Drops, not square feet
A 42 foot wide elevation off a 16 foot roll with a 6 inch overlap does not take 42 divided by 16. The first drop covers 16 feet and every drop after it covers 15.5, so it takes three: 16 + 15.5 + 15.5 = 47 feet of covered width for a 42 foot wall. Five feet of that is paid for and not used, and it cannot be recovered by measuring more carefully. The only lever is the roll width you buy.
The clips are the job
The three default elevations are 1,560 square feet of face. They carry 312 feet of perimeter cable, 96 feet of intermediate line at a 10 foot maximum span, and at 12 inch spacing that is 408 clips. Every one is placed by hand at height. Estimating this job on square feet of net produces a number that is wrong by more than it is right, because the net is the cheap, fast half.
Grouping drops between rolls
Drops of the same cut length are packed onto a roll together; drops of different lengths are not, which is the conservative assumption and the one a supplier will make when they cut. If two elevations happen to be the same height they share, and the roll count falls. That is worth knowing when a facade could be netted at one line rather than two.
Related pages
For netting stretched over a sports enclosure rather than a wall, the batting cage net calculator and the goal net layout page work from a box. For a tensioned fabric on cables, shade sail sizing and hardware covers the corner loads. For the openings on the same building, the gap survey page ranks them by area and the vent screening take-off handles vents and soffits.
Questions people ask
Why does a wall one foot wider cost a whole extra drop?
Because a roll cannot be split lengthways on site without leaving a raw edge along the whole drop. Drop count is a ceiling function of the width, so it steps rather than slopes. A 47 foot wall and a 48 foot wall off a 16 foot roll both take four drops; a 47 foot wall and a 46.5 foot wall may not. It is worth entering the real widths before ordering rather than a rounded set.
What overlap should I use between drops?
Whatever the net maker and the installer work to, which is why it is a field. The number matters more than it looks: at a 6 inch overlap a 16 foot roll covers 15.5 feet per drop after the first, and doubling the overlap to a foot takes another half foot off every drop on the building.
Where does the maximum span figure come from?
The net specification or the person installing it, not from this page. It depends on the mesh, the material, the wind exposure and how the net is being carried. The page uses it only to count intermediate horizontal lines, so if you have no figure, leave it out and the intermediate cable drops to zero rather than being guessed.
Does the slack allowance change the roll count?
It changes the drop length, which changes how many drops come off a roll, so it can. A 5% slack allowance turns an 18 foot drop into 18.9 feet, and a 100 foot roll then gives five drops rather than five and a half. Slack is not optional in practice — a net hung tight loads every clip and tears at the fixings — so it belongs in the arithmetic rather than in the waste percentage.
Is netting something I should be installing myself?
This page counts material and takes no position on that, but two things are worth saying plainly. The work is at height, in wind, with something that behaves like a sail, and that is where the injuries come from. And birds and bats are legally protected in most cases, with seasons in which closing a facade traps animals behind the net. Whether netting may go up at all, and when, is a question for your state wildlife agency and a licensed wildlife control operator.