Pitched roofs shed water, flat roofs hold it
That single sentence explains most of the difference. On a pitched roof, gravity is on your side. Water is moving, and it only gets in where something interrupts the path: a lifted tile, a failed flashing, a blocked valley, a parapet that lets water in from above.
On a flat roof, gravity is working against you. Water stands, sometimes for days. A flat roof is not really flat either, it is meant to have a slight fall to an outlet, and when that fall is wrong or the outlet is blocked, water sits in a pond and waits for the smallest weakness in the membrane.
How each one actually fails
Pitched roof failures are usually mechanical and local. Slipped or cracked tiles, rusted or badly formed flashings around chimneys, vent pipes and geyser stands, perished ridge mortar, valleys full of leaves. Fix the detail and the leak stops.
Flat roof failures are usually about movement and standing water. Membranes crack where the structure flexes, seams and overlaps let go, blisters form where moisture was trapped underneath, and upstands fail where the membrane was never taken high enough up the wall. Ponding water finds every one of those weaknesses.
The upstand detail is the one most often skipped. A membrane needs to run up the parapet or wall well above the water line and be properly terminated. When it stops short, the roof leaks in heavy rain no matter how good the flat area is.
What a proper flat roof job includes
Checking and correcting the fall to the outlet before anything is coated. Cleaning and drying the substrate properly. Reinforcing the movement joints, the upstands and the outlets with membrane or tape rather than relying on liquid alone. Then building the specified film thickness across the whole area, not just where it looks thin.
A seamless liquid system such as Rubber Sealant liquid rubber suits flat roofs well, because there are no overlaps to let go. It still only works on a substrate that has been prepared and on a roof that drains.
Maintenance is not optional on a flat roof
Clear the outlets twice a year, before winter and after the worst of it. Look for ponding that is still there two days after rain, because that tells you the fall or the outlet needs attention.
A pitched roof will usually forgive a year of neglect. A flat roof with a blocked outlet can put water into your ceiling in one storm.
No pressure
If you would like a second opinion on your own property, we will come and look.
The inspection is free, you get the findings in writing with photographs, and you are under no obligation to do anything with them.