Calculating Hold-Down Strength

In order to ensure that your PV array is not going to fly away, you need to effectively ensure that it's connected adequately to the structure it's installed on.

That's done by using a chart like this one, which is provided by the Standard Handbook For Mechanical Engineers:

Each screw, usually a lag that fastens into the support of a rafter inside of the roof, has a different diameter. Different types of wood have different resistance strengths.  Each amount of allowable withdrawal is determined by 1" of threading inside that particular kind of wood.  For example:
  • A 3/8" lag through Southern Yellow Pine that is 2" long will have a withdrawal strength of 381x2, or 762 lbs.  
  • A 1/4" lag through Douglas Fir that is 3" long will have a withdrawal strength of 232x3, or 696 lbs.  
Let's also say that the plywood that separates the roof surface from the rafters is 3/4" thick.  You would need to subtract that 3/4" from the length of the screw.  

So how do you estimate whether a system has enough support to keep from flying away?  Wind speeds with force that is greater than 75 lbs/ft will cause the glass to shatter, thus creating a limit to how much hold-down strength the array needs.  

You can take the area of your array, in square feet.  Multiply the 75 lbs/sq.ft by a safety factor of 4.  That will help you determine that the array will require 300 pounds per foot of hold-down strength.  

Let's say that there are 30 solar panels, each with an area of 15 sq. feet.  That's 450 square feet. 
  • Determine the number of feet, attached with 3/8" screws 3" in length, that are required to hold the array in place to satisfy the requirements of hold-down strength.  The wood type is Southern Yellow Pine.  
450 x 75 = 33,750 lbs hold down strength required. 

Each screw is 3" at 3/8" diameter in yellow pine (381 x 3 = 1143 lbs per screw).  
Divide 33,750 by the 1143 per screw and you get 29.5, which is essentially 30. 

This particular example requires 30 footings in order to satisfy the requirements of hold-down strength.

Use this formula and these calculations for any array! 

You will find the example to use 75 lbs per square foot as the panel's weight load.  Find the information on panel specifications as listed by "load resistance."  Officially, you must also account for the depth of the plywood as not included by the amount of thread that is actually fastened into the rafters.

Counter-Ballast Systems
Instead of using the hold-down strength of screws, just calculate the weight of your counter-ballast (usually cinderblocks) and then determine how many cinderblocks you need, with the same formula.  (Square area of array, pounds per square foot of required hold-down strength determining number of required blocks).

Nabcep Study Guide
Handbook For Mechanical Engineers