Gutter systems that fail to catch rainwater can result in rotting soffits, moldy siding and even a damaged structure. To make sure rainwater flows seamlessly from the roof to the inside of the gutter, many roofers and gutter hangers install gutter aprons. 
While protecting the roof sheathing from rot, a gutter apron also keeps water from getting behind the gutter. They are particularly useful when roof shingles don’t extend far enough out to direct water into the gutter.
Similar to a drip edge, a gutter apron is a long piece of metal folded at about 120 degrees. It starts under the first course of shingles and hangs over the roof sheathing. Gutter aprons take on an additional role by hanging down an inch or so into the back of the gutter to prevent water from dripping or splashing behind the gutter. While a drip edge is about 2 1/2 inches wide on both sides of the fold, a gutter apron will usually measure 4 inches or more above and below the fold. The bottom edge usually has a little fold for rigidity.
To install, the top section is slipped under the starter shingle course — the layer directly above the sheathing. The apron is held fast by carefully raising the shingles and attaching with roofing nails. While this is a job that intermediate do-it-yourselfers might tackle, it’s best left to licensed professionals who have the equipment and know-how to do the job safely. Flat tar or membrane roofing, as well as slate and tile shingles, cannot be lifted, so gutter aprons are best installed during new roof installation.