Installing L-feet on metal roof?

[Mike90250]

Hi, i had to install an array on a metal roof (corrugated) and the problem that i ran into was that in order to mount a few of the L-feet i had to use a hammer to flatten down the ridge (about 1" high) that runs down the metal sheeting. The ridge just happen to be directly above the desired truss and moving to the truss on either side was not an option at the time so flattening the area where the L-foot needed to be was the solution to a well supported mount.
I was successful in attaching the L-foot however it didnt seem that it was in good craftsmanship to bang out the ridge with a hammer and i would hate to have to do it again.
I am looking for suggestions or alternatives to doing it this way.

Thanks.

Maybe you can get a wood spacer with a thru-hole that will leave the roof metal un-bent, and have a helper mount it from the inside, and you drill thru roof, and pass bolt thru spacer and onto the truss under it ? Mashing the roof is "less than optimal".

[Phil Factor]

Hi, i had to install an array on a metal roof (corrugated) and the problem that i ran into was that in order to mount a few of the L-feet i had to use a hammer to flatten down the ridge (about 1" high) that runs down the metal sheeting. The ridge just happen to be directly above the desired truss and moving to the truss on either side was not an option at the time so flattening the area where the L-foot needed to be was the solution to a well supported mount.
I was successful in attaching the L-foot however it didnt seem that it was in good craftsmanship to bang out the ridge with a hammer and i would hate to have to do it again.
I am looking for suggestions or alternatives to doing it this way.

Thanks.

I can include a photo if anyone needs a visual aid.

I've always used two layers of 2x6 blocking fastened between the rafters or trusses with 3 or 3-1/2" exterior screws. Admittedly, I'm not crazy about mounting L-feet to metal roofs since they are difficult to flash properly. Lately I've been considering the best solution for metal roofs that ldon't have standing seams. S-5! and a few other companies have spanning blocks that span the "ridges" and have holes to accomodate mounting of L-Feet.

[russ]

Hi Phil - Welcome to Solar Panel Talk!

Russ

[Vandergraaff]

DayStar,

I seem to have the same roof as you on my new garage, right down to the color!
Mine has purlins (2x4) running perpendicular to the ribs in the steel. I imagine you have something horizontal as well, or the roof surface wouldn't be very stiff.

I talked with Jean at Unirac about this just yesterday. She recommends their Flattop or Aluminum two-piece standoffs for my application. The L-Bracket mounts to the top of the standoff.

I also considered Unirail running vertically, but decided that utilizing components from a single manufacturer would make my Engineering easier, and make things easier on our Code Official

How are you making out on this project?
(No affiliation with Unirac, Unirail, or Unicycles)

Mod note. what have you got against Unicorns ?

[Naptown]

DayStar,

I seem to have the same roof as you on my new garage, right down to the color!
Mine has purlins (2x4) running perpendicular to the ribs in the steel. I imagine you have something horizontal as well, or the roof surface wouldn't be very stiff.

I talked with Jean at Unirac about this just yesterday. She recommends their Flattop or Aluminum two-piece standoffs for my application. The L-Bracket mounts to the top of the standoff.

I also considered Unirail running vertically, but decided that utilizing components from a single manufacturer would make my Engineering easier, and make things easier on our Code Official

How are you making out on this project?
(No affiliation with Unirac, Unirail, or Unicycles)

Mod note. what have you got against Unicorns ?

Or more importantly Mimes
You know a mime is a terrible thing to waste.

[Casandrita]

Power House Fastrack solar PV Racking system

I've experienced over 200 kWatts worth of solar PV installs on standing seam metal roofs here in Ontario, Canada. Natural Resources Canada ("NRCan"), in conjunction with the Canadian Solar Industry Association ("CanSIA") and the Truss Plate Institute of Canada ("TPIC") are developing a 'solar-ready' truss design, including recommended blocking practices for solar PV racking system 'feet' installation (Ref: http://www.cansia.ca/sites/default/f...ign_report.pdf)

Mounting anything directly into a truss is not recommended. What is recommended is installation of blocking into which the lags would be installed i.e., blocking (typically 2X6) is installed either horizontally from truss to truss or 10-12" lengths are sistered on either side of the trusses.

