Homemade roof mount help

Steve961 · February 2010

I am planning to make my own roof mount, but since I have no experience in this area I would appreciate some feedback on my design (see pics below).

Here are the parameters:

Galvanized steel roof, corrugated with 1" flats, 4:12 pitch
Uni-Solar US64 panel (54"x29")
1"x2"x1/8" Aluminum angle, bolted or welded in triangular shape
4" gap under panel to prevent snow buildup

Some questions?

Will the 1/8" Aluminum angle be strong enough?
Will it need any additional bracing (diagonal?) to hold up to the wind?
Due to the spacing of the flats, the panel cannot be bolted along the frame sides - just on the top and bottom panel frame. Is that OK?

Any other suggestions would be appreciated.

Thanks.

Steve

dwh · February 2010

Re: Homemade roof mount help

Looks potentially wobbly. I would cross brace.

BB. · February 2010

Re: Homemade roof mount help

Generally, for best strength against wind, the rails should be 1/4 the panel width in from each end -- rather than at the edges. Your current design could let the center of the panel bow in and out.

You could also do the mounting you have and place one more dead center too (three mounts total).

-Bill

niel · February 2010

Re: Homemade roof mount help

i'll agree with dwh's suggestion of crossbracing. aluminum does not like movements or vibrations that could weaken and crack the frame and the crossbracing will help this too in addition to dwh's point of it being capable of pancaking. i picture an x configuration on the back of the framework.
the angle is generally your latitude, however, many go with latitude + 15 degrees to reap more in the winter when there is less sunshine. to verify your understanding of the angle, it is 0 degrees latitude at the equator and the pvs would be flat on the ground or horizontal for 0 degrees. the increase with + 15 degrees may be a good compromise for you if you are actually in wisconsin as the steeper angle would allow snows to slide off easier in the winter. i suppose ideally you could devise a way of adjusting the angle for optimal winter/summer angles.
you will find that far more than 4 inches clearance above a surface will be needed. picture a 4 inch snowfall. now you have snow right up to the bottom of the pv right? now what happens with the snow that slides down the pv to the surface below? it will bunch up and wind up shading the bottom of the pv as it slides downward. don't forget that snows that have already accumulated can be far deeper than 4 inches and can result in the part pvs being buried for a good part of the winter unless you have access to clear the snows when needed.
hope this helps and know that i have mounted my us64s to the face of my brick wall using al angle mortared straight into the wall. i should've braced better myself as when the stainless screws are tightened this pulls on the al angle and has twisted it some being i gave a bit of leeway to fit the pv. it's still holding fine in spite of the twist.;)

niel · February 2010

Re: Homemade roof mount help

bb,
that bowing is not so much of a problem with unisolar pvs as there's no glass.

Steve961 · February 2010

Re: Homemade roof mount help

I kinda thought I would need cross bracing, it's just that I did not see it on the commercial mounts that I've looked at. My cabin, while it is surrounded by some trees, is still in the middle of a 220x440 yard farm field. The winds can be somewhat strong there.

Niel:
Is their a structural reason for the panel to be higher on the roof? That specific panel will be mostly for keeping the battery charged during the winter months when I am not there. It also won't be hard for me to clean it off if I need to. BTW, the panel will be at a 60 degree angle (my location is 45 lat).

BB. · February 2010

Re: Homemade roof mount help

If you have snow--placing the array closer to the roof edge may help shedding of snow.

Regarding position of frame work--could still see a collapsing of the metal sides to to lack of support (depending on the winds in your area).

-Bill

Steve961 · February 2010

Re: Homemade roof mount help

Bill:

What about this? BTW, I need to keep the panel as close to the peak as possible to deter theft - an unfortunate problem where I am.

niel · February 2010

Re: Homemade roof mount help

steve,
any shading will impede even a float charge to the batteries. eventually the snow melts, but just how long until that happens may be too long at times to be good for the battery. whatever your worst average ground cover for snow is i would double that for the minimum clearance. say if january is your worst month and you average 7 inches on the ground at all times then multiply that by 2 because of the doubling effect of the snow sliding off of the pvs. with 7 inches that would mean 14 inches for your clearance. i have about 2 feet of clearance on mine which is far overkill for my snowfall here in pittsburgh. there will be times of the pvs being snow covered, but this would lessen the times of no charge to the batteries.
regarding your last photo now you could slide the pvs up the framework so that some of the pvs stick up above the framework. assuming both pvs are mounted it would work well with the 3 triangular legs and the backbracing as shown. you could run another brace along near the top of the a frame or even better yet about at the middle of the hypotenuse of the a frame triangle connecting all of the a frames together for added sway resistance.
hmm. thinking further take the idea of your crossbracing shown in your photo which looks like this ^ and invert it in the center portion of the a frame. i wish i could verbalize this better or show you what i mean. if you would, put this to an illustration for me to show me if you understand.

mike95490 · February 2010

Re: Homemade roof mount help

Electrolysis Corrosion.

