How to install roof flashing: why we stop at the ground and what to ask a roofer
Installing flashing means working on a sloped roof with shingles lifted. We explain how it should be built so you can judge the work, not copy it.
If you searched for how to install roof flashing, you will find dozens of guides with the same six steps. This page does not have them, and we say so up front. Installing flashing means standing on a sloped roof and lifting shingles. We do not publish instructions for that.
What we can do is show you how flashing is supposed to be built. Then you can tell whether a roofer did it properly.
The one rule behind every flashing job
Water only runs downhill. Every piece of flashing is laid so the upper piece overlaps the lower one, like shingles. If a piece sits under the one below it, water gets behind it. Almost every failed job comes back to this.
What a correct installation looks like
You can recognize it without touching the roof. Here is what to look for.
- Metal that follows the wall or chimney in clean, straight lines.
- Separate small pieces along a sloped wall, one per shingle course, not one long strip nailed flat.
- An upper layer, the cap flashing, covering the top of the lower layer.
- A lower layer, the base flashing, lapped under the shingles.
- Sealant used as a small backup at the edges, never as the main barrier.
Different spots need different pieces. A valley, a wall, a chimney and a pipe each use their own shape. For an overview, read what is roof flashing.
Red flags in someone else's work
These are visible from the yard or from a photo.
- A thick line of tar or caulk where metal should be.
- Flashing nailed through its exposed face.
- Metal that is bent flat against a wall with no upper cover.
- Shingles cut short, leaving gaps beside the metal.
If you see any of these on a new job, take a photo and ask the roofer about it before the final payment.
Why we do not publish the steps
Flashing work happens at the roof edge, beside walls and around chimneys. A fall from a roof is the most serious risk in home repair, and a step-by-step guide invites people to attempt it. A wrong piece of advice also hides a leak for months. We would rather you hire the job out and know enough to check it.
What to ask before the work starts
Ask which pieces will be replaced and which will be reused. Ask whether the old flashing will be shown to you. Ask what happens to the shingles around each piece. A roofer who answers clearly is easy to trust. One who says "we will seal it" without naming a metal is not.
Ask for photos of the finished work from the ground, and compare them with the list above.
Frequently asked questions
How do I install roof flashing?
We do not publish installation steps because the work happens on a sloped roof with shingles lifted. This page explains how flashing should be layered so you can judge a roofer's work.
What is the most important rule of flashing?
Upper pieces must overlap lower pieces, so water always runs over the joint and never behind it.
Is sealant enough to flash a roof?
No. Sealant is a small backup at the edges. If it is the main barrier, the flashing is not doing its job.
How can I check new flashing from the ground?
Look with binoculars for straight metal lines, a covering layer over the lower layer, and no thick bands of tar or caulk.
Should I do flashing work myself?
We recommend against it. Flashing work means working at height on a slope. Have a licensed roofer do it and check the result from the ground.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NRCA. National Roofing Contractors Association
- US OSHA. 1926.501 — Duty to have fall protection (OSHA)