We know that for a given depth, say... 6 inches, the weight of snow will vary with moisture content. Do architects ever make recommendations concerning the point at which snow removal should take place?
I assume any such recommendation would be tied to some method for determining- at least to some degree of accuracy- the moisture content of the snow.
Non Sequitur
Jan 23, 19 7:16 pm
no. You’re more likely to ruin your roof by shoveling the snow that it’s weight causing structural damage.
OneLostArchitect
Jan 23, 19 7:21 pm
no structural should have covered this. Especially with snow drifts in mind.
OneLostArchitect
Jan 23, 19 7:24 pm
Had an architectural lawyer tell me one of his clients built a house in Arizona. He wanted this same house and design in Wisconsin. So he took the same construction documents and built it The same house in Wisconsin. The roof collapsed after a heavy snow fall. This guy is now suing the architect IN Arizona.
Miles Jaffe
Jan 23, 19 10:50 pm
.
Inspectorman
Jan 24, 19 10:21 pm
Guess he was in a part of Wisconsin where they didn't bother too much with plan review.
OneLostArchitect
Jan 24, 19 11:48 pm
It is a small rural town... wont say which one as this is currently ongoing...
BulgarBlogger
Jan 23, 19 7:42 pm
Be careful with two gables side by side at different elevations... when snow melts, the lower gable will get additional snow.
whistler
Jan 24, 19 2:17 pm
Get your Structural Engineer to support the snow load. Technically whether you have a sloped roof or flat both have to be designed to withstand the snow load ++++ for drifting snow and the worst case scenario of a large snow fall followed by heavy rainfall ( not untypical in western north america )
The advantage of the flat roof is that it will hold the snow vs dumping onto adjacent walks / driveways / cars and people. Many cases of that over the years and in some situations with deadly consequences.
JLC-1
Jan 24, 19 2:26 pm
6"? this load is for about 36"
mightyaa
Jan 24, 19 3:39 pm
IRC has tables for snow load (prescriptive) (R301.2 at least in the 2012 IRC I have open). First number is elevation, second is snow load.
Generally 1 inch of snow is 1psf. So a snow load of 80 psf means about 80" of snow stacking.
JLC-1
Jan 24, 19 4:12 pm
2.2.1 Range in Snow Weight
The weight of 1 foot of fresh snow ranges from 3 pounds per square
foot for light, dry snow to 21 pounds per square foot for wet, heavy snow
(Gooch, 1999).
So switched to 'per inch of thickness' that's 0.25 psf per inch of light dry snow to 1.75 psf per inch of wet heavy snow... Which means that 80 psf load would be just under four feet of heavy wet snow.
Non Sequitur
Jan 24, 19 4:56 pm
There's almost 2' of snow on my 60y old roof... and it's been raining all day. I'm not the least worried, but I know there are a few wankers on the street right now trying to take that snow off.
JLC-1
Jan 24, 19 4:56 pm
yes, missed it by 12, but still rarely happens in one event. at least here in colorado high rockies.
Inspectorman
Jan 24, 19 10:26 pm
2006, Nederland, CO, 10' in three days. The only roofs that collapsed were really badly built.
Inspectorman
Jan 24, 19 10:13 pm
I'm writing courses on commercial roof inspection for a professional association for home inspectors, so the roofs of main concern are low-slope. Steep-slope snow loads are generally not so much of an issue depending on the type of roof covering material and the pitch, although roof avalanches are a related issue with them.
Anyway, InterNACHI has about 25,000 members and I know some of them are going to be asked this, so I wanted to be able to tell them how to reply.
Inspectorman
Jan 24, 19 10:29 pm
Here's an excerpt:
If instead of rain, a roof load from a storm consists of snow or hail, the load will continue to accumulate and will remain on the roof until the outside temperature rises enough for it to start melting and running off the roof; a much slower process than with rain.
FIGURE: Lower roof accumulation
The lower roof is on the north side of the building, in shade much of the winter.
If a downspout from an upper roof that is exposed to the sun discharges meltwater onto a lower, shaded roof, that meltwater may re-freeze, adding even more weight to the lower roof.
