BLD2023-0202_Other_2.16.2023_10.16.19_AM_3373108CONSSLING
SULTING
Scott E. Wyssling, PE
Coleman D. Larsen, SE, PE
Gregory T. Elvestad, PE
Revolution Solar
9981 West 190th Street, Unit K
Mokena, IL 60448
To Whom It May Concern:
76 North Meadowbrook Drive
Alpine, UT 84004
office (201) 874-3483
swyssling@wysslingconsulting.com
February 3, 2023
Re: Engineering Services
Moss Residence
19220 76th Avenue W, Seattle, WA
10.350 kW System
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: Prefabricated wood trusses at 24" on center. All truss members are
constructed of 2x4 dimensional lumber.
Roof Material: Composite Asphalt Shingles
Roof Slope: 18 degrees
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
• Live Load = 20 psf (reducible) — 0 psf at locations of solar panels
• Ground Snow Load = 20 psf
• Wind Load based on ASCE 7-16
o Ultimate Wind Speed = 97 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 2018 International Residential Code, 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
The solar panels shall be mounted in accordance with the most recent Sunmodo 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.
The maximum allowable withdrawal force for a #14 wood screw in 1/2" plywood/OSB board is 55
Ibs per screw (per APA technical note E830d). Considering the variable factors for the existing roof
framing and installation tolerances, we require that a minimum (1) screw be attached to the existing
rafter/truss minimum 1 1/2" penetration (378 Ibs) with the balance of the screws (3) connected into
the existing decking (3 x 55 = 165 Ibs) Total uplift = 543 Ibs). Based on this, the connections will
be adequate and will include a sufficient factor of safety.
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.
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 2018 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.
V truly
`ly rs,
Scott E. Wyssli , PE
Washington License 111163
Signed 2/03/2023
AWYSSLING
CONSULTING
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