Exellent advise. We replaced the shingles on our roof last year that were 20 years old. But the most of the damage had been done by the roof being shoveled in the past.

This is heresay but apparently some of the old roofs that were properly constucted to building code could with stand a snow load of 600# per square foot.

Cheers

The building code requirement is that the minimum standard is to design the roof structure for houses to 60% of Ground Snow Load. GSL in PG used to be 60 lbs/SF, thus the minimum "live" load capacity had to be for 36 lbs/sf. As I recall, the GSL was reduced slightly about 15 to 20 years ago. Thus, roofs on newer buildings do not have to be designed for quite as heavy a snow load as those built in the 70's and 80'.

Roofs also have to take drifting snow into consideration. Thus, the high parts of roofs will typically have the least amount of snow on them, while roofs lower to adjacent high roofs, such as the old style flat carport roofs, will have a greater amount of snow on them.

That is where the "problem" comes in. Some of those areas were not constructed with sufficient structural capacity and thus there were several collapses in the city of those types of carport roofs. There were also others. There were a couple of collapses of freestanding "carports", not because the roof was inadequate, but because the columns holding up the roof were inadequately braced against shear. The old "carhop" style A&W on 18th was one of those, as was one of the carports at the condos on Killoren just north of 5th.

The more famous one in recent history was a roof collapse of part of Esther's Inn which was caused by water backing up underneath the snow not being able to escape through a roof drain in the middle of the roof. Due diligence dictates that such roof designs should be able to hold the weight of water the roof area can contain, should the drain malfunction.

In my opinion, the contemporary style of houses with all sorts of dormers, varying roof heights, etc. are more prone to collapse in high snowfall years, especially when snow can get saturated with rainfalls in between. These roofs are "snowcatchers", especially when we get higher winds and the snow is still light enough to cause it to drift.

Also, as was brought out in the article, the deeper the snow, the less insualtion in the ceiling immediately below the roof structure, the less the ventilation of the attic space, and the wider the overhang which is unheated, the more the bottom layer of the snow will melt, flow towards the eaves area, and begin to freeze at the lowest part of the slope, causing an ice dam which will simply start building over the winter season. If the dam is about 3ft wide, water will begin to penetrate the roofing and typically impact the outside wall.

That is why some of the older houses have sheet metal at the lower part of the roof slope.

As with many things, we have lost the knowledge developed over generations of practical experience. Those who have done so, are doomed to rediscovering it.

Those who feel they have ice dams (extensive icicles) have an interesting decision to make ... should they remove the ice dam carefully and possibly damage the roofing near the edge of the roof .... or should they leave it and risk the possibility of the dam building even further which could end up with water penetrating the inside of the house, potentially causing even more expensive damage, including the build-up of undetected mould for years, until it penetrates the inside of the house.

I often wonder why houses do not come with a User's Manual.

[url]http://www.extension.umn.edu/distribution/housingandclothing/DK1068.html[url]

"What about the homes built in the 70’s, which covers most of the homes in Prince George? Those homes were built before there were engineered trusses."

Don't tell the engineers that. They will wonder who put the stamps on the drawings....... LOL ..

how quickly we forget ....