Roof Slope Question

[CurlyDave]

The one area we see felt in a completely failed state is on the slate roofs we work on. After 80 to 120 years especially on the south sides it literally turns to dust and pretty much just leaves a black residue/dust, making those roofs susceptible to wind and ice causing leaks, even as steep as they are.

I would suggest that under shingles the felt wouldn't last that long either. The only reason you don't see felt failures under shingles is that the shingles fail first...

 

[dfkrug]

The only reason you don't see felt failures under shingles is that the shingles fail first...

Yeah, but now we have some manufacturers selling "lifetime" rated comp
shingles (TimberLine). I have heard that some of the these triple-laminated
shingles can be quite lumpy, however.

In our area, Dave, there are many roofs as low as 2.5/12 since we have
no ice damming or snow loads. Many of these roofs get converted from
tar/gravel to comp shingles and legally do so by doubling up the
underlayment and cutting off the "Dutch gutters". My new house is
mostly 3:12 with dual-laminated comp (class A) 40-year shingles. My
barn I did with standing seam steel (Western Metal), which required
a 3:12 minimum pitch.

Some of our jusidictions in the Bay Area still allow shake roofs with NO
underlayment. Menlo Park did, as recently as the late 90s.

Your 4:12 is more than adequate.

 

[Coyote machine]

You mentioned the insulation factor and your roof pitch. What insulation method/material are you going to use?
Also, Grace ice and water shield is a great product but you don't need to use it on your whole roof. There are other similar products to use on the rest of the roof surface that cost much less and get the job done. You would still use the Grace on the valleys etc. There is roof top guard 2 that comes in 5 foot rolls and gets cap nails from a gun, vs sticky backing. It is better than felt paper by far and keeps out water,etc. I just did my entire 1200sq.' addition and my 30' .deep x 40' wide barn/apartment in the combo of the two products. Since then we've had all kinds of high wind driven rain, snow sleet, etc. with no problems. We used an IKO Harvard slate 30yr. architectural shingle. Don't waste your money on a 40 year shingle- you won't be using it, and it will not be worth anymore to a potential buyer. I've yet to see anyone get a useful 40 years out of a shingle job. As far as insulation goes if you do a sealed airtight roof with no soffit or ridge, or gable end vents and use closed cell spray foam you can do preconstructed trusses and get exactly what pitch you want by having your local lumber yard do the formulation for you and print a set of construction drawings. You will pay more for the spray foam upfront but it will be a big selling point to your buyers and it will be returned on your investment when you sell.

 

[crbr]

Rebuilding my burned out house, I am having to jump through a lot of hoops the Town is putting up.

One is that in many places I can not increase the overall height of the house, not even by an inch.

The problem that I am facing is that my old roof was built 100 years ago from 2x4 rafters. To get the right insulation in I have to go to 2x10, which is 6 inches thicker. Combine this with 2x10 floor joists instead of 2x6 and a couple of other problems, and I have to lower the roof peak by ~12".

One relatively painless way to do this is to just lower the roof slope from 5 in 12 to 4 in 12.

Does anyone have any experience here. I suspect that a 4 in 12 roof is going to be more leak prone than a 5 in 12, but is it a big difference or a small one?

Foregiveness is easier than permission. A 4-12 isn't that much different in functionality vs the 5-12. I would not worry about leaks because of slope. Construct any roof (slope) correctly and leaks will not be a consideration, regardless.

 

[yooperdave]

Have you considered a fractional roof pitch - say 4.5 on 12?
Whole numbers are nice - but not required.

Can you put sheets of thermaCAL over shallower rafters
ThermaCal Nail Base Insulation -- Cornell Corporation - Building Product Manufacturer in Sweets Network

Just providing some suggestions outside the box.

Yooper Dave

 

[xrjohndeere]

Are you going to use fiberglass insulation to achieve your r-39? If you use high-density spray foam you could probably use the 2x4 rafters and 2x6 joist. The spray foam will give higher r-factor per inch. Spray foam I believe is used alot on the west coast because of the weird restrictions. Good luck, codes are always a pain and enforced by idiots who do not know what the **** they are talking about.

 

[radioman]

May I ask why bother rebuilding if you are just gonna turn around and resell it ? will you actually realize a profit ?

 

[tallyho8]

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.

 

[CurlyDave]

May I ask why bother rebuilding if you are just gonna turn around and resell it ? will you actually realize a profit ?

My options are very limited.

As a buildable lot, but with a foundation already in place, the value of the property is minimal. The only real potential buyers are contractors building spec homes, who are pretty scarce these days. And, who are going to negotiate hard for a bargain basement price for the house.

Plus a lot of the previous structure did not conform to current setback requirements. It is not at all clear to me that a buyer could build a home nearly as large as the one I can build, since I am allowed to duplicate the previous structure.

In most real estate markets, the most common item for sale is a single family home. This is the item of commerce which will command the highest price.

By building, I get to put the largest possible home on the lot and I believe that I will make a substantial profit.

 

[Hannibalbill]

I built my own home years ago from plans from a catalog. The plans called for a 4/12 pitch which as others have said is common for ranches. I live in a heavy snow area so I used 5/12 trusses 2X4 truses. In 35 years with winters with as much as 12 feet of total snow, I have never had a problem. Why the need for 2X10's for insulation. My insulation is laid between the trusses and sticks up into the air. It doesn't matter how thick you go. Am I missing something. Vaulted ceilings, maybe?

 

[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?