[GroundUrMast]

Because we can't see the attic/roof construction it's not reasonable for us to give you a definitive number. If you're lucky, you may only loose half the signal power and not be troubled by multipath symptoms. If the attic/roof is like some, you might not see any usable signals. The range is that extreme.

Given your specification, "would like all channels possible", use the largest antenna you can fit into the available space. The Winegard antennas are well built, but they can be very difficult to re-fold if you find the need to remove them from an attic. Antennacraft and Channel Master products are another choice that can be used if you would like to avoid the risk of an antenna trapped in the attic.