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Recommending snow removal (flat roof) at a certain depth

Inspectorman

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.

 
Jan 23, 19 6:58 pm
Non Sequitur

no. You’re more likely to ruin your roof by shoveling the snow that it’s weight causing structural damage. 



Jan 23, 19 7:16 pm
OneLostArchitect

no structural should have covered this. Especially with snow drifts in mind. 

Jan 23, 19 7:21 pm
OneLostArchitect

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. 

Jan 23, 19 7:24 pm
Inspectorman

Guess he was in a part of Wisconsin where they didn't bother too much with plan review.

OneLostArchitect

It is a small rural town... wont say which one as this is currently ongoing...

BulgarBlogger

Be careful with two gables side by side at different elevations... when snow melts, the lower gable will get additional snow. 

Jan 23, 19 7:42 pm
whistler

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.  


Jan 24, 19 2:17 pm
JLC-1

6"? this load is for about 36"

Jan 24, 19 2:26 pm
mightyaa

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.  

Jan 24, 19 3:39 pm
JLC-1

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...

mightyaa

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

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

yes, missed it by 12, but still rarely happens in one event. at least here in colorado high rockies.

Inspectorman

2006, Nederland, CO, 10' in three days. The only roofs that collapsed were really badly built.

Inspectorman

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.

Jan 24, 19 10:13 pm
Inspectorman

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.”


Jan 24, 19 10:29 pm
mightyaa

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

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

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

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.



Jan 25, 19 2:37 pm
Inspectorman

You have a brutally strong roof! What's your location?

whistler

in the mountains, north west of vancouver, canada !

whistler

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

^beautiful story to read as I look at -33c walk in to work this morning.

Volunteer

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.

Jan 25, 19 3:01 pm
Inspectorman

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!

Jan 26, 19 7:16 pm
mightyaa

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...


Jan 27, 19 6:15 pm
mightyaa

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

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.

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