As the roof surface ages, it wears and becomes less and less reliable. Eventually it may leak.

But not only old roofs leak. One of the most common causes of roof failures is poor workmanship during installation. The reason this is not readily apparent is that it often takes a few years for a poor installation to manifest itself in a leak. By this time it is all too easy to point the finger at wear and tear.

A leaking roof can be disruptive and costly, causing damage to interior finishes. In some cases it is easy to determine the cause and in other cases it can be difficult to diagnose. In some cases a roof leak will only occur with specific weather conditions. Let’s try to make some sense of this mystery.

Sloped Roofs Shed Water
Sloped roof systems are designed to shed water from one shingle to the next down to the roof edge. Sloped roof systems are not waterproof. Understanding this concept is the first step to understanding how a roof can leak. Flat roof systems, on the other hand, are designed to be waterproof.

It’s All About the Flashing
Roofs don’t normally leak in the middle of a field of shingles or tiles. They leak where there is a roof penetration such as a skylight, chimney, dormer or roof/wall intersection. These critical areas are kept from leaking with flashing. Flashing is usually made up of pieces of metal that are configured so water will shed across the gap between the roof penetration and the roof surface. In many cases, roof leaks can be traced to poorly installed or worn flashing. If you have a roof that leaks, the flashing is the most likely culprit.

Wind and Rain
Roof systems should be designed and installed to accommodate your local climate. On the other hand, it is possible for a perfectly installed and maintained roof to leak given the right combination of wind and rain. Recall that sloped roof surfaces are not waterproof but shed water down the roof.

Ice
In cold climates, ice can cause a perfectly good roof surface to leak. Ice can block the flow of water to the edge of the roof or to the drain. Water can then back up under the shingles and start to leak into the house.