Rav Moshe Feinstein and the String Fence on the Roof
A homeowner hoped to save money by putting an eruv-style string doorframe around his roof instead of a fence. Rav Moshe Feinstein's answer was short and plain.
Now, dear son, someone asked Rav Moshe Feinstein, zt”l, the following question: He had a flat roof that could only be accessed by adults. It was used occasionally. He knew he had a biblical obligation to build a fence–a ma’akeh–around the roof. Wanting to save some money, he asked Rav Moshe, zt”l, if perhaps he could build a tzuras ha’pesach instead of a physical wall. He would construct poles at the corners of the roof and attach them at the top with string. Would he fulfill his mitzvah of ma’akeh? Rav Moshe, zt”l, probably struggling to keep a straight face, replied, ‘Absolutely not. A tzuras ha’pesach won’t prevent you from falling!’”