Alton, if the shingles are as brittle as you say, they will break during the install, laying over the roof is an option, then blackjack the nail hole. Also get up into the attic, figure out the rafter layout, and nail into them. One more thing, when you install the jack, take a piece of new roof shingle, cut it into a strip, place it under the heel of the jack, this will keep the jack from diging its heel into the roof . G