Eric Rea-LetterSubject: Proposed Solar Panel Installation
Eric Rea Residence, 609 Trump Ave. NW., Yelm, WA 98597
To Whom it May Concern,
Design Parameter Summary
Governing Building Code: 2018 Washington Building Code (2018 IBC)
Risk Category: II
Design Wind Speed: 97 mph (per ASCE 7-16)
Ground Snow Load: 18 psf
Roof Information
Roof Structure: 2x4 Manufactured Trusses @ 24" O.C.
Roofing Material: Asphalt Shingles
Roof Slope: 22 degrees
Note: Required embedment length excludes the tapered tip of the screw, and embedment into sheathing.
Roof Connection Details
Our engineering department has reviewed information, gathered by our field crews, related to the proposed solar
panel installation at the above-referenced address. The purpose of our review was to determine the structural
adequacy of the existing roof. Based on our review and analysis of the available information, and in accordance with
governing building codes, it is our professional opinion that the existing structure is permitted to remain unaltered
for the proposed solar installation.
Locations per design drawings
October 07, 2021
Analysis
The proposed installation - including weight of panels, racking, and mounts - will be approximately 2.63 psf. In the
areas where panels are installed, roof live loads will not be present. Also, where applicable, roof snow loads will be
reduced because the slippery surface of the panels will cause the roof to shed additional snow loads relative to the
existing roof surface. The reduction of roof live and snow load is adequate to fully or partially compensate for the
addition of the panel installation. Because the member forces in the area of the solar panels are not increased by
more than 5%, the stresses in the members are not increased by more than 5%, and so per section 807.4 of the
International Existing Building Code (IEBC), the structure need not be altered for gravity loading.
Lag Screws: 5/16" diameter min., with min. 1.5" embedment into framing members, at 48" O.C. max, staggered
The proposed installation will be 6" max. above the roof surface (flush mounted) and parallel to the roof surface.
Therefore, any increase in wind loading on the building structure from the solar panel installation is expected to be
negligible. Wind is the governing lateral load case. Because the increase in lateral loading is not increased by more
than 10%, per section 805.3 of the adopted IEBC, the structure need not be altered for lateral loading.
Sincerely,
Trevor A. Jones, P.E.
Wind uplift on the panels has been calculated in accordance with the relevant provisions of ASCE 7-16.This loading
has been used to verify the adequacy of the connection specified above. Connection locations should be in
accordance with design drawings.
Limitations and Disclaimers
10/7/2021
If you have any questions or concerns, please contact our office at (509)-931-1663, or email me directly at
Trevor.Jones@solgenpower.com.
Electrical engineering is beyond the scope of this analysis. Solar panels must be installed per manufacturer
specifications. Structural design and analysis of the adequacy of solar panels, racks, mounts, rails, and other
components is performed by each component's respective manufacturer and the undersigned makes no statement
of opinion regarding such components. This letter and the opinions expressed herein are rendered solely for the
benefit of the permitting authority (city or county building department), and may not be utilized or relied on by any
other party.
The opinion expressed in this letter is made in reliance on the following assumptions: the existing structure is in good
condition; the existing structure is free from defects in design or workmanship; and the existing structure was
code-compliant at the time of its design and construction. These assumptions have not been independently verified,
and we have relied on representations made by the property owner and his or her agents with respect to the
foregoing. The undersigned has not inspected the structure for patent or latent defects.
The roof structure need not be altered for either gravity or lateral loading. Therefore, the existing structure is
permitted to remain unaltered. Connections to the roof must be made per the "Roof Connection Details" section
above. Copies of all relevant calculations are enclosed.
Conclusion
Note: All calculations per ASCE 7
Load Comparison
Without Solar With Solar
Dead Load
Asphalt Shingles 5 5 psf
1/2" Plywood 1 1 psf
Framing 3 3 psf
Insulation 1 1 psf
1/2" Gypsum Ceiling 2 2 psf
M,E, & Misc 1.5 1.5 psf
Solar Panel 0 2.63 psf
Total Dead Load 13.5 16.13 psf
Snow Load
Ground Snow Load, Pg psf
Exposure Factor, Ce
Thermal Factor, Ct
Importance Factor, Is
Flat Roof Snow Load Eqn. 7.3-1 or jurisdiction min.
Slope degrees
Unobstructed Slippery Surface?No Yes
Slope Factor, Cs 1.00 0.80
Sloped Roof Snow Load 13.9 11.1 psf
Live Load
Roof Live Load 20 0 psf
Load Combination
D + Lr 33.5 16.1 psf
D + S 27.4 27.2 psf
Max. Load 33.5 27.2 psf
% of original 81%
Result:
This calculation justifies the additional solar load by comparing existing to proposed gravity loads
in the location of the solar panels.
Because the total forces are decreased, per the relevant code
provisions stated in the body of the letter, the existing roof
structure is permitted to remain unaltered.
18
1.1
22
1
1
13.86
Lag Screw Calculation (per ASCE 7-16 Ch. 30 and NDS)
Connection Demand
Spacing perpindicular to rail 32.5 in 1/2 panel length
Spacing parallel to rail 48 in Max spacing
Effective Wind Area on each connection 10.8 ft2
Roof Angle 22 degrees
Wind Speed 97 mph
Exposure Coefficient, Kz 0.85
Topographic Factor, Kzt 1
Directionality Factor, Kd 0.85
Elevation Factor, Ke 0.99
Velocity Pressure, qz 17.2 psf
Prying Coefficient 1
Zones 1, 2e Zones 2n, 2r, 3r Zone 3e
GCp (max)1.5 2.5 3.6
Exposed Panels? (γE = 1.5)No Yes Yes
Pressure Equalization Factor, γa 0.79 0.79 0.79
Uplift Force 20.3 50.7 72.9 psf
Max. Uplift Force / Connection (0.6 WL)131.7 329.2 474.1 lbs
Solar Dead Load (0.6 DL)17.1 17.1 17.1 lbs
Max. Uplift Force (0.6 WL - 0.6 DL)114.6 312.2 457.0 lbs
Connection Capacity
Fastener Type Lag Screw
Fastener Diameter 0.3125 in
Embedment Length, min 1.5 in
Lumber Species & Grade DFL #2 (Assumed)
Lumber Specific Gravity 0.5
Nominal Withdrawal Capacity, W 399.0 lbs NDS 12.2-1
# of Screws 1
Load Duration Factor, CD 1.6
(Note: CM = Ct = Ceg = 1.0)
Adjusted Withdrawal Capacity, W'638.4 lbs
Compare Adjusted Withdrawal Capacity to ASD Factored Demand
Zones 1, 2e Zones 2n, 2r, 3r Zone 3e
O.K.O.K.O.K.
This calculation justifies the connection of the solar panels to existing roof members, by showing the
connection capacity is equal to or greater than the uplift force demands.