Neither the IRC nor the IBC give maximum depth accumulations for roof ponding. They specify that drains/scuppers be installed in a manner that will “…prevent the depth of ponding water from exceeding that for which the roof was designed…”
Inspection Note: Inspectors may be asked at what depth of snow on the roof should removal operations begin. The answer is “Because snow varies in water content, its weight per inch of accumulation will vary, so snow depth is not a good criteria on which to base snow removal decisions. Roofs are designed to support expected snow loads using historical data and design calculations include a safety margin.”
mightyaa
Jan 25, 19 4:12 pm
Commonly related defects we see with this stuff... Low sloped roofs with drains in shadow zones. So you get the original snow load, but as it melts in the sun, that water goes to the shade, and creates massive ice dams. Along those lines sort of... roof replacements have become a huge issue with condensation. EPDM, black, absorbed enough heat to vaporize the condensation that might form below it.... TPO does not. And with low slope, you don't normally have ventilation... We're seeing a lot of rot. Adding Ice & Water (non-permeable) to a poorly ventilated, no vapor barrier building, has the same effect. Condensation below the barrier forms, saturates the sheathing, and rot can take hold. We're going to see more of this with these big climate shifts... used to just be an issue in the mountains where they'd see the extremes.
mightyaa
Jan 25, 19 4:15 pm
Ventilation is the key to control icing and snowmelt. If I had advice for inspectors, start really looking at ventilation; main roofs are normally fine, but lower roofs, garages, projections, etc. I commonly have a defect of no ventilation.
Inspectorman
Jan 26, 19 6:57 pm
Good post, mightyaa! I've covered vaporized condensation within roof systems, and lower roofs in shade fed by downspouts from upper roofs in direct sunlight, but good point... the source of shade doesn't really matter. A drain located in constant shade is likely to suffer ice damming in cold climates.
whistler
Jan 25, 19 2:37 pm
So we received 384 cm ( 12'6" ) of snow in December and another 120 cm ( 4' ) in January. I currently have a compacted depth of about 5' sitting on the roof of my home.... with no worries ( our design snow load is 205 lbs/sf and the additional rain load is another 20 lbs/sf) also have a green roof up there too to add to the weight!
There will be no snow removal. Roof designed by Structural Eng. to handle full load.
Inspectorman
Jan 26, 19 6:39 pm
You have a brutally strong roof!
What's your location?
whistler
Jan 28, 19 12:11 am
in the mountains, north west of vancouver, canada
!
whistler
Jan 28, 19 12:16 am
Roof structure is 6 x 12 glulams @ 24" o.c. ( max span about 16') 2" d. fir decking w/ 3/4" ply sheathing as diaphragm, 6" rigid insulation w/ 2 ply torch on. 6" green roof system over entire roof area.... for those interested.
Non Sequitur
Jan 28, 19 7:26 am
^beautiful story to read as I look at -33c walk in to work this morning.
Volunteer
Jan 25, 19 3:01 pm
This was in Poland a few years ago. After a heavy snowfall high winds drifted the snow onto a localized roof area where it failed. 65 people died and a lot more injured.
Inspectorman
Jan 26, 19 7:16 pm
Just so y'all know, the roof courses I'm writing are online courses and consist of the present general course, covering aspects common to all low-slope roof-covering systems, and additional courses on the individual membranes; built-up, mod-bit, single ply, SPF, and liquid-applied.
The general course is a prerequisite to the individual membrane courses and is now at just under 40,000 words, so it's pretty comprehensive.
I gotta say, aside from the NRCA Manuals and Low-slope Roof Systems, by Griffen and Fricklass, locating sources of good information has been tough, so I appreciate any help. Especially photos that illustrate common failures clearly. Roofing companies and building owners won't let me onto roofs to take photos for liability reasons.
CCPIA is a sister organization to InterNACHI, which has around 25,000 members, so there are a lot of people looking forward to having courses on low-slope inspection available.
