Residential roofs have slope, and water flows downhill. The slope of your roof provides a mechanism for the roof to shed water (gravity). However, there are areas of the roof that may allow water to accumulate. Think about a skylight or chimney; water is running down your roof and hits a chimney or skylight frame, what happens next? The flow of water off the roof is disturbed and there is potential for water accumulation; that water has to go somewhere. If you can’t direct water off your roof, it gets into your house via leaks in the roof. The picture I have included provides a perfect example of a chimney disturbing this water.
Flashing is material used to waterproof the roof at penetrations through the roof deck or transitions in the roof deck. When you look at your roof, common penetrations would be chimneys, roof vents, plumbing vents, and skylights. Transitions are where the plane of the roof changes and would commonly be at crickets, vertical wall transitions, and valleys. The rest of the roof deck has other waterproofing materials (i.e. ice and water shield and underlayment); flashing works in conjunction with these elements to reinforce and supplement waterproofing at these critical areas.
Gravity is very efficient, water is very damaging, and the combination of the two will leak water into every nook and cranny on your roof, which necessitates and that your roofing contractor install an effective waterproofing system. What flashing does is to ensure that any water that gets into these nooks and crannies gets efficiently redirected back out to the topside of the roofing material, where it belongs. Flashing is often installed in multiple layers (called counter-flashing), and seams are frequently sealed (especially important around chimneys and vertical wall transitions). With waterproofing, redundancy and details matter. With masonry chimneys, even the best flashing may not be enough as masonry itself is porous and absorbs water; even a minute amount of water behind the waterproofing system needs to get out. On the picture, the chimney flashing is typical but far from correct and will allow for water leak between the chimney and the flashing itself as water sheds down the vertical chimney. A proper installation would have the flashing embedded into the mortar, so water running down the outside of the chimney would be redirected to the roof surface. This is not an inexpensive house; these homeowners are going to be very disappointed.
If you experience a roof leak, more often than not it is because of improperly installed, improperly detailed, or deteriorated flashing. When I inspect roofs to repair leaks (vs. replacing roofs), the first place I inspect when I get on the roof is the flashing. This element is critical to get right, and to get right from the beginning.
The next question becomes what is the best. That is easy, heavy gauge copper, overlapped and lead-soldered at any joints. Look at old slate roofs (>50 years old), and this is what you will find. Copper is expensive, lead solder is dangerous for the installer, and roof materials that aren’t intended to last hundreds of years don’t warrant flashing material that does. In residential roofing, the flashing I use is galvanized steel.
When selecting a roofing contractor, finding one that gets uber-excited when talking flashing details should be one of your selection criteria.
We are an experienced Lees Summit roofing company with over 15 years experience in repair, service and installation.