SNOW! Check your roof!.

Here's a brief explanation of where residential roof trusses fail and why.

The top chords of a truss are 2x4 or 2x6 compression members & are the ones that the roof sheathing
(usually 4' x 8' panels of either OSB or plywood) is attached to.

The bottom chords are tension members and are the ones to which ceiling materials (usually sheetrock, also known as GWB) are attached.

The web members are the 2x4s that run diagonally between the top & bottom chords, & the points at which they connect are called panel points.

Almost always these framing members are not what fail when a roof collapses. What usually fails are the galvanized steel connector plates that hold various framing members together at the panel points. It is also conspicuous that a common point of connector plate failure is where the bottomm chord (a tension
member) joins the top chord (a compression member) where they rest upon the double plate of bearing walls.

Why do they fail?

Because they are subjected to a load that exceeds their structural capacity, at which point they either pull out of the framing members or they simply rip apart. The next time you see a residential trussed roof failure on TV look at the ends of the 2x4's and you will rarely see them
broken. In most instances what you will see is connector plates that have either ripped in half or torn loose from the 2x4's.

Flat roof structures common in commercial buildings such as grocery stores etc. have different issues. The trusses are usually much longer than home trusses. The top and bottom chords are almost parallel, and of a configuration that requires special on-site bracing to unify and distribute the loads especially the
tensile load on the bottom chords. It is not uncommon to trace commercial building truss failures
back to inadequate or improper on-site bracing.

A sq. ft. of water 1" deep weighs 5.2083 lbs. A sq. ft. of water 7.68" deep weighs 40 lbs. which
is the live roof load design criteria for several local municipalities.

The easiest way of determining how much the snow weighs at your house is to weigh it. Take your shovel & cut some snow on your driveway into a 12"x 12" square 12" high. Take your bathroom scale to your driveway & with shovel in hand step on it & check your weight. Then carefully lift the cubic foot of snow
with your shovel & step on the scale again. Subtract the first weight from the second & that's the weight of the snow. Then to see how much additional weight some rain might make, take a gallon jug of water, which weighs 8.333 lbs/gal. See how much of it you can gently sprinkle onto the cubic ft. of snow before it
runs through onto your shovel. After you've determined the total weight, divide by 12 to get weight per inch. Divide 40 by that weight to find the number of inches of snow that would produce a 40 p.s.f.load.

Thank you to Michael Jacobs of Hope Realty in Coeur d'Alene, Idaho for this information!