I cut the 1st pipe and checked the measurement... Perfect 5'. I did the other 12 pipes the same way. I got some off by 1/8th", but generally, worked quite well. The slight difference won't make any difference (I'll match side by side pieces to match).
So now I have all my 1.25" pipes cut to the 5' and 7' lengths I need. I'll construct the bottom part 1st. Then I can try out a few roof arch lengths to see how to cut the tops pieces.
The reciprocating saw worked GREAT! I found one sawblade at the Home Depot marked for PVC (marked as "8/12"). But they were sold in packs of 10, and I sure didn't need that many. Individual blades were not marked for PVC. But the 10 pack was 8/12 so I bought a single 8/12 Rigid blade. That's the one to use. Cuts through like a hot knife in soft butter, no casting of PVC dust at low speeds, and a nice clean cut! The DeWalt saw gave a good square cut with almost no vibration. I was utterly pleased! No hint of melt, and the burrs rubbed off by hand (I"ll run a file inside and out anyway).