Linda Overfield Letter SS
76 North Meadowbrook Drive
Scott E. Wyssling, PE Alpine, UT 84004
Jon P. Ward, SE, PE office (201) 874-3483
Gregory T. Elvestad, PE swyssling@wysslingconsulting.com
June 7, 2022
Ken Trappen
Advanced Solar Solutions
2372 Morse Avenue #912
Irvine, CA 92614
Re: Engineering Services
Overfield Residence
27 Higgins Way, Northampton, MA
12.240 kW System
To Whom It May Concern:
We have received information regarding solar panel installation on the roof of the above referenced
structure. Our evaluation of the structure is to verify the existing capacity of the roof system and its ability
to support the additional loads imposed by the proposed solar system.
A. Site Assessment Information
1. Site visit documentation identifying attic information including size and spacing of framing
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 will be utilized for approval and construction
of the proposed system.
B. Description of Structure:
Roof Framing: 2x10 dimensional lumber at 24” on center.
Roof Material: Composite Asphalt Shingles
Roof Slope: 17°, 18°, 39°, 41°, & 46°
Attic Access: Accessible
Foundation: Permanent
C. Loading Criteria Used
• Dead Load
o Existing Roofing and framing = 7 psf
o New Solar Panels and Racking = 3 psf
o TOTAL = 10 PSF
• Dead Load
o Existing Roofing and framing = 7 psf
o New Solar Panels and Racking = 3 psf
o TOTAL = 10 psf
• Live Load = 20 psf (reducible) – 0 psf at locations of solar panels
• Ground Snow Load = 40 psf
• Wind Load based on ASCE 7-10
o Ultimate Wind Speed = 117 mph (based on Risk Category II)
o Exposure Category C
Analysis performed of the existing roof structure utilizing the above loading criteria is in accordance
with the 2015 IRC, including provisions allowing existing structures to not require strengthening if
the new loads do not exceed existing design loads by 105% for gravity elements and 110% for
seismic elements. This analysis indicates that the existing framing will support the additional panel
loading without damage, if installed correctly.
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D. Solar Panel Anchorage
1. The solar panels shall be mounted in accordance with the most recent IronRidge installation
manual. 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. The maximum allowable withdrawal force for a 5/16” lag screw is 235 lbs per inch of penetration as
identified in the National Design Standards (NDS) of timber construction specifications. Based on
a minimum penetration depth of 2½”, the allowable capacity per connection is greater than the
design withdrawal force (demand). Considering the variable factors for the existing roof framing
and installation tolerances, the connection using one 5/16” diameter lag screw with a minimum of
2½” embedment will be adequate and will include a sufficient factor of safety.
3. Considering the wind speed, roof slopes, size and spacing of framing members, and condition of
the roof, the panel supports shall be placed no greater than 48” on center.
4. Panel supports connections shall be staggered to distribute load to adjacent framing members.
Based on the above evaluation, this office certifies that with the racking and mounting specified, the existing
roof system will adequately support the additional loading imposed by the solar system. This evaluation is in
conformance with the 2015 IRC, current industry standards, and is 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