James Stillwaggon Letter SS v2
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
Ken Trappen Advanced Solar Solutions
39650 Mallard Bass Lake, CA 93604
Roof Material: Composite Asphalt Shingles Roof Slope: 45 degrees Attic Access: Accessible
Lumber type: Assumed Douglas Fir 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 rafters will support the additional panel loading without damage, if installed correctly.
Re: Engineering ServicesStillwaggon Residence10 Hampton Terrace, Northampton MA
5.600 kW System
To Whom It May Concern:
We have received information regarding solar panel installation on the roof of the above referencedstructure. 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 raftersfor the existing roof structure.2. Design drawings of the proposed system including a site plan, roof plan and connectiondetails for the solar panels. This information will be utilized for approval and construction
of the proposed system.
B. Description of Structure:
Roof Framing: 2 x 5 rafters spaced at 21” on center
November 29, 2021
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D. Solar Panel Anchorage
1. The solar panels shall be mounted in accordance with the most recent SnapNrack 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 rafters, and condition of the roof, the panel supports shall be placed no greater than 42” on centers. 4. Panel supports connections shall be staggered to distribute load to adjacent rafters.
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