would let it shed.
It never did and accumulates snow up to my chin, and I'm 6 ft.
That roof is insulated to about R-32 and vented.
Never forms icicles.
But come spring and it decides to shed, well it sounds like a freight train passed 10 ft away.

I reccon that the reason is that the cabin in snugged in mature trees and there is little sun (north slope) and also very protected from prevailing winds.

Did you know that roofing felt and most shingles is recycled newspaper? impregnated with tar? and as the tar dries ir becomes brittle and that is when the shingles start to fail.
Same with 'felt underlay', it dries up and turns to powder with age.

Personally I believe underlay's origional existance was to even out the surface back in the days when tounge and grouve was used as decking and since we nowdays use plywood or sheet decking the requirement no longer exists.

Shingle makers being faced with rapidly rising costs of recycled paper looked to alternatives to get better cheaper finished products and that is what brought on the flexible membranes to the marketplace.

Also shingles used to be rated by years of expectancy; eg 10 year, 20 year etc but nowdays they wish to be rated by the weight/sq.
Guess the mfgs got fed up defending lawsuits for roofs that leaked prior to 'advertised expected life span'

Standard bundle covers 33 sq ft, 3 packs per 100 sq ft so calculating roofing is real easy (in most cases) however I once had the client purchase the shingles and he bought the heaviest 'double' type.
Due to OSHA requirements, the heavy shingles need to be 6 bundles/square because of weight.
Well 1/2 way into the job I needed to re-order as we'd be short 50%.
Learning curves!

 

[radioman]

When you cut a rafter, the width of the bird mouth should be the same as the width of the top plate. So yes, a 2X10 rafter would be taller than a 2X6 rafter.
You can cut the bird mouth on a 2X10 so it will be the same height as a 2X6 but that's not the proper way to do it. You'd end up with a really big cut out on the bird mouth.

yup, I was gonna suggest this but wasn't sure of the term. birdmouth. Learn something new. I did the same thing recently for my folks. We ripped out a old bedroom and attic space to make one big huge bedroom. The old ceiling was 2X4's but my dad wanted more insulation . We "bird mouthed" a 2X6 to fit down the sill plate and ran 6 all the way to ceiling to a new hip. We basically built a new roof inside a old one and tied it together so its square and more insulating value. perhaps this is a possibility curly wants to consider?