Roof-Mounted Racks

for Solar Water Heating Collectors
Intermediate

Inside this Article

A rack of SHW collectors
Three collectors on a typical roof- or ground-mount rack
Illustration showing wind loads on a rack
Collectors in landscape position.
Installing collectors in a landscape position can improve the aesthetics of the installation and reduce the wind loads on the roof.
Side of house mounted collectors
When the roof orientation is less than ideal, tilt racks can often be rotated 90° to allow for a wall-mounted array.
Illo of typical tilt-mount design
Tilt-up rack
This aluminum tilt-up rack for flat-plate collectors (by SunEarth) makes the most of a shallow roof pitch.
Cross-braced tube collector
Evacuated-tube collectors often need cross-bracing, seen here through and behind the tubes.
Tube clamps for rack
A close-up of the clamps that hold Apricus tubes to the rack.
Rack attachment clip
This rack attachment clip is designed to work specifically with the frames of Solar Skies collectors.
Clamps for standing seam roof
Seam clamps are often used when mounting collectors on standing-seam metal roofs. It is important to determine how the metal roof is attached to the roof structure to ensure that it can transfer the collector loads.
A rack of SHW collectors
Illustration showing wind loads on a rack
Collectors in landscape position.
Side of house mounted collectors
Illo of typical tilt-mount design
Tilt-up rack
Cross-braced tube collector
Tube clamps for rack
Rack attachment clip
Clamps for standing seam roof

The installation of solar water heating (SWH)  collectors is an investment in one’s energy security. Proper installation protects that investment.

A roof-mounted array has the economic benefit of utilizing an existing structure—your home or garage’s roof—rather than having to build a separate structure to support the collector array, which can be costly (see “Ground-Mounted SWH Systems” sidebar).

Even so, a roof-mounted SWH system should not affect the building’s structural integrity. Examine the roof structure and be certain that the rack can be attached to the roof framing—not the sheathing! If the roof already looks like it is having a difficult time doing its job—i.e., it is sagging or bounces when you walk on it—you may need to reinforce it. If you plan to use a tilt rack (see “Tilt or Parallel?” sidebar), be sure to understand collector loads on the roof. It may be necessary to consult a professional for advice or have the system engineered.

Design Loads

This article provides a glimpse into the mind of a structural engineer and discusses the impact of a collector array on a structure. This insight should help you determine the various forces that can affect a project, and to appreciate the expertise that an engineer can lend to your project.

The forces upon a collector and roof are the major consideration when designing a rack or selecting mounting hardware. These forces are categorized as dead loads, live loads, and environmental loads.

Dead loads result from the weight of the collector, the mounting hardware, and the collector fluid—and remain constant. For SWH systems that use glazed flat-plate or evacuated-tube collectors, the collector dead load is approximately 3 to 5 pounds per square foot (psf). For comparison, a layer of shingles has a dead load of 2 to 3 psf. The exact empty and filled weights of a particular collector can be obtained from the manufacturer’s specification sheets. Integral collector storage (ICS) units and thermosyphon systems contain tanks and may have dead loads that range from 25 to 70 psf.

Live loads are intermittent, resulting from movable objects. The weights of the staged equipment and of solar installers working on a roof would be considered live loads. These loads typically don’t affect rack design, but the roof itself must be able to support live loads.

Environmental loads come from rain, snow, earthquakes, and wind. Rain is rarely considered when designing the mounting for a solar array unless the design causes pooling on the roof. It is best practice to avoid such a situation by orienting the attachments to the roof in a manner that permits normal drainage of the roof surface.

Typical snow loads in the United States range from zero in areas such as Florida and Southern California to 100 psf in northern Maine to up to 400 psf in some locations in Alaska. Drifting and sliding snow can increase these loads significantly. Snow loads for collector racks are affected by local siting—whether they are in an exposed area where wind will readily blow snow off, or they are in a sheltered area where the snow is unlikely to be blown away.

