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6
Installation Manual 801.1
SunFrame™ Code-Compliant Planning and Assembly
3. Determine the design wind pressure required for your
installation.
Design wind pressure is the amount of wind pressure that a
structure is designed to withstand, expressed here in pounds
per square foot (psf). To determine the design wind pressure
required for your installation, apply the following factors using
Table 1:
• your basic wind speed (determined in step 1),
• your exposure category (determined in step 2), and
• the height of your roof above the ground.
If your values fall outside the range of the table, or if your
design wind pressure exceeds 50 psf, consult UniRac, a profes-
sional engineer, or your local building authority.
Module manufacturers provide wind pressure rating for
their modules. Confi rm that they meet or exceed the wind
speed rating for your installation. If in doubt, contact the
module manufacturer.
Table 1. Design Wind Pressure (psf)
by Wind Speed and Ex po sure Cat e go ry
Basic wind speed (mph)
70 80 90 100 110 120 130
Category B
15' roof height 10 13 17 21 25 30 35
20' roof height 11 14 18 22 27 32 38
25' roof height 12 15 19 24 29 35 41
30' roof height 12 16 21 25 31 36 43
Category C
15' roof height 17 23 29 35 43 51 60
20' roof height 19 24 31 38 46 54 64
25' roof height 19 25 32 40 48 57 67
30' roof height 20 26 33 41 50 59 69
Category D
15' roof height 23 30 38 46 56 67 78
20' roof height 24 31 39 48 58 70 82
25' roof height 25 32 41 50 60 72 84
30' roof height 25 33 42 51 62 74 87
Source:
These design wind pressure (P) values are based on the formula
P = C
e
* C
q
* q
s
* I
w
( UBC 1997, Vol. 2, Structural Engineering Design
Provisions, Chapter 16, Div. III, Wind Design, p. 7). Assumptions: I
w
= 1 and
C
q
= 1.3.
Figure 3. Foot spacing and overhang defi ned.
4. Determine minimum design dead and live loads for
standard rafter spacing.
Foot spacing refers to the space between L-feet (or standoffs,
if used) along the same SunFrame rail (Fig. 3). Foot spac-
ing may not exceed 48 inches. For the rafter spacing at your
installation, consult Table 2 (facing page) to determine your
minimum design live loads and design dead loads per footing.
Locate the manufacturer and model of the PV module that you
plan to install and the rafter spacing at your installation site.
Read or interpolate live loads for the design wind pressure you
determined in Step 3. For assistance on this point, consult a
local professional engineer.
SunFrame
foot spacing
Overhang
6. Ensure that live loads do not exceed pull-out limits.
Based on the characteristics of your roof truss lumber and the
lag screws, consult Table 3 to determine the lag pull-out value
per 1-inch thread depth. Compare that value to the minimum
design live load per footing determined in step 4. Based on
these values, determine the length of the lag-screw thread
depth you require to resist the design live load. To ensure code
compliance, the lag pull-out value per footing must be greater
than the footing design live load.
If your SunFrame rails require splices, see also “Splicing re-
quirements,” page 9, before beginning your installation.
If your SunFrame requires standoffs, always use at least two
lag screws to secure the standoff to the rafter. Bolt the L-foot
to the standoff through the slot nearest the bend in the L-foot
(see Fig. 1, p. 4).
Verify that roof framing has adequate capacity to support
these design loads.
If they do not, try a smaller footer spac-
ing. If the result is still not acceptable, relocate the array to a
stronger area of the roof or strengthen the inadequate framing
elements.
5. Verify acceptable rail end overhang.
Rail overhang (Fig. 3) must equal 50 percent or less of foot
spacing. For example, if foot spacing is 48 inches, the rail over-
hang can be up to 24 inches. In this case, two feet can support
a rail of as long as 96 inches (48 inches between the feet and
24 inches of overhang at each end).
