Leaks can occur just about anywhere, but certain patterns show up again and again. This is by no means a comprehensive list, but the majority of leaks will fall into one of the following categories:

1. Flat Roofs. A flat roof is not supposed to have areas that are truly level. Everywhere should have a slight slope that guides water to one or more drainage points, but as the roof ages it can warp, creating pockets of standing water. This is an extreme test for a water barrier, and it only takes a pinhole to be a serious problem. As a general rule, the lower the pitch of a roof, the easier it is to interfere with water runoff and create a potential leak.

2. Remodels and Additions. In a structure that has been perfectly sound since it was built, leaks have been caused by something as simple as installing a new light fixture in a wall or ceiling. Add on a whole new kitchen or bedroom and the possibilities are multiplied.

3. Valleys. The angle where one down-sloping roof surface meets another is called a valley. Valleys have a higher volume of water flow because runoff is channeled into them from two directions. The valley should have metal flashing that goes up under each surface. Where the valley is blocked by leaves or other debris, the water will tend to back up under the roofing, and if it backs up enough to go over the flashing it can get into the structure.

4 . Roof and Vertical Wall. Where the roof slopes down into a wall, this is similar to a valley, and the worst case is when they meet at an angle parallel to the ground. However, it’s also possible to have problems when the roof slopes up to a wall. This occurs especially when an addition brings a new roof up against an existing stucco wall, because the new flashing is often put against the outside of the stucco, which is porous and lets moisture get behind the flashing.

5 . Vents. The roof in most buildings is penetrated by a number of vents. There are vents for hot water heaters and furnaces that let gases escape. There are vents for plumbing that equalize pressure in the drains. There are ducts for roof-mounted air conditioners. One thing these have in common is that they break through all layers of the roof membrane (see Note below), and wherever that occurs you increase the potential for leaks. Also, vents should have flashing that goes under the top layers of roofing on the uphill side, and over all layers of roofing on the downhill side.

6. Stucco walls. Many people don't realize that stucco is actually quite porous. Like roofing, it relies on a 'tarpaper' underlayment as a water barrier. And at the bottom of the wall, there should be a 'weep' system to let the water out.

7 . Chimneys. Unfortunately, chimneys combine the leak hazards of several sources we’ve already mentioned. Like vents, they usually break through the roof membrane. Because they have vertical sides, they duplicate the disadvantages of #3, Roof and Vertical Wall. And unless the chimney goes through at the ridgeline, on one side the roof and chimney will create a valley parallel to the ground. It’s common practice to put a little gable or v-shaped flashing, called a cricket, on the up-slope side so that water will run off and around the chimney, but this is by no means universal. Chimneys are also often made of bricks and mortar, which is porous. The exterior surface of a brick chimney should be water-sealed every few years, which is seldom done, and every chimney, regardless of materials, should have a rain cap.

Here in Southern California there is an additional factor that causes problems. We have earthquakes periodically, and when the house shakes, even if the masonry chimney doesn't crack it will sway at a different rate than the rest of the structure because it is so much denser. This leads to separation of the flashing between the chimney and the roof. Again and again we've seen this patched with roofing tar, which gets brittle over time and then the patch itself cracks in the next tremor.

8 . Deteriorated roofing materials. When your roofing material is damaged or worn out, leaks can occur anywhere on the roof surface. Asphalt (composition) shingles will usually last between 10 and 50 years, depending on quality. Wood shingles and shakes also wear out after a few years. If you have an old composition or wood roof that has gone past its life expectancy, you may be better off replacing it rather than patching the leaks. Other roofing materials such as slate, clay tile and concrete shake can last almost indefinitely but are subject to breakage, especially by someone who doesn’t know the special techniques for walking on these surfaces. And the underlayment still gives out after a few years (see Note).

9 . Decks and balconies. These are flat and nearly level, so water is slow to run off and easily ponded by the slightest barrier, just like the flat roof. Unlike the flat roof, they are frequently walked on, which increases the stress. Consequently decks and balconies are one of the most common places for leaks.

10. Groundwater. This is actually a whole class of problems by itself, but most groundwater problems are one of two types. First is intrusion from surface water, whether from rain run-off or sprinkers. Moist soil is in contact with the building walls and the moisture is seeping through. The second type of problem is where hydrostatic pressure is forcing water up through the foundation or footings. This water may originate some distance away from the structure. In either case, the solution requires careful analysis of the causes, and usually a combination of remedial actions. It's not quite like plugging a leak.

Note: In each of the situations above (except sometimes groundwater), it's important to know that most leaks involve at least 2 layers of roofing or siding. One layer is called the underlayment, and as its name suggests it goes underneath the top layer. It is usually made out of roofing 'felt', also commonly called tar paper. It's the underlayment that must be waterproof and has the ultimate responsibility of keeping out moisture. The top layer can be made from a wide variety of materials, some of which are actually quite porous. While the top layer might shed most of the water, it is really there to protect the underlayment from damage, especially from sunlight, and for aesthetics.

If you see evidence of water intrusion inside the building, that moisture has gone through not only the top layer but through the underlayment, and in the case of a roof, through a plywood subroof as well. Sometimes water will get through the top layer and then travel for many feet before it finds a break in the underlayment, and then up to 8 additional feet to the dge of a plywood sheet, which is where your leak shows up inside. That's one of the things that keeps our job interesting. (Top)

Leaves in the valley.

Before: An ugly raincatcher.

After: A nice cricket.