Christopher Bascomb Letter SS
Wyssling Consulting
76 North Meadowbrook Drive
Alpine, UT 84004
Scott E. Wyssling, PE, PP, CME office (201) 874-3483
swyssling@wysslingconsulting.com
June 25, 2020
Ken Trappen
Advanced Solar Solutions
39650 Mallard
Bass Lake, CA 93604
Re: Engineering Services
Bascomb Residence
205 Emerson Way, Northampton MA
11.520 kW System
Dear Mr. Trappen:
Pursuant to your request, we have reviewed the following information regarding solar panel installation on
the roof of the above referenced home:
1. Site Visit/Verification Form prepared by Advanced Solar Solutions representative identifying
specific site information including size and spacing of rafters for the existing roof structure.
2. Design drawings of the proposed system including a site plan, roof plan and connection
details for the solar panels. This information was prepared by Advanced Solar Solutions
and will be utilized for approval and construction of the proposed system.
3. Photographs of the interior and exterior of the roof system identifying existing structural
members and their conditions.
Based on the above information we have evaluated the structural capacity of the existing roof system to
support the additional loads imposed by the solar panels and have the following comments related to our
review and evaluation:
Description of Residence:
The existing residence is typical wood framing construction with the roof system consisting of truss system
with all chords constructed of 2 x 4 dimensional lumber at 24” on center. The attic space is unfinished and
photos indicate that there was free access to visually inspect the size and condition of the roof rafters. All
wood material utilized for the roof system is assumed to be Doug-Fir #2 or better with standard construction
components. The existing roofing material consists of a composition shingles. Photos of the dwelling also
indicate that there is a permanent foundation.
A. Loading Criteria Used
117 MPH wind loading based on ASCE 7-10 Exposure Category “C” at a slope of 32, 40 degrees
7 PSF = Dead Load roofing/framing Live Load = 20 PSF Snow Load = 40 PSF
3 PSF = Dead Load solar panels/mounting hardware
Total Dead Load =10 PSF
The above values are within acceptable limits of recognized industry standards for similar
structures in accordance with the Massachusetts Uniform Construction Code (2015 IBC).
Analysis performed of the existing roof structure utilizing the above loading criteria indicates that
the existing rafters will support the additional panel loading without damage, if installed correctly.
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B. Solar Panel Anchorage
1. The solar panels shall be mounted in accordance with the most recent “SnapNrack Installation
Manual”, which can be found on the SnapNrack website (http://snapnrack.com/). If during solar
panel installation, the roof framing members appear unstable or deflect non-uniformly, our office
should be notified before proceeding with the installation.
2. Maximum allowable pullout per lag screw is 235 lbs/inch of penetration as identified in the National
Design Standards (NDS) of timber construction specifications for Doug-Fir (North Lumber)
assumed. Based on our evaluation, the pullout value, utilizing a penetration depth of 2 ½”, is less
than what is allowable per connection and therefore is adequate. Based on the variable factors for
the existing roof framing and installation tolerances, using a thread depth of 2 ½” with a minimum
size of 5/16” lag screw per attachment point for panel anchor mounts should be adequate with a
sufficient factor of safety.
3. Considering the roof slopes, the size, spacing, condition of roof, the panel supports shall be placed
no greater than 48” o/c.
4. Panel supports connections shall be staggered to distribute load to adjacent trusses.
Based on the above evaluation, it is the opinion of this office that the standard installation will adequately
support the additional loading imposed by the solar panels. This evaluation is in conformance with the
Massachusetts Uniform Construction Code, current industry and standards, and based on information supplied
to us at the time of this report.
Should you have any questions regarding the above or if you require further information do not hesitate to
contact me.
Very truly yours,
Scott E. Wyssling, PE
MA License No. 50507