There are many things that cause a roof to leak.
Obstructions: Moss, Needles, Leaves and Tree Branches
These materials slow water flow down the roof, hold water and cause water to flow sideways on a roof. They need blown off the roof. Roofs are only designed for water to flow straight down the roof. The roof is water resistant, not water proof. This is probably the biggest cause of roof leaks. And it's also why home insurance companies may require you to remove and treat your roof for moss before they will begin coverage or renew a home owner's insurance policy.
Wind-Driven Rain Along with Excessively Gapped Chimney and Skylight Step Flashing
Wind driven rain may find its way past flashing that was installed with only gravity pulled rain and minor side travel in mind. However in heavy winds, sheets of rain can be blown sideways and even upwards during heavy gusts. Common areas for leaks during storms are step flashing around chimneys and skylights. Overflowing gutters and improper drip cap flashing do not cause roof leaks. The solution to wind driven rain is to caulk in the "it would never leak there" areas of the flashing. The areas to look at in particular are the high side of the chimney or skylight where the step flashing on the side meet the top flashing. Corner flashing can be installed and each of the joints of the step flashing pushed tight together and caulked at each overlap. The goal is to stop the wind and rain from getting past the joint. If the step flashing is loose, then it will need to be mechanically secured first with screws, rivets, metal forming or crimping.
Crowding Roof Jacks and Roof Exhaust Vents with Shingles
Another leak area caused by storms are roof jacks where the roofing installer runs the shingles toor far onto the roof jack so part of the shingle slopes up. This keeps the shingles from laying flat together and creates a side slope. Water then flows or is driven sideways and penetrates the roof. Shingles are meant for water to flow straight down, not side wsys. For roof jacks, the shingles should be cut away from the roof jack flashing to leave a 1/2 to 3/4 inch trough were water can flow away and the shingles stay on the flat part of the roof jack. You may find caulking from when it leaked before and someone tried to fix it with a mastic or caulk instead of letting the shingles lay flat. The caulk should just be removed and the shingles cut back. A heavy duty pair of old tin snips and a strong wrist will do the trick. Please do not caulk a roof leak from the attic/ bottom side. It may stop the drip, but the water will continue to flow between the tar paper and the plywood roof. This will rot the plywood roof. The roof needs to be dry and have good air flow. Leaks must be taken care of from the top side using well proven flashing, caulk and roofing material installation techniques.
Excessive Water Flow Across Mossy Roof
If an upper roof contains large volumes of water, it will saturate and cause a lower roof to leak if the lower roof is used as a drain pipe. The water should be contained in an "overspout" or downspout that carries the upper roof water over the roof and into the lower gutter.
This cedar shake roof was leaking because the moss was slowing the flow of water and there was a lot of water running over the roof. It quit leaking when the downspouts was extended across the roof to the gutter below.
Improper Installation by Contractors
I think the biggest cause is by improper installing of a roof by contractors. This may include flashing, not filling nail holes left by roof jacks, improper valley flashing and also improper roof design by the builder.
Cable Guys and Dish Antennas
Many times I find dish antennas marks on roofs. The dish has been removed from a roof location, but the contractor has not properly sealed the holes. As a result, a lag bolt hole of about 1/4 inch diameter allows water to go from the shingle top directly down through the shingles, tar paper and even through the plywood roof into the homeowner's insulation. It may take some time before this leak finally shows up. Holes near the peak of the roof are not as bad as holes near the gutter (as in picture below). This is true because more rain water will wash into the holes that are lower on the roof.
The TV Dish was removed, but the holes were not properly filled.
Nail Holes from Old Zinc Strip Installations
When people install zinc strips, they hammer nails through the roof. When the zinc strip stress fractures and flaps in the wind, the homeowner may remove it, but leave behind the nails. Some of the nails come out when they pull of the strip. This can cause very slow leaks over long periods of time. Since the holes are near the peak of the roof, not much water flows over or through the holes. Staining and wet spots may be seen near the peak from inside the attic. A roof treatment of zinc-sulphate works better than zinc strips and doesn't create holes in the roof. If you remove zinc strips, then pull all the nails and caulk the holes (lift up shingle and squirt caulk underneath, push down on shingle so caulk oozes up through hole) as you go. Double back to make sure you got all the holes.
Worn or Damage to the Roof Ridge Cap
Another cause of roof leaks is a worn ridge cap. This may get worn by contractors walking on it or dragging ropes across it (for rope and harness setups) without protecting it with carpet until they are down with the job. Not much water lands on the ridge, but everything that does will wet the ply wood and flow under the roofing material below. The damage is often hidden and goes undetected for years. The ridge cap should be inspected before and after a contractor is on the roof. Ask the contractor to take pictures of your roof ridge cap for you.
This roof ridge cap shows clear signs of wear from a rope.
Close up of rope wear in the roof ridge cap that has cut through the tar paper and exposed the plywood to weathering.
Using the Roof as a Pipe
The roof is intended to shed rainwater, not to act as a drain pipe. If downspouts that drain upper gutters are just emptied onto the roof, then it will wear the granules off the shingles. This is just poor design. The solution is to put down some metal shingles to take the brunt of the wear, or to extend the downspout across the roof. It's appropriate to match the downspout across the roof to match the color of the roof.
This poor downspout design uses the roof as a downspout. Consequently the granules have worn off the shingles prematurely. Extending the downsput a few more inches is appropriate to stop this problem.
Blown Off Shingles
During high winds, shingles may be blown off your roof. If the shingles are on a part of the roof that is difficult to see from where you normally walk, you may not notice they are missing until the roof begins to leak. Because of the installation method of composite shingles, it may be a slow leak that can cause rot before you notice damage to your ceiling.
Insufficient Flashing and Wind-Driven Rain
Around skylights and dormers, flashing is important. Flashing may be fine during normal rains, but wind-driven rains may push water where gravity would not take it alone. Careful inspection is useful if you have roof leaks in your attic around dormers and skylights.
BulletsAmazingly enough, about 1 out of 100 homes that I walk on has a bullet embedded in the roof. Bullets that are fired straight up usually tumble, but most of these (and they are about all) 9mm bullets have been shot from over 1000 yards away and could kill if they hit someone. The bullets usually penetrate through one layer of shingle and only cause very slow leaks at worse. Mention "bullets" at the time I write up your bill and receive a $5 savings. :)
A Portland area customer holds a 9mm bullet that was imbedded in his mother's roof