A flashing material must be impervious to water, tough enough to withstand construction abuse and yet flexible enough to conform to the desired shape, and not deteriorate while in service. It should also not react with mortar or corrode and should be compatible with joint sealants.
Traditionally, copper sheet has been used as flashing in masonry walls. Stainless and galvanized steel sheet have also been used as flashing. More recently, plastics and combination flashings have found their way into masonry walls. While there are pros and cons to each type of flashing and a range of cost associated with each, they will all perform satisfactorily as long as they meet the basic criteria for a flashing material.
For additional information on flashing, see Technical Notes 7A.