Roof flashing is one of the most critical parts of a roof. Unfortunately, it is also one of the least understood. Some homeowners, and inexperienced roofers, might just dab some roofing cement on leaking flashing and call it done. However, this is only going to stop the leak temporarily. Roof flashing needs special treatment and, when leaking, to simply be replaced. Here is what you need to know about roof flashing and some clarity on some common misconceptions about it.

What is Roof Flashing?

Roof flashing is a thin material (usually metal) that roofers use to repel water from some of the most important areas of your roof. This especially includes any spot where a vertical surface projects up from your roof, like a dormer or a chimney. Flashing may be stainless steel, copper, or other materials.

Types of Flashing

While most flashing looks the same, like a thin sheet of metal, it can vary in shape depending on where it is installed. These are the types of flashing across the roof:

  • Chimney flashing
  • Dormer flashing
  • Valley flashing
  • Plumbing boot flashing
  • Edge flashing

How Does Flashing Work?

Why does adding a bit of metal around roof projections help waterproof them? If water met the joint between the roof and the chimney, it would work itself into the small gaps in this area and run down into the roof. Flashing blocks this (on the chimney, especially counter-flashing does so) by keeping the water running downhill towards the shingles.

Flashing requires a straight profile to accomplish its goal. If it is twisted or bent, it creates gaps in the very gap it is trying to fill. This leads to some counter-intuitive installation methods. For example, step flashing should only have one nail in it, although you could be tempted to put two in. However, if there are two nails, the flashing is restricted and can’t shift. Under pressure, it bends, and then water will get into that gap.

Similarly, applying roofing cement to a flashing leak is unlikely to work for the long term. The point of flashing is to keep water running downward. Adding cement creates another bump in the path of the water which, while it may be very small, increases water retention and erosion in the area. This can wear down the cement itself and reopen the leak. Or, it can actually create a new leak by redirecting the water improperly.

Should Flashing Be Replaced?

When you are getting a new roof, you can either replace the current flashing or remove it and carefully reinstall it. Which is better? It depends. Your roofer should immediately toss any flashing that has been bent, warped, rusted or otherwise damaged. If the flashing is in great condition and the roofer feels that they can drive the nails back in the exact same spot as before, they may reuse them. However, it is typically better to simply replace the flashing to ensure the new roof is as high-quality and resistant to leaks as possible.