Comments (5)

norm rostocki's picture

i'd just like to comment on this dialog about the shw evacuated tube array and the pv panel in the picture. I would say that a PV panel for backup should not be unusual, it should be mandatory. I have an ac controller also, but what happens when the grid goes down? its an immediate crisis, as I found out the hard way. now i have a small dc pump that connects to a pv panel via an ice cube relay to run that pump in a no grid power situation. I would strongly recommend anyone considering an evacuated tube array to do the same.

Tom Lane's picture

Another issue with this article is that in one dr awning and in one picture of multiple collectors the STUTS DO NOT FORM A RIGHT ANGLE TO THE REAR MOUNTING HARDWARE . THE STRUT ANGLE IS OBTUSE to the plane formed by the collector's front mounting hardware AND THE LOWER MOUNTING HARDWARE holding the strut to the roof . The mounting hardware is weaken when either obtuse or oblique angles are created by the strut . The strut should always be at a RIGHT ANGLE or form a right angle . All manufacturers show this in their mounting guides for struts . Tom

Vaughan Woodruff's picture

Tom,

You are correct that the back legs should form a right angle with the collectors when using struts. All of the photos (many of which were supplied by manufacturers) show this. The only image that is a bit off is the one produced for this article, which appears to be off by 5 degrees. There are tolerances at play here - a 5 degree deviation from perpendicular results in a stress increase of 0.3% versus a leg that is perpendicular. That said, the drawing could have been more accurate and I will be sure to do a better job of reviewing the drawings in the future.

Vaughan

Tom Lane's picture

Two very serious errors with this article . One the awning mount shown on page 68 where the use of the strut is use to tilt the collector from the wall of the house . All awning mounts should be near the top of the roof of near the peak on the gable end of the wall to keep from being shaded in the early morning and afternoon sun . This picture shows an awning mount that would be seriously shaded in the Northern Hemisphere from early spring throughout the summer into late fall . Another serious error on page 69 is showing evacuated tubes RAISED ON THE ROOF using a solar electric module to power the pump . THIS WILL NOT WORK BECAUSE IN THE EARLY SPRING TO LATE FALL THE RISING SUN will hit the evacuated tubes on the back or North side and vapor lock the evacuated tubes BECAUSE the PV module is a flat surface facing south and receives no solar radiation to power the DC pump as the back of the evacuated tube is receiving enough radiation to vapor lock the system and PREVENT IT FROM OPERATING ALL DAY . Another salient point is that no evacuated tube manufacturers recommends a PV module wired to a DC pump because CURVED SURFACES DO NOT RECEIVE DIRECT RADIATION the same as flat plate collectors and i.e. The flat surfaces of PV modules . Tom Lane

Vaughan Woodruff's picture

Tom,

Thanks for your comments.

I respect your difference of opinion, but I do not consider either of these details as "serious errors." As you are aware, there are factors within any solar water heating installation that must be consider, including orientation of the building, local shading issues, building structure, aesthetics, and the hot water use patterns in the household.

As you have identified, any awning mount will receive shading from the building when the sun is north of the east-west ordinate. In this particular application, moving the collectors towards the peak would have resulted in significant accessibility issues (they would have been 25-30 feet off the ground rather than 15 feet) for a very minor increase in performance. The increased amount of solar radiation for this array (mounted at a 50 degree angle) would be minimal. Considering the hot water load, the desires of the owner, and their satisfaction with the system (it has been working beautifully for four years - the owners' backup is wood, so they know exactly how it is meeting their needs), I think it is reasonable to say that this isn't an error, nor a "serious" error. Perhaps I could have touched more on your concern of the shading that occurs from the building in the caption. I opted not too considering the major topic of the article is the rack, not the mounting location.

Your observation of the PV control with evacuated tubes is correct. However, in this particular system it is used in tandem with an AC-powered differential control as a second level of security. This is rather atypical. That said, the illustration is to illustrate the rack, not the control. I do understand your concern and perhaps in this article its inclusion might infer that the PV module as a singular control is sufficient. I appreciate you pointing this out and will be more cognizant of this in the future.

Vaughan

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