As I address in another article, a wide majority of roof leaks occur where 'things' stick up through roofs. Shingles in and of themselves rarely leak, except when worn or damaged by wind. Flashings are almost always the first place I look when a leak has developed in a roof.

Skylight technology has advanced to a point where the skylight itself is virtually leakproof. The only way a leak will develop is where the roof butts up against the skylight. This, of course, is where the flashing is!

Most roofing materials such as shingles, slate, tile and cedar shakes depend upon gravity to keep water from entering your house. The roofing materials generally overlap one another in a staggered fashion to shed water down the roof. The steeper the pitch of the roof, the better the system works. Low pitched roofs are more prone to developing leaks because the water moves down the roof at a slower rate.

Flashings work using the same principles. They overlap one another beginning at the bottom of the object where it penetrates the roof. Each course of roofing material has a piece of flashing (step flashing) on top of the last piece of roofing which abuts the object which is penetrating the roof. The next (higher) course of roofing materials then covers this flashing. The flashings are 'laced' into each course of shingles, slate, etc.

The only way that water can enter alongside the 'object' is for it to travel uphill (against the force of gravity). This phenomena can happen in areas that receive snowfall. They are called ice dams. If you live in such an area, special products need to be included in your skylight installation to prevent leaks from ice dams. This linked article talks about these special products.

If you decide to hire an individual or company to install a skylight for you, ask them pointed questions about flashings. If you don't know the questions to ask, read up on flashings. Most of the skylight manufacturers have helpful literature explaining their individual flashing kits or systems.

Following Directions

I have installed many different brands of skylights. While all are similar, there are specific steps involved with each skylight in order to ensure a leak-free installation. It is important for you to make sure that you and your installer read the instructions before proceeding.

This is especially true if you are installing a skylight which opens and closes. Examples of this are 'ventilating' skylights or roof windows. The installation of these units is very critical. If directions are not closely followed, there is a good chance that the skylight will bind or close improperly. If this happens, be prepared for a leak!!

Caulk and Roofing Cement

Caulking and roofing cement are materials that generally should not be found on your jobsite. Some installation instructions call out for sealants to be used in hidden locations as an extra precaution or insurance policy against leaks. These materials should never be used in exposed areas of the skylight installation or alongside the frame or flashings. If these materials are used in this fashion on your skylight, something is wrong!

Caulking and roofing cement work well for emergency or temporary repairs. They are not to be used as permanent solutions to a leak. Don't let someone tell you differently.