When installing roof brackets on existing shingled surfaces, Best practice is to lift up the shingle and install the roof bracket underneath the existing course above. If need be or conditions warrant it can be installed on top of the existing row of shingles but this will warrant the sealing of the nail holes when bracket is removed. Please call our customer service @ 1-800-231-5647 if additional information is needed for a particular situation. Thx.
The model # 2503 - 6" x 60 degree fixed angle roof bracket is not currently stocked at any Home Depot locations, this item can only be obtained by placing an order on line @ Home depot.com or by special order at the store location or your choice. However you also mentioned an " adjustable" roof bracket, this would be our model # 2500 10" adjustable roof bracket and these are available in store in most locations.
See the picture below, the long tail points toward the peak of the roof