The racking system we have been using http://www.heshomeenergy.com/hesprod...%20Rack_V1.pdf includes a lag screw / threaded rod assembly with a domed rubber gasket at the top of the lag screw portion ("ejot")http://www.ejot.com/ejot.de/Der_EJOT...teLang__en.htm , which seals to the roof surface. We position mounting points in off-ridge/metal seam locations to achieve a good seal and then redundantly seal over top of the gasket with exterior grade silicon.

The challenge with this system comes from the fact that the ejots need to be installed perpendicular to the roof surface in both planes to ensure that the individual modules of the finished array are flush with each other and parallel to the top of the rails to ensure good WEEB bonding.

Does anyone have a good guide/jig to ensure that pre-drilled lag screw holes are perpendicular to the roof surface in both planes? Foot-print of jig/guide needs to be quite narrow when drilling close to a standing seam.

Hoping to learn from others in this forum during this exciting time for the industry here in Ontario

[russ]

Hi Casandrita - Welcome to the forum!

Good post.

Russ

[Vandergraaff]

Mounting anything directly into a truss is not recommended. What is recommended is installation of blocking into which the lags would be installed i.e., blocking (typically 2X6) is installed either horizontally from truss to truss or 10-12" lengths are sistered on either side of the trusses.

Do you have a sketch of this configuration?
I am mounting feet into 2x4 purlins (laying on their sides) running between trusses and found that a single lag screw, engaging 1.5 inches of purlin, would not have the withdrawl strength neeed for uplift. For that reason I'm using a foot with 2 mounting holes (2 lags). It looked to me that the single lag products, with their long threaded sections, were best suited to go directly into a truss.

Why the aversion to attaching to trusses?

[Casandrita]

Truss Intrusion Aversion Reasoning

Based on what I've been reading in the proposed standards for new construction: "The attachment of screw to truss chords will reduce the cross-section of the top cord
and hence reduce top chord capacity by a factor that depends on the diameter of the screw.
...
For example, the use of 3/8" lag screws ...(typical) ... will decrease axial and bending strength by 25%"
[Source: Solar-ready truss design for solar PV racking system 'feet' installation http://www.cansia.ca/sites/default/f...ign_report.pdf]

Hoping this clarifies the reasoning.

As for images of the blocking: We have been photo documenting all of our work through our Facebook site: http://www.facebook.com/#!/media/set...53585347994374 www.excessenergy.net There are shots of the blocking sisters to the trusses but unfortunately no shots of the double L-brackets mounted on the threaded lag screws we were required to install due to the structural assessment. Imagine 2 X 3/8" lag screws 3" centre to centre into the blocking sistered to the truss.


Hope this helps.

[Vandergraaff]

Casandrita,

This publication makes a good point. I doubt many Engineers are considering the lag screw's effect on the truss, and it isn't even mentioned in the Unirac Code Compliant Installation Manual. I forwarded a copy of this report to Unirac and they stated that it is the responsibility of the Design Professional to determine the adequacy of the structure. Still, since most of their mounting methods involve driving a lag into a truss, there should be some caution given in their manual.

In my case the lags are holding mounting feet into 4x2 purlins (lag threads engage 1.5 inch of wood). This is similar to the "blocking" depicted in photo 3. The Engineer that designed the structure certified that the trusses could carry the load of the modules, but he did not assess point loads on these purlins. When I did the analysis myself I found that nearly twice the allowable load would be applied in a worst case snow. He saw the error in his ways and gave me reinforcement options.

Also, a single lag would not have sufficient retention in such a shallow joint, so I chose to use a mounting foot with 2 holes to spread the load between 2 lags.

BTW, my roof reinforcing is complete, and the permits are expected today. Hope to start installing mounting feet tomorrow if the weather holds!

Art

[russ]

Well done Art!

