BLD2025-0097_Comment_Response_Letter__1.22.2025_12.37.06_PM_4723280SSLING
CONSULTING
Scott E. Wyssling, PE
Coleman D. Larsen, SE, PE
Gregory T. Elvestad, PE
January 22, 2025
Northwest Electric and Solar LLC
18001 73rd Ave NE,
Kenmore, WA 98028
76 North Meadowbrook Drive
Alpine, UT 84004
office (201) 874-3483
swyssling@wysslingconsulting.com
Re: Engineering Services
Derry Residence
16107 74th Place West, Edmonds, WA
11.500 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: Prefabricated wood trusses at 24" on center. All truss members are
constructed of 2x4 dimensional lumber.
Roof Material: Tile Roofing
Roof Slopes: 23 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 = 15 psf
• Wind Load based on ASCE 7-16
o Ultimate Wind Speed = 98 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 2021 Washington State Residential Code. This analysis indicates that the existing framing
will support the additional panel loading without damage, if installed correctly.
Page 2 of 2
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 229 Ibs per inch of penetration as
identified in the National Design Standards (NDS) of timber construction specifications. Based on
a minimum penetration depth of 2%", 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
2'/" 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 72" 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 2021 Washington State Residential Code, current industry standards and practice, 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 yours,
Scott E. Wyssli , PE
Washington License 111163
Washington UBI # 60 81384
Wyssling Consulting, PLLC
76 N. Meadowbrook Drive, Alpine UT
Washington UBI # 604981384
Signed 1 /22/2025
NUYSSLING
CONSULTING