Force of Snow on Roof

[frank1492]

I took a physics course once in which force vectors were discussed. We all intuit that the greater the slope of a roof the less chance it will collapse from snow weight. But isn't this more than the issue of it will slide off sooner and therefore have less chance to do damage? My recollection is that you can break the vertical force on a roof into two components: One perpendicular to the roof, the other parallel to the roof surface. This is important because the component that does the damage is the perpendicular component. By my reasoning, a roof with a 45d angle would have 1/2 the perpendicular component of a flat roof and the risk of collapse would therefore be 1/2 the risk of a flat roof. Any websites that address this? Am I missing something? Or does everybody figure it out by this logic? Thanks. Frank

[frank1492]

...actually .707 of the perpendicular component of a flat roof...

[dpb]

If the same amount of snow is on a given roof area, the total weight borne by the roof is also the same. The difference is that the load is transferred into the structure differently depending on the roof design (and the slope is, of course, part of that difference).

--

[frank1492]

I think the ladder analogy is a good one. As the ladder becomes more and more vertical, the sides of the ladder bear more and more stress longitudinally which lessens the chance of their buckling. The sides of the ladder are analogous to the roof trusses. Your first point I would think would be invalidated by the above, but it's a tricky point I feel.

[trader4]

Sorry, wrong again harry....

A pitched roof is typically supported at each end of the rafters which cover a considerable span. And those rafters are most definitely subjected to bending. Deflect them more than a certain amount and they break, which is the principle failure mechanism. The amount of load on the rafters does depend on the angle, but to state that there are "few bending moments" is incorrect.

[Higgs Boson]

Harry, in the States we call it "torque"

in a pitched roof, mostly tension and

HB

[HeyBub]

Those vectors are used in determining frictional force and have nothing to do with roof load.

If you have a ton of snow on the roof, you have a ton of snow on the roof, period.

[trader4]

Well, that just isn't so. The forces on the roof rafters vary according to those force vectors, which in turn depend on the roof angle. Imagine a perfectly flat roof with a snow load. All the force is acting straight down. The forces on the rafters is a bending force that causes them to deflect. Put enough snow on it and they will fail by breaking from the sideways force on them.

Now imagine the same roof but tilted up almost to vertical. Now you have little force causing the rafters to bend, but instead a force running linear along the rafters. The loads and failure mechanism are now very different.

[Wayne Whitney]

Actually, in the US, if you are a physicist you call it torque; if you are a civil engineer you call it bending moment.

Cheers, Wayne

[RicodJour]

Thanks, you saved me the trouble of having to post to point that out. Oh, I just posted. Damn! ;)

For a far more detailed discussion than probably anyone here would want:

R

[krw]

That's true, but Bub has a point, though perhaps not well stated. Divide the total weight of the snow by the area of the house (not the roof) and you get the vertical loading. It doesn't matter what the pitch of the roof.

You're assuming the same depth of snow. In fact the depth won't be the same, normal to the roof since snow falls vertically, give or take. That is, given a steeply pitched roof, the same amount of snow will fall over the larger area of the roof (the same area of the house). Snow won't pile up as easily on a more steeply pitched roof, though.

[RicodJour]

I lost track of who said this, but it's simply wrong.

Now I know who said it. Sigh.

Not the roof area? Huh? Elaborate. What else contributes to rafter loading if not roof area? I believe you mean to say the horizontal projection of the roof area. If you'd like to engage in discussions of roof loading please use the established terms so you aren't confusing yourself and others. There's no reason to create new terms or attempt to redefine the existing ones.

Your first three sentences in the paragraph above are correct. The last one confuses the issue.

I love these theoretical modelings where people say things like, "Assume that all snow is distributed to an equal depth." That makes it simple to figure out an answer - just not necessarily the correct one. A shallowly sloped roof is more likely to have more snow on one side than on the other. Unless, that is, you live in one of those theoretical locations where there is no wind and no snow drifting.

R

[krw]

You used silly two-bit words to say the same thing. "Horizontal projection", my ass.

The "area of the house" is too complicated for you? I suppose...

I can't help it if you can't read.

Simple is good. You're only going to get a first-order estimate from these sorts of discussions.

[RicodJour]

No, it is too nebulous. Does "area of the house" mean floor area or footprint? Houses have overhangs. Maybe you don't believe a load on an overhang should count towards a roof load.

Do you not get wind along with snow? We do. In fact some silly structural engineers actually believe that you can get _combinations_ of loads! Can you imagine thinking that dead and live loads could happen at the same time? Incredible isn't it? They also have this silly idea that wind loading (wind is the thing that causes snow to drift, which apparently doesn't happen in your imaginary uniform world) can cause positive pressure on one area of a roof and negative pressure (aka uplift) on another.

In other words a single rafter may have different forces acting on it in different directions at the same time. Wind uplift at the overhanging eave can contribute to the downward pressure on the section of the rafter interior to the house.

It's obvious from your comments and sketchy terminology that you have never bothered to learn any of that stuff, yet feel qualified to argue about it. That's okay, just ignore those things that trouble you - they can't possibly matter and it's simpler for you to understand that way. Stick with your limited knowledge and maintain your belief system. Your ego will thank you.

R

[krw]

I guess it is too complicated for you. I shouldn't be surprised.

Ever heard of a "first order approximation"? No, I suppose not, dumbass.

Now *that* is funny. Many forces at once. Engineers understand this concept called a "vector".

It's obvious that you're too stupid to breathe. The fact that you apparently do, is one of the wonders of the universe.

What's the matter ricky, tired of being beat up by Swingman?

[krw]

Wow, your monthlies are really bad this time around! ...but you missed with the tampon. Do come back when you're feeling better. Oh, and tell Swingman "hi".

[krw]

Oh, and since you're copying articles from other newsgroups: