Roof Curvature

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Roof Curvature

January 27, 2017, 03:43:21 pm

I am installing solar panels on the roof of my 1997 U270. The dimensions are approximately 78' x 38". I need to mount standoffs on the roof and would like to know the geometric shape of Foretravel roofs in order to figure out the proper length of the standoff. Would they follow the mathematical formula for a:

- Circle
- Lens
- Parabola

The standoffs will provide 3.5 inches of clearance between the solar panel and the roof. The solar panels do not bend much so they will not follow the curvature of the roof.

Re: Roof Curvature

Reply #1 – January 27, 2017, 05:35:10 pm

Tim,

Best bet is to take them up there and see how you want to orient them. Standoff if orientated along coach long axis will be totally different than if they are better laid out transversely. Be sure to leave yourself walk room to get to A/C's, and any other roof-mounted stuff. And, you may choose to have one on the long axis and one transversely mounted. Much of the answer depends on where other roof mounted stuff is (vents, satellite dish/dome, shower bubble, etc differ with coach and interior layout and where toys were installed).

Re: Roof Curvature

Reply #2 – January 27, 2017, 05:44:41 pm

Tim, Math Ahead!
I would be surprised if it was anything other then just a radius. Look at the back wall of your coach on the inside. You should see the arc of the roof line on the inside. Measure the width of the arc between two points and the height of the arc at the mid point between the two points. You can also do this on the roof with a long straight edge and equal distances from the ends to the roof for the height. The total roof thickness is about 1.75-2"

If your panels are framed then wouldn't you just put equal length standoffs on each corner? Following the roof curve will let then drain off better.

I measured mine. If the height for a 76.5" width is 2-1/4" then the radius is 326.25"

So if your panels were 78" long (78' would present a whole different set of issues) and you mounted then transversely with 3-1/2" standoffs at the corners you would have about 1.2" of clearance.

Re: Roof Curvature

Reply #3 – January 27, 2017, 09:23:17 pm

These 320 watt panels are frameless and can flex about 2.5 inches, length wise. The three heights of standoffs, referring to your drawing, will be 2 inch cuts of pressure treated wood fastened with VHB tape:

1.55x3.55 inches (standard 2x4) at the center

TBD inches (cut 2x6) mid-way between center and the ends. I'll use your formula.

1.55x4.55 (cut standard 2x6) at the ends

This is based on my measurements.

Thanks for the formula.

Re: Roof Curvature

Reply #4 – January 28, 2017, 09:27:16 am

I'm confused. What is the need for the "extreme" roof clearance? Secondly, I would never use treated wood to mount anything on my roof. There are such nice aluminum or stainless things that would do a better job and porbably live longer.

Re: Roof Curvature

Reply #5 – January 28, 2017, 11:15:38 am

Quote from: lgshoup – January 28, 2017, 09:27:16 am

I'm confused. What is the need for the "extreme" roof clearance? Secondly, I would never use treated wood to mount anything on my roof. There are such nice aluminum or stainless things that would do a better job and probably live longer.

X2. An inch and a half roof clearance allows for good air flow underneath and lets you route a flex conduit from panel to panel easily. I used rectangular aluminum tubing to space the panels above the roof. Not expensive and permanent. Think the tubing and the small mounts with the rounded corners were about $100. Aluminum rectangular tubing is available in twenty foot lengths and is about $40 if you purchase from an aluminum supplier. I used two lengths for all the panels.

Since the roof has over one hundred holes already, I used an expanding fastener, drilled holes and then sealed them with GE silicone. A stud finder locates the aluminum underneath so I screwed directly into that when the spacing was convenient. I cut about a 1 1/2" piece of aluminum angle from tubing to roof as shown in the photo and used stainless hardware everywhere I could. The two screws shown go directly through the roof into the aluminum roof structure.

Pierce

Re: Roof Curvature

Reply #6 – January 28, 2017, 11:16:28 am

What is the plan to attach the frameless solar panel to the roof? I was thinking you might be able to drill through the glass and make an attachment? or? Havnt seen the panels to experiment.

Re: Roof Curvature

Reply #7 – January 28, 2017, 12:01:20 pm

Quote from: DavidS – January 28, 2017, 11:16:28 am

What is the plan to attach the frameless solar panel to the roof? I was thinking you might be able to drill through the glass and make an attachment? or? Havnt seen the panels to experiment.

No need to drill. These are flexible for a curved roof and make it a snap: http://i.ebayimg.com/images/g/lhsAAOSw-kdXy5s2/s-l500.jpg

http://i.ebayimg.com/images/g/lhsAAOSw-kdXy5s2/s-l500.jpg

Pierce

Re: Roof Curvature

Reply #8 – January 28, 2017, 09:17:43 pm

My research revealed a recommended stand-off clearance of 4 inches for cooling under the solar panels. Cells can get very hot when there is strong sun and shade on a single panel due to solar AND electrical heating. Two 320 watt panels in series is a lot of power to cool if a part of a panel is shaded.

CONSIDERATIONS
Wood
Pressure treated with spar varnish: 20 year life, easily machinable, forgiving and cheap.

Aluminum extrusions and brackets
Not easily machinable to fit roof curvature and 4 inch stand-off height requirement. Expensive.

VHB tape: Yes. Very strong.

Drilling holes into roof or glass panels
No more holes!

Silicon
No. Yuk. Messy. Very hard cleanup. Use self-leveling Di-cor instead.

Re: Roof Curvature

Reply #9 – January 28, 2017, 09:32:50 pm

VHB has held our panels on for 4 yrs and no movement. Would never dream of drilling into roof- like asking for trouble when tape does it fine.
But then I seem to be a renegade or such!!!
JohnH

Re: Roof Curvature

Reply #10 – January 28, 2017, 09:52:28 pm

Quote from: Tim – January 28, 2017, 09:17:43 pm

My research revealed a recommended stand-off clearance of 4 inches for cooling under the solar panels. Cells can get very hot when there is strong sun and shade on a single panel due to solar AND electrical heating. Two 320 watt panels in series is a lot of power to cool if a part of a panel is shaded.

CONSIDERATIONS
Wood
Pressure treated with spar varnish: 20 year life, easily machinable, forgiving and cheap.

Aluminum extrusions and brackets
Not easily machinable to fit roof curvature and 4 inch stand-off height requirement. Expensive.

VHB tape: Yes. Very strong.

Drilling holes into roof or glass panels
No more holes!

Had 1200 watts on one inch brackets. Shot interior roof temps on hot texas day. [95 f] found less than 2 degree difference between panel and non panel areas. Thinking 4 inches gives lots of lift effect at speed and subsequent stress on the panels.

Re: Roof Curvature

Reply #11 – January 28, 2017, 10:03:34 pm

Here are my opinions for what they are worth.

Spar varnish on wood has a two year life in the sun. Pressure treated wood needs a minimum of one year per inch to air dry sufficiently to let spar varnish adhere well and it must be thoroughly sanded to remove the surface machining marks (surface compression due to planing knives). I make curved structural wooden beams from premium 5/4" treated decking (southern yellow pine) dried indoors in stickered weighted stacks for at least a year. Glue and finished just don't work if you don't dry the wood.

Doesn't it make more sense to run the panels lengthwise?

Use 3M 4000 UV on the roof. Dicor self leveling is for lap seams, dries much slower and will attract more dirt.

JohnH is not a renegade, maybe a bit opinionated but that is OK.

Re: Roof Curvature

Reply #12 – January 28, 2017, 10:11:35 pm

JohnH is not a renegade, maybe a bit opinionated but that is OK.
Quote Roger.
Yeah I guess I am Rog' but when I see the crap that people pay thousands for and compliment the place I shake my head.
RV shops are out to make a very good living on the backs af people who do not know how to "drive a nail straight", IT MAKES ME MAD.
fwiw
JohnH

Re: Roof Curvature

Reply #13 – January 28, 2017, 10:22:31 pm

Right with you there John. I will do anything I can myself and do my best to learn how to do the rest. And If I can help someone learn how to do something or be more efficient or productive at it then I am just paying back for all the help I have received from folks like you. Thanks!

Re: Roof Curvature

Reply #14 – January 29, 2017, 12:38:11 am

Tim the proposed plan with height/wood is unusual, mostly because none of us have ever seen it before on any sort of MH/trailer. The only wood structures I've seen with solar are off-grid residential "solar farms" that are on wooden built frames in the yard. Most of your points are quite accurate in theory, but a few things to note:

- You stated some of your preparations are for keeping the panels cool when the panel is partially shaded and partially receiving sun. Two points here: 1) The reason to keep the panels cooler is that they are more efficient at energy producing, let's arbitrarily call that 10% more efficient, which is on the generous side of in the ballpark. Otherwise, there's no real danger in a solar panel that gets a little hotter. 2) But, if even one single full cell (6 in x 6 in?) is shaded on your panel, it will not produce energy. You'll lose 90% of the full-sun output, even if the rest of the panel receives full sun. Same goes for all panels wired in a series. One cell on one panel fully shaded drops the entire series. I would suggest that even though clearance and underneath airflow for when a panel is a factor, it is more important to make your goal an arrangement in which a partially shaded panel scenario never exists, - ensure none of the other roof hardware can cast partial shadows on any of your panels at any time of day that your other panels would be receiving full sun.

- Depending on the wattage/vmp, several smaller panels wired together can function exactly the same as a single larger panel. That's essentially all a larger panel is - more cells in a row wired under the same frame. If you ever run into height/clearance/curvature/arrangement/shading difficulties with a larger panel, just go with several smaller panels, wired in the method that would most closely resemble the large panel. It comes down to more holes/securing points vs. better layout, and personal preference.

Re: Roof Curvature

Reply #15 – January 29, 2017, 11:08:14 am

Quote from: Tim – January 28, 2017, 09:17:43 pm

Pressure treated with spar varnish

Agree with Roger vanish will not stick well to PT lumber. Adhesive most likely will stick to varnish but i would not trust bond strength between varnish and wood to secure solar panels going down the road. Consider getting some aluminum plate, angle, and tubing along with a few self tapping screws then fab a standoff bracket that will both be strong and stay stuck to the roof.

Re: Roof Curvature

Reply #16 – January 29, 2017, 11:17:54 am

Wood is great for the interior but has no place on the roof. If you have ever owned a boat, you know what water and sun do to wood. To adjust for any roof curvature in mounting a frameless panel, the clamps that I posted yesterday will allow for curvature with the flexible rubber that secures the glass. Do a Google search for solar suppliers and then ask them what they recommend.

You do need to mount the panels a little above the roof for a couple of reasons. If you want to do a neat, and trouble free job, you don't want to have the solar wires exposed when they run from panel to panel. That's why I used the vinyl flex conduit to run the wires inside. It's about 1 1/4" in diameter so that's the minimum distance the panel has to be above the roof.

The panels will produce more if cooler and since the dark blue panels take up a significant area of the roof, cooler panels mean less heat radiated down into the coach.

While at low sun angles, the ACs and vents will shade a panel or two. This does not seem to make much difference in the output. A half hour before the sun comes up, the panels will start trickle charging the batteries. The higher in the sky, the more watts they put out. We were fortunate that our interior design allowed the four large commercial panels (65 lbs each) to be mounted on our coach's roof. There is certainly nothing wrong in mounting a larger number of smaller panels where they will fit. Voltage delivered to the controller should be as high as possible but kept about 8-10% below the controller's maximum rated input. I could have wired all panels in series for 144 volts total into the controller but Midnite felt that on a cold morning, the input could exceed the 150 V maximum so I opted for a series/parallel connection for 72 volts down to the controller.

Pierce

Re: Roof Curvature

Reply #17 – January 29, 2017, 11:57:30 am

Couldnt you just buy the track rail and pie cut it in a couple areas to get the desired affect? If you are running the across the roof and not long ways.. easy as a couple pie cuts to bend the material?

IF I understand what you are trying to do here...

Re: Roof Curvature

Reply #18 – January 30, 2017, 06:36:03 am

Decided to go with 2x4x2inch aluminum angle. I will machine it with slots to provide variable angle, variable height capability. With stainless fasteners, it should be virtually weather-proof.

Thanks for your unput.

Re: Roof Curvature

Reply #19 – January 30, 2017, 02:20:14 pm

Quote from: Tim – January 30, 2017, 06:36:03 am

Decided to go with 2x4x2inch aluminum angle. I will machine it with slots to provide variable angle, variable height capability. With stainless fasteners, it should be virtually weather-proof.

Thanks for your unput.

Take pics plz.

Re: Roof Curvature

Reply #20 – February 07, 2017, 10:03:25 am

Here's a photo of the "Care Free" RV solar panel mounting system using aluminum brackets. It will securely fasten the frameless, semi-rigid 78x38 inch panels to the U270, regardless of roof curvature. It includes:

- Adjustable height
- Adjustable angle
- No holes. Uses 3M VHB tape.
- Maintenance-free. Uses aluminum and stainless steel bolts and nuts.

I spent the last few weeks procuring and machining the brackets. This is my first experience with using a Bridgeport milling machine. The soft aluminum was gouged a little in the process, but deburring produced acceptable results. For four 320 watt panels, there are 120 brackets, providing support for the most snow loads, winds and G-forces.

Thanks for you suggestions related to the use of wood. I love wood, but it would probably crack and rot over time.

Next step, wait for warmer weather to mount the panels.

Re: Roof Curvature

Reply #21 – February 07, 2017, 10:13:06 am

\Tim, you mention Bridgeport and it takes me back centuries to my old Machining days.
JohnH

Re: Roof Curvature

Reply #22 – February 07, 2017, 10:33:01 am

Tim, might I suggest a "little bit of insurance"? The bolt, lockwasher, and nut allow adjustment and locking up after positioning, but--me being a bit paranoid or anal, I would, after getting a panel locked into desired location, drill a 11/32 diameter hole thru each angle and use a nut/bolt/lockwasher assembly as a position locking key.
This way vibration wouldn't cause the panels to possibly slide out of the original location.
Again, just me being paranoid and liking a bit of "extra" insurance.
Forgot to mention one of my reasons: Aluminum is soft so fasteners can "coin" in and you lose location holding ability.

Re: Roof Curvature

Reply #23 – February 07, 2017, 10:39:40 am

I used to mill Tungsten carbide tip pockets in end cutting tools on a Bridgeport Mill, then also a Cincinnati horizontal mill milling tip pockets using Woodruff keyway cutters.
Fun at first, but way boring after a day or so. Sure wanted to make other things way more creative, but it was my job at the time.
Had a lot more fun trying to machine a machine column on a VERY LARGE horizontal boring machine with a 50 Hp motor!

Re: Roof Curvature

Reply #24 – February 07, 2017, 10:53:02 am

Take a look at this and then consider if you really want to drill more holes in your roof.

3M™ VHB™ Tapes | 3M United States

Also consider on down the road when panels are obsolete, damaged etc. There's a good chance mounting brackets will not be right for the replacement panels due to mounting dimensions, shape etc. With VHB, you run a piano wire between two handles and cut the tape. No holes left.

Has anyone thought of placing some reflective insulation (mylar bubble wrap) under the panels? If you're worried about heat might mitigate. It can be taped down as well.

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