This is usually because low quality leak prone decking materials such as epdm rubber are used often in an effort to save money.
Re decking roof.
Is it necessary to space the 7 16 osb on a re deck over a 1x12 lumber deck i think it needs room for expansion and contraction i say yes but a roofing contractor i know tells me it s not necessary re decking a roof general discussion contractor talk.
Of roof decking costs 800 2 800 to install depending on the material you choose.
Unfortunately many homeowners who have a flat deck roof have to deal with frequent leaks.
Until the 1940s exterior grade plywood was not available.
Since the boards used for shiplap were manufactured here in the pacific northwest it was still cheaper than plywood up through the 1960s.