So... thank you for your help!
mightyaa
Jan 27, 19 6:15 pm
You might see if you can dig up HAAG Certified Inspector handbooks and reference documents.
They have some helpful links to various other organizations too
Oh, and since you are familiar with the NRCA, I assume you have access to the the NRCA Roofing and Waterproofing Manual(s)... Expensive little books, but pretty much the bible of reference documents on roofing.
Inspectorman
Jan 28, 19 12:57 pm
I've taken the Haag commercial course and have the NRCA manuals, thanks. The NRCA manuals are very helpful but the NRCA looks at some things (like air/vapor barriers) a little differently from the ICC standards. And then there's the Building Science Corp.
Working on this section now. I prefer books when I can find them because so much of the online stuff is geared toward sales. Manual of Low-slope Roof Systems, Griffin and Fricklas is pretty good too.
We know that for a given depth, say... 6 inches, the weight of snow will vary with moisture content. Do architects ever make recommendations concerning the point at which snow removal should take place?
I assume any such recommendation would be tied to some method for determining- at least to some degree of accuracy- the moisture content of the snow.
no. You’re more likely to ruin your roof by shoveling the snow that it’s weight causing structural damage.
no structural should have covered this. Especially with snow drifts in mind.
Had an architectural lawyer tell me one of his clients built a house in Arizona. He wanted this same house and design in Wisconsin. So he took the same construction documents and built it The same house in Wisconsin. The roof collapsed after a heavy snow fall. This guy is now suing the architect IN Arizona.
.
Guess he was in a part of Wisconsin where they didn't bother too much with plan review.
It is a small rural town... wont say which one as this is currently ongoing...
Be careful with two gables side by side at different elevations... when snow melts, the lower gable will get additional snow.
Get your Structural Engineer to support the snow load. Technically whether you have a sloped roof or flat both have to be designed to withstand the snow load ++++ for drifting snow and the worst case scenario of a large snow fall followed by heavy rainfall ( not untypical in western north america )
The advantage of the flat roof is that it will hold the snow vs dumping onto adjacent walks / driveways / cars and people. Many cases of that over the years and in some situations with deadly consequences.
6"? this load is for about 36"
IRC has tables for snow load (prescriptive) (R301.2 at least in the 2012 IRC I have open). First number is elevation, second is snow load.
Generally 1 inch of snow is 1psf. So a snow load of 80 psf means about 80" of snow stacking.
2.2.1 Range in Snow Weight The weight of 1 foot of fresh snow ranges from 3 pounds per square foot for light, dry snow to 21 pounds per square foot for wet, heavy snow (Gooch, 1999).
https://www.fema.gov/media-lib...
So switched to 'per inch of thickness' that's 0.25 psf per inch of light dry snow to 1.75 psf per inch of wet heavy snow... Which means that 80 psf load would be just under four feet of heavy wet snow.
There's almost 2' of snow on my 60y old roof... and it's been raining all day. I'm not the least worried, but I know there are a few wankers on the street right now trying to take that snow off.
yes, missed it by 12, but still rarely happens in one event. at least here in colorado high rockies.
2006, Nederland, CO, 10' in three days. The only roofs that collapsed were really badly built.
I'm writing courses on commercial roof inspection for a professional association for home inspectors, so the roofs of main concern are low-slope. Steep-slope snow loads are generally not so much of an issue depending on the type of roof covering material and the pitch, although roof avalanches are a related issue with them.
Anyway, InterNACHI has about 25,000 members and I know some of them are going to be asked this, so I wanted to be able to tell them how to reply.
Here's an excerpt:
If instead of rain, a roof load from a storm consists of snow or hail, the load will continue to accumulate and will remain on the roof until the outside temperature rises enough for it to start melting and running off the roof; a much slower process than with rain.
FIGURE: Lower roof accumulation
The lower roof is on the north side of the building, in shade much of the winter.
If a downspout from an upper roof that is exposed to the sun discharges meltwater onto a lower, shaded roof, that meltwater may re-freeze, adding even more weight to the lower roof.
