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How much of a load does a solar array add to a roof?

Evaluating the ability of a roof to support solar panels requires assessing the condition and construction of the roof, calculating the weight impact of the solar panels and support structures.

Evaluating the ability of a roof to support solar panels requires assessing the condition and construction of the roof, calculating the weight impact of the solar panels and support structures, and taking into account the potential impact of snow and wind.  Typical solar panels weigh 20 to 50 pounds each and are distributed evenly across a roof along with the racks that support them.  By dividing the weight of the panels and underlying racks by the area of the panels, one generally finds that the combined weight of solar panels and the rack that supports them puts about 3-4 pounds of weight per square foot on a roof.  Most structures built after 1970 are designed to support loads far greater than this.  Local permitting rules must be consulted, but generally such loads are acceptable.

However, this assumption of a distributed load is generally not valid.  The weight of the panels is actually distributed to a limited number of base mounts and installers generally try to minimize this number and its resultant roof penetrations, thereby reducing the opportunity for roof leaks. This strategy has the downside that it can create large point loads on the trusses that support the racks.  

Assessing the magnitude of these point loads involves adding up the contributions of the roofing materials themselves as well as the solar equipment.   The uniform dead load on a rafter (expressed in pounds per linear foot or PLF) is calculated by multiplying the uniform dead load pressure (in pounds per square foot or PSF) by the rafter spacing and adding the weight of the rafter.  A typical value for the roofing material itself is 10 psf.  Thus, a typical roof with 16-inch rafter spacing and rafters that weight 2 PLF would have a dead load of 10 x (16/12) +2 = 15 PLF before the solar equipment was added.   For a 16-foot rafter, this then results in a 240 lbs. dead load.  Adding the panels would increase the dead load according to the same sort of calculation.   The PSF of the panels (typically 3) is multiplied by expanse of panels supported by each rail times the rail spacing.   As an example, if 5 feet of panels sit on two rails spaced 4 feet apart, the resultant point load would be 3 x (5/2) x 4 = 30 lbs.  Thus, in this example, the dead load is increased by 12.5%.

Residential applications typically involve a pitched roof on which solar panels are mounted parallel to the pitch. If the slope of the roof is not too high, the panels can hold snow and the weight loads can increase dramatically.  Such a live load can be much greater than the previously calculated dead load because snow can weigh much more than the panels.  In addition, there are circumstances under which snow loads will be greater than what would be calculated from the amount of snow on the ground (these additional loads are called “surcharges”) including the effects of drift or sliding snow.  The actual calculation is the same as above but estimating potential snow loads requires analysis of the roof structure and detailed local climate information.

An even greater concern is wind loading caused by uplift accumulated through the solar array and acting on the posts that support the solar panel. In some cases, solar panels can act as a sail and the wind from under the panels can create very high uplift loads. With enough upward force, solar panels can come loose from the roof and it is even possible that the roof itself can be pulled off along with the solar panels.  Calculating wind uplift forces (Fw) is similar to other load calculations (it is a product of the local pressure times the panel length times the rail spacing) but is complicated by the fact that these forces vary significantly depending upon where the panels and rails are located on the roof.  Details of such calculations are presented in the reference cited below.

Unirac, and other solar racking companies, provides a number of tools for calculating such parameters as the total uplift from wind and down force from snow.  These values vary enormously from location to location and must be considered carefully for the specific site of the system.  In places where there may be extreme winds (such as in Florida) or where there can be heavy snow (such as near the Great Lakes), the solar contractor, architect or engineer may need to design in additional and specialized support for solar panels.  Solar panels are intended to stay in place for decades; properly attaching them to the roof is essential for this long operating life.

References

http://www.structural101.com/Solar-Panels-on-Sloped-Roof.html

http://us.pv-contractor.com/us_store/index.php/us_us/aitdownloadablefiles/download/aitfile/aitfile_id/781/