The gutters must extend beyond the roof tiles, not align with them. Water naturally flows through the shingles, along the edge of the shingles, and even beneath them. When gutter sections are installed directly in line with roof shingles, the shingles impair the natural flow of water. In most areas, gutters will be installed against the fascia, with approximately 3 to 4 inches of the width of the gutter extending beyond the edge of the shingles.
Depending on the inclination of the fascia, the gutters may need a support installed behind the rear gutter wall to make them level. The rear edge of the gutter should rest below the drip edge that is covered to prevent water from entering behind the gutter. An improperly installed gutter will not pick up stormwater runoff from the roof as intended and could cause damage to the home. The gutters should extend beyond the roof to trap and drain water without splashing the roof or siding, especially when it rains a lot.
Without this extension, water will begin to cascade down the roof instead of dripping down the downspout and may not reach the gutter. If you continue to run the risk of water seeping down the tile and overflowing down the gutter. Normally, the gutter does not extend beyond the slanted edge of the roof. It is often kept an inch or two behind.
In almost all cases, gutter installers take the path of least resistance and instead place the gutter just below the roof shingles. A good rule of thumb is to make sure that the gutters lean toward the downspouts at a quarter of an inch for every 10 feet of the gutter. The gutters should be hung in a place where a straight edge placed above the roof tiles can easily extend over the front edge of the gutter (this is the proper installation location for the highest point of the gutter). A professional gutter installation company will use a level during installation to ensure that the gutters are tilted.
Before jumping into a DIY gutter installation, consider hiring a professional to hang your new gutters properly. When exposed to all the water that can flow through a gutter (worse if there is a blockage and the gutter fills up like a bathtub), the shingles will absorb the water like a straw. This is how far a gutter should extend beyond the edge of the roof, as well as other tips, to get the most out of gutters. The space between the gutter and the roof may vary in size because the gutters must be tilted towards the downspout to ensure proper water runoff.
I read that the start of a gutter must be low enough so that, if you take a level and place it on the roof, it clears the front edge of the gutter. I understand that if the gutters are placed too high, a layer of snow could slip and if it gets stuck in the front of the gutter, it can tear it off.