[RE-wrenches] Roof flashings on L-feet in high snow load conditions

[Troy Harvey]

Hi Benn,

Usually the solar array is not the limiting factor in my structural calcs. 

The rails could span farther between feet, it is the point loads on the building structure that is a problem. Unless it was designed for PV, the uniform load of the snow is now getting transferred to the structure below at only points which can easily overload the building structure. Often we have to design for every rafter to be attached to keep the uniformity of the load on the rafters. That easily gets to be a lot of feet in an install.

On the shingles, we do use a good flat bar, but have found that some shingles out there are just not that good. When facing 100 feet in a residential install, the chances of ripping shingles in my mind needs to be balanced with the benefits... I am not all that certain of which is really higher risk.



thanks,

Troy Harvey
---------------------
Principal Engineer
Heliocentric
801-453-9434
taharvey at heliocentric.org



On Nov 13, 2012, at 8:08 PM, Benn At DayStarSolar <benn at daystarsolar.ca> wrote:

> Troy,
> Maybe adding additional support to the trusses would allow you to spread out the Lfeet a bit more?  ...but then there is still that added cost!?!
> 
> QuickmountPV for one, makes a pretty solid case that any roof penetration needs to be flashed if the waterproof integrity of a roof is to be maintained. Especially when the roof is under warranty. 
> Yeah the added cost can be tough to get used to, but it is the cost of doing it right.(my perception)  I was always 100% confidant that none of my Lfeet-straight-to-shingle penetrations would ever leak but when the flashed mounts came along it didn't take long to accept them as a better method. 
> 
> Regarding working with the shingles... Do you use a good flat roofing bar to separate the shingles?  That is a must!  Sometimes you have to be patient and dont rush it, just break the seal a bit at a time or they will crack/tear for sure. Also the working temperature is key. Shingles have a recommended working temp and above/below that temp greatly increases the chance of damage.  
> If the temp is to cold and the shingles are brittle then a heat gun may help soften them and loosen the adhesive. Not to much, not to little
> 
> William,
> Yeah the term 'gravity flashing' is new to me too.  
> 
> While adding Lfeet for support without lagging them into the roof seems like a reasonable solution to supporting longer rail spans, are you not concerned that the Lfoot may wear thru the shingle after years of shifting around with wind lift and snow load?
> Maybe I'm thinking to cautiously? Do you put anything btwn the Lfoot and the shingle?
> benn
> Sent from a 'smart' phone, with touch screen keys. Please excuse shortcuts and typos. 
> 
> On 2012-11-13, at 5:51 PM, William Miller <william at millersolar.com> wrote:
> 
>> Troy:
>> 
>> I do not know what a gravity flashing is.  Can you elucidate?
>> 
>> If you have adequate upload mounting strength and need more download support, you can consider using additional L-fee that simply rest on the shingles without penetrating the roof.  We do that sometimes to take the sag out of some DPW racking products if the span is long.  This should be an easy way to provide more snow load support.  The caveat is that, on pitched roofs, the resting feet do not provide any support down pitch.
>> 
>> William Miller
>> 
>> At 08:04 AM 11/13/2012, you wrote:
>>> Hi all,
>>> 
>>> We have been doing PV installs for years with L-feet and silicone without problems ever. Even still, gravity flashings seem attractive in shingle roofs, to provide a second level of security, and a more professional install (at least in perception). But the cost of these systems in significant in high snow load areas where we often have L-feet every 2 feet on center, to evenly load the structure below. In todays costs, the feet could cost as much as 33% of the panel value.
>>> 
>>> I'm wondering what other people are doing in high snow load areas?
>>> 
>>> Also. I have noticed that there is a flip side to the risks. We have found that unless you have good quality shingles, on a preexisting roof, that sometimes the adhesive sticking the shingles together is stronger than the low quality shingles themselves - adding risk of trying to shoe horn flashing in after the fact. Your experience?
>>> 
>>> thanks,
>>> 
>>> Troy Harvey
>> 
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