[silverhorsefarm]

That's worrisome. My install is lag'd into the 2X4 purlins that cross the trusses with single lags through a corrugated Ondura roof using hangar bolts. Did your calculations reveal that 1.5" of penetration did not provide sufficient strength to bear expected loads or stay put with expected wind-lift?

Casandrita,

This publication makes a good point. I doubt many Engineers are considering the lag screw's effect on the truss, and it isn't even mentioned in the Unirac Code Compliant Installation Manual. I forwarded a copy of this report to Unirac and they stated that it is the responsibility of the Design Professional to determine the adequacy of the structure. Still, since most of their mounting methods involve driving a lag into a truss, there should be some caution given in their manual.

In my case the lags are holding mounting feet into 4x2 purlins (lag threads engage 1.5 inch of wood). This is similar to the "blocking" depicted in photo 3. The Engineer that designed the structure certified that the trusses could carry the load of the modules, but he did not assess point loads on these purlins. When I did the analysis myself I found that nearly twice the allowable load would be applied in a worst case snow. He saw the error in his ways and gave me reinforcement options.

Also, a single lag would not have sufficient retention in such a shallow joint, so I chose to use a mounting foot with 2 holes to spread the load between 2 lags.

BTW, my roof reinforcing is complete, and the permits are expected today. Hope to start installing mounting feet tomorrow if the weather holds!

Art

[Vandergraaff]

Silverhorse,

My calculations were done via the Code Compliant Manual on the Unirac website. They also have an on-line calculator. I encourage you to play with it.

In my case I found that uplift loads (100 mph winds) could apply as much as -375 lb of uplift IF my mounting feet were 3.5 feet apart. IF 3/16 lags are used in 1.5 thick wood, and IF that wood is pine, there could be as little as 300 lb of retention. Notice there are a few "IFs" involved. I believe you said you used Hanger Bolts, which are 3/8 dia. instead of my 5/16's.

I also found that the builder's Engineer did not consider that the loads were concentrated at points on the purlin. I needed to reinforce these after the fact.

See what you get from the Unirac calculator. Let us (and your Engineer) know if you find anything disturbing. Also, Jean Araya at Unirac is very helpful: jeana@unirac.com. See if she can tell you the retention force for those Hanger Bolts.

Good luck,
Art
(No affiliation with Unirac, Unicycles, Unicorns, Unirail, Monorail, Conrail, ConEdison, etc.)

[silverhorsefarm]

The purlins are 2' OC, but the calculations were based on a 3" thread depth which is impossible with a flat 2X4. The calculator required 1.7" of thread depth.. Plus, some of the lags clearly nick the purlin near the edge, making me wonder if those calculations are still good. The purlins were listed as larch or fir, but is that a safe assumption?

Another oddity, the installer said they would use hangar bolts owing to the corrugation, but all the docs given read an install for composite shingle, 5/16 lags, and standard L-feet. Now I am confused.

Silverhorse,

My calculations were done via the Code Compliant Manual on the Unirac website. They also have an on-line calculator. I encourage you to play with it.

In my case I found that uplift loads (100 mph winds) could apply as much as -375 lb of uplift IF my mounting feet were 3.5 feet apart. IF 3/16 lags are used in 1.5 thick wood, and IF that wood is pine, there could be as little as 300 lb of retention. Notice there are a few "IFs" involved. I believe you said you used Hanger Bolts, which are 3/8 dia. instead of my 5/16's.

I also found that the builder's Engineer did not consider that the loads were concentrated at points on the purlin. I needed to reinforce these after the fact.

See what you get from the Unirac calculator. Let us (and your Engineer) know if you find anything disturbing. Also, Jean Araya at Unirac is very helpful: jeana@unirac.com. See if she can tell you the retention force for those Hanger Bolts.

Good luck,
Art
(No affiliation with Unirac, Unicycles, Unicorns, Unirail, Monorail, Conrail, ConEdison, etc.)

[Vandergraaff]

Sounds like you have some quality issues to take up with the installer. Did the local building inspector see the off-center lags?

What calculator are you referring to? The purlins should have some ID stamps visible. From these you can decode the wood type.