Sheet metal products are sold in ten foot lengths because most sheet metal brakes are ten feet long. (notice how foot and feet went with ten in both cases? Boy am I glad English is my first language.) Also there is a problem with transporting something over ten feet long that when stacked together tightly can weigh hundreds of pounds when only a few inches high. So "W" valley, metal edge, and commodity patio cover panels come in ten foot lengths.
But there is more: winter and summer.
when I was an installer and we put on concrete tiles, we separated the tiles with dimes on hot days and nickels on cold days. If you put the tiles together tightly in the winter, in the summer they would stand up, side to side, because of the expansion of the concrete in the sun.
Metal too. And valley metal comes in 50 feet lengths. See the problem? When exposed to the sun it can really expand but doesn't seem to do so when used as a valley underlay. When some jack leg tries to caulk long panels of patio metal together you'll see the dance of the snails. To them it is the meringue but to us it is just bulging and twisting.
On flat roofs, where the perimeter metal is part of the water proofing process, whereas on sloped roof it is part of the "water turning" (away) process, the end laps will leak if the metal is not properly enveloped by the roof underneath and strip in felts on top. There are nailing patterns to keep the metal from rocking or pivoting back and forth, and to minimize overlap movement.
Metal memory is strong too. Any 90 degree metal forced flat with nails will lift the strongest of nails over time but not back to the full 90 degree bend. That's why the pieces are opened up some prior to installation and the memory pushes down rather than pulling up.
So the ten foot metal length is both practical and physical. I'm not sure and Mr. Google couldn't easily find the answer but I've always heard that sheet metal expands and contracts about 1/4" per ten feet. In a fifty foot roll that is more than an inch of movement.
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