Leaks occur primarily at the points of penetrations and transitions. If the roof leak occurs in the open field of the roof, it is probably because of some form of damage or it is just simply worn out. Most of the repairs that are made on roofs are around the penetrations and transitions.
Penetrations are things like vents, skylights and chimney chases. Transitions are your valleys, wall abutments like dormers and addition tie-ins. All of these require special flashings to insure your roof's water tightness. Improper or inadequate flashings mean roof leaks sooner rather than later.
Flashings are typically metal pieces formed and sealed into the roofing materials where the roof stops, turns or changes elevation. They also tie into siding walls and fascia returns. They are the strength of your roof that takes rushing water where it may build up, prevents it from entering your home or attic, turns it down the slope and directs it off of your roof at the eves. The integrity of the roof flashings has a direct impact on the mechanical functioning of your roof and how well it will shed water.
Most roof flashings are typical in their design and their use applications. An important point to be made here is that the flashing specifications that are spelled out in the codes establish a baseline requirement. This means that what is accepted is a bare minimum requirement. What gets used most times is not improper but may very well be inadequate. Most roofs have their own specific needs and how the details of these needs are treated establish the
roof's weather tightness.
Discussing with your roofer the different flashing points around your roof can go a long way in making sure you get the type of flashings necessary to improve the ability of your roof to shed water. We get 52+ inches of rainfall a year in Florida and are subject to extremely high wind driven rain conditions. Any discussion about your roof flashings should be made with this information in mind. Let your roofer know that you are concerned about how he intends to flash your roof and are interested in increased flashing procedures where it is necessary or just a good idea.
A professional roofer will be impressed at how well informed you are and should enjoy taking the opportunity to explain in detail how he can improve the weather tightness of your new roof. Don't let any roofer deflect your questions or shrug off your concerns. If they do, they are not the roofing contractor you want to use. Don't wait till after the roofing work has begun. This is an important topic and should be discussed in the initial meeting.