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
Any websites that address this? Am I missing something? Or does
everybody figure it out by this logic?
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).
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.
On Sat, 5 Feb 2011 08:21:37 -0800, "Bill"
Sorry, wrong again harry....
A pitched roof is typically supported at each end of the rafters which
a considerable span. And those rafters are most definitely subjected
bending. Deflect them more than a certain amount and they break,
is the principle failure mechanism. The amount of load on the
depend on the angle, but to state that there are "few bending moments"
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
Well, that just isn't so. The forces on the roof rafters vary
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
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
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
linear along the rafters. The loads and failure mechanism are now
On Sun, 6 Feb 2011 06:45:41 -0800 (PST), firstname.lastname@example.org wrote:
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.
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.
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.
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