Aluminum, steel, galvi steel, nuts & bolts. They all need to play together.

niel · February 2010

Re: Homemade roof mount help

mike90045 wrote: »

Electrolysis Corrosion.

Aluminum, steel, galvi steel, nuts & bolts. They all need to play together.

quite true so use aluminum all throughout with stainless nuts, bolts, and washers.

BB. · February 2010

Re: Homemade roof mount help

Mounting looks OK.

-Bill

icarus · February 2010

Re: Homemade roof mount help

The disadvantage of mounting the entire assembly near(er) the eve is the tendency for the steel roof to hold snow, until it can shed the entire snow load in one fell swoop! I have seen 12/12 pitch roofs shed the entire winters worth of snow let go in 1 second, with damaging results to things that happen to be below. I have also seen a 6/12 roof, with only ~15' of run, let it's snow go hard enough to knock off the Metal-bestos chimney right off the roof!

Snow breaks help, but don't always work well enough. I would avoid PV on a steel roof except near the peak if you are in snow country. I was going to put steel on our new house when we built it, but I realized that I need to be on the roof on a regular basis during the winter. Sweep the panels, clean the chimney etc.

Tony

Steve961 · February 2010

Re: Homemade roof mount help

Niel:

Is this what you were thinking?

BB. · February 2010

Re: Homemade roof mount help

See, don't listen to a guy from California about how to manage snow. :roll:

'-Bill

icarus · February 2010

Re: Homemade roof mount help

Bill,

You just need to visit the Sierras more often!

Tony

niel · February 2010

Re: Homemade roof mount help

Steve961 wrote: »

Niel:

Is this what you were thinking?

not exactly as the back crossbracing was fine. firstly i was only saying that you could slide the pvs up the mount as to allow the top of the pvs to extend past the top of the mount. there is not reason both the tops of the frame and pvs must meet together as the pvs could hang over somewhat. this also would raise the pvs off of the bottom allowing more clearance.
you could also keep the tops to meet each other and to make the bottom clearance higher would mean to use more al angle to make the frames larger. in any case you better have more clearance than 4 inches.
as to the added bracing i was talking about it would be in the ^ shape (if the back bracing remains in the v shape opposite of the other bracing as shown in the above pic) and connected in 3 spots also as follows,
1 midway along the bottom al angle of the 1st frame to
2 the middle of the top (hypotenuse) al angle of the 2nd frame to
3 midway on the bottom al angle of the 3rd frame.
the added extra brace is like the ^ back bracing but slid forward and inverted to a v shape.

bill,
he said he didn't want the pvs on the edge as an added deterrent against theft.

dwh · February 2010

Re: Homemade roof mount help

I think you could get by with a single diagonal brace and just two of the triangle supports...

IF you supported the PV frame all the way around. I.e., a rectangular box facing the PV frame, with two triangles (one at each end) and a single diagonal cross brace.

Steve961 · February 2010

Re: Homemade roof mount help

dwh:

That would definitely be easier to build. I also moved the triangles in from the ends 1/4 length like Bill had suggested. Do you also think the ends of the panels would need reinforcement? It is a thin film panel without glass after all.

BB. · February 2010

Re: Homemade roof mount help

The Unisolar panels (PDF) appear to be fully framed--so supporting in 1/4 from each end should be optimum.

-Bill

niel · February 2010

Re: Homemade roof mount help

as shown that will not be sufficient to brace the pvs as it could still collapse in one direction. an x pattern on the rear between triangular frames would be best being you don't quite understand what i was driving at before. you don't want to depend on the pv framework to act as crossbrace supports either.

solarvic · February 2010

Re: Homemade roof mount help

Me living about 43 degree I know I don,t want panels on my roof. With a 5-12 pitch roof I had at least a foot of snow on my buildings and my garage. We had a meltdown that lasted about 10 days before all the snow melted off my roofs. I was worried about roof caveing in and there were some that did. Solar Vic

lotek · February 2010

Re: Homemade roof mount help

as pictured, the panel will catch a lot more wind off the back side roof. this wind will be excellerated by compression as it moves up the slope. You might consider moving the entire unit down the slope a little, putting it in the eddy wind currents.

tallgirl · February 2010

Re: Homemade roof mount help

niel wrote: »

as shown that will not be sufficient to brace the pvs as it could still collapse in one direction. an x pattern on the rear between triangular frames would be best being you don't quite understand what i was driving at before. you don't want to depend on the pv framework to act as crossbrace supports either.

That's true if flat bars are being used instead of L's or C's. But if the cross braces have stiffness in compression, it will resist collapse in either direction -- in one direction the brace is in tension, in the other the brace is in compression.

I do agree that the frames shouldn't be used as structural members and that a horizontal member is needed.

icarus · February 2010

Re: Homemade roof mount help

lotek wrote: »

as pictured, the panel will catch a lot more wind off the back side roof. this wind will be excellerated by compression as it moves up the slope. You might consider moving the entire unit down the slope a little, putting it in the eddy wind currents.

See the previous posts regarding snow issues on the roof,

Tony

Homemade roof mount help

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