Neither the IRC nor the IBC give maximum depth accumulations for roof ponding. They specify that drains/scuppers be installed in a manner that will “…prevent the depth of ponding water from exceeding that for which the roof was designed…”
Inspection Note: Inspectors may be asked at what depth of snow on the roof should removal operations begin. The answer is “Because snow varies in water content, its weight per inch of accumulation will vary, so snow depth is not a good criteria on which to base snow removal decisions. Roofs are designed to support expected snow loads using historical data and design calculations include a safety margin.”
Commonly related defects we see with this stuff... Low sloped roofs with drains in shadow zones. So you get the original snow load, but as it melts in the sun, that water goes to the shade, and creates massive ice dams. Along those lines sort of... roof replacements have become a huge issue with condensation. EPDM, black, absorbed enough heat to vaporize the condensation that might form below it.... TPO does not. And with low slope, you don't normally have ventilation... We're seeing a lot of rot. Adding Ice & Water (non-permeable) to a poorly ventilated, no vapor barrier building, has the same effect. Condensation below the barrier forms, saturates the sheathing, and rot can take hold. We're going to see more of this with these big climate shifts... used to just be an issue in the mountains where they'd see the extremes.
Ventilation is the key to control icing and snowmelt. If I had advice for inspectors, start really looking at ventilation; main roofs are normally fine, but lower roofs, garages, projections, etc. I commonly have a defect of no ventilation.
Good post, mightyaa! I've covered vaporized condensation within roof systems, and lower roofs in shade fed by downspouts from upper roofs in direct sunlight, but good point... the source of shade doesn't really matter. A drain located in constant shade is likely to suffer ice damming in cold climates.
So we received 384 cm ( 12'6" ) of snow in December and another 120 cm ( 4' ) in January. I currently have a compacted depth of about 5' sitting on the roof of my home.... with no worries ( our design snow load is 205 lbs/sf and the additional rain load is another 20 lbs/sf) also have a green roof up there too to add to the weight!
There will be no snow removal. Roof designed by Structural Eng. to handle full load.
You have a brutally strong roof! What's your location?
in the mountains, north west of vancouver, canada !
Roof structure is 6 x 12 glulams @ 24" o.c. ( max span about 16') 2" d. fir decking w/ 3/4" ply sheathing as diaphragm, 6" rigid insulation w/ 2 ply torch on. 6" green roof system over entire roof area.... for those interested.
^beautiful story to read as I look at -33c walk in to work this morning.
This was in Poland a few years ago. After a heavy snowfall high winds drifted the snow onto a localized roof area where it failed. 65 people died and a lot more injured.
Just so y'all know, the roof courses I'm writing are online courses and consist of the present general course, covering aspects common to all low-slope roof-covering systems, and additional courses on the individual membranes; built-up, mod-bit, single ply, SPF, and liquid-applied.
The general course is a prerequisite to the individual membrane courses and is now at just under 40,000 words, so it's pretty comprehensive.
I gotta say, aside from the NRCA Manuals and Low-slope Roof Systems, by Griffen and Fricklass, locating sources of good information has been tough, so I appreciate any help. Especially photos that illustrate common failures clearly. Roofing companies and building owners won't let me onto roofs to take photos for liability reasons.
CCPIA is a sister organization to InterNACHI, which has around 25,000 members, so there are a lot of people looking forward to having courses on low-slope inspection available.
So... thank you for your help!
You might see if you can dig up HAAG Certified Inspector handbooks and reference documents.
They have some helpful links to various other organizations too
https://haagcertifiedinspector...
Oh, and since you are familiar with the NRCA, I assume you have access to the the NRCA Roofing and Waterproofing Manual(s)... Expensive little books, but pretty much the bible of reference documents on roofing.
I've taken the Haag commercial course and have the NRCA manuals, thanks. The NRCA manuals are very helpful but the NRCA looks at some things (like air/vapor barriers) a little differently from the ICC standards. And then there's the Building Science Corp.
Working on this section now. I prefer books when I can find them because so much of the online stuff is geared toward sales. Manual of Low-slope Roof Systems, Griffin and Fricklas is pretty good too.