A common area of misunderstanding for many trying to install OSHA recommended fall protection guard rails on roof tops is simply this: what length rail kits do I need and how many? This short article will provide a simple way to figure this out. Please note, however, that large or complicated jobs should submit drawings to us so an engineer can assist.
What Length and How Many Guard Rails
One first has to determine the layout of the rooftop guard rail system. Will it be an enclosed design that connects back onto itself like a square or rectangle or something similar--a closed layout? Or will it be a design that does not connect back on itself, like a straight run--an open layout?
The number and lengths of rail kits for a closed layout configuration is easy to determine. This is due to the fact that no extra bases or returns are needed. More on those in a bit.
Let's use an example layout of a 20' x 40' rectangle to demonstrate how the correct number of kits is obtained. We are assuming that the dimensions can't be exceeded due to distance constraints on the roof.
(1) First, we need to account for the distance between each rail when a kit is installed. This distance is 5 inches.
(2) Second, we add that 5 inches to the longest rail kit available which is the 10' rail kit. This gives us 10.5. (We use the longest rail kit available in order to use the fewest rail kits and bases possible. This saves money. Note that in California the longest length that can be used is 7.5').
(3) Third, we then divide the lengths of each of the sides of the closed layout configuration - our 20' x 40' example - by this 10.5 number. So 20 divided by 10.5 gives us 1.9 for the 20' sides and 40 divided by 10.5 gives us 3.8 for the 40' sides.
(4) Fourth, these two numbers - 1.9 and 3.8 - tell us how many and what length kits we need to get the closest to our 20' x 40' layout without going over.
This is how we interpret these numbers.
The number on the left side of the decimal point is the number of 10' kits needed. The "1" in our 1.9 number for our 20' sides means that we need 1 each 10' rail kit per side - for a total of 2 each 10' rail kits (there are two 20' sides).
The "3" in our 3.8 number for our 40' sides means that we need 3 each 10' rail kits per side - for a total of 6 each 10' rail kits (there are two 40' sides).
The number on the right of the decimal point is the length not to be exceeded by the final kit needed. So the ".9" in our 1.9 tells us we need a 9' rail kit. That doesn't exist. So we go with the nearest kit to that number which is a 7.5' rail kit. This means we need 2 each 7.5' rails kits for our 20' sides - 1 for each side.
And same goes for the ".8" in our 3.8 number. It again tells us we need 2 each 7.5' rail kits for our 40' sides - 1 for each side.
So the kits needed for our 20' x 40' rectangle are: 8 each 10' rail kits and 4 each 7.5' kits.
To determine the least number of kits for an open layout is the same as above but with one important adjustment--returns and extra bases.
So let's say you have a 85' straight(ish) run. We do the above calculations and our 85 divided by 10.5 gives us 8.09. As shown above, this means we need 8 each 10' kits and 1 each 7.5' kit.
However, on straight runs, returns are required at each end. A return is simply an additional kit that comes off at a 90 degree angle from the first and last rail of the run. Moreover, because this is not a closed run, one extra base will be needed to terminate the run. The rail kits used for returns are generally the 5' kits. But it doesn't matter. Below is picture showing a short straight run with the two returns.
This means that for a 85' run we need: 8 each 10' kits, 1 each 7.5' kit, 2 each 5' kits (the returns), and 1 each extra base to terminate the entire run.
Be assured we are always willing and able to help you properly configure you roof safety guard rail system. Please do not hesitate to contact us for help.