Can I use mylar sheets to keep my roof cool?
would this even work?


It would be moderately effective at reflecting sunlight but would not be durable.

Thin sheets are likely to get torn in high winds, if the wind gets under the sheets, your bricks are likely to be moved around. Worst case strips of mylar sheeting get blown across the neighbourhood and in really high winds an occasional brick gets thrown off the roof. If there's an unexpected storm at 4 a.m just before the normal onset of winter do you want to be climbing the roof in darkness to put away your mylar sheeting?


  is this better or would white paint be the better option


White paint would probably require less maintenance.