Deck Flashing Code Requirements

December 18, 2018


The IRC is a thick book, but there is relatively little aimed at decks. A few sections have a few sentences dedicated to construction of safe and durable decks.

But little sentences can do big things (just ask the US Constitution), for example, two little sentences in chapter 7 specify four broad requirements:

  1. Required locations for flashing
  2. Required flashing materials
  3. Required methods for installing flashing
  4. Expected performance of the flashing

"Flashing is required where exterior porches, decks, or stairs attach to a wall or floor assembly of wood frame construction.

Flashing should be approved corrosion-resistant flashing shall be applied shingle-fashion in a manner to prevent entry of water into the wall cavity ... the flashing shall extend to the surface of the exterior wall finish.'"

—IRC Section 703.8

Breaking down the sentences to get to their underlying meaning, we get this:

  1. Required locations: " ... where exterior porches, decks, or stairs attach to a wall or floor assembly of wood frame construction ..."
  2. Required materials: " ... approved corrosion-resistant flashing ..."
  3. Required methods: " ... shingle-fashion ... extend to the surface of exterior wall finish"
  4. Expectation of performance: " ... prevent entry of water into the wall cavity ..."

A lot of work for two little sentences, yes—but not a lot of specificity, so Glenn looks a little deeper at the meaning behind these words.

Required materials: Corrosion resistant

Defined by IRC as "The ability of a material to withstand deterioration of its surface or its properties when exposed to its environment." A common type of deterioration is rust. This is solved by using materials that are non-iron, such as galvanized.

The more interesting, and common, type of corrosion is between two dissimilar metals simultaneously in contact with water. This causes one metal to give up electrons to the other metal which weakens the 'donor' metal. 


Materials that are corrosion resistant and accepted by code.
  • Galvanized steel flashing
  • Stainless steel flashing
  • Aluminum flashing
  • 'Copper flashing
  • Vinyl Flashing
  • Self adhered membrane (compliant to AAMA 711)
Where do deck materials fall on the anodic chart?
  • Copper, used in most pressure treated wood, is high on the chart. With the highest number being zero (Gold) copper comes in at -0.35
  • Stainless steel, a high quality (and expensive) flashing option, is close by at -0.55
  • Aluminum comes in at -0.90
  • Hot-dipped zinc plated galvanized fasteners come in toward the bottom, at -1.20

Wait. What?

This is curious because we are told to use hot-dipped galvanized steel for contact with PT lumber. Is this a mistake? No, it is a trick of chemistry. When copper is in contact with the galvanized steel, the zinc corrodes. In doing so, it provides a protective shield for the structural steel behind it. Strangely, the least corrosive product to use is covered with the most corrosive material.


Required methods: How to install it 

All the flashing has to do it stop water from getting into the wall. IRC specifies 'shingle-fashion' and that is a term we hear a lot. Shingle-fashion is a simple-sounding term that makes a lot of sense when you think about shingling a roof.

But it is more difficult to visualize on a complex shape. 

POP Quiz: How would you flash this shape?


Give up?

ANSWER: It is flashed like this:


Seems easy when the flashing is drawn in, doesn't it?

So how do all of these ideas translate into actual deck ledger details? There are at least a couple of good references for designing deck ledgers.

Design Sources:
EPA Indoor air and Moisture Control Guidance

Glenn likes the L-shaped flashing and building paper covering the flashing top. Both code requirements. EPA adds a self adhered membrane behind the ledger and another L-shape piece of flashing below, probably to deflect water away from siding below. Neither of these are required by code, but that doesn't mean they're not a good idea. Another detail from EPA eliminates metal flashing, using just self-adhered membrane. All good details to include.

Glen does not like the way EPA specifies cutting the flashing at each joist, so that it can lay atop the joist rather than folding over the ledger. He feels that this will cause water to wick under the flashing, defeating its purpose to some extent. It also creates a pocket for debris.

Building Science Corporation 

Good details for both walls with and without house wrap. Glenn likes that there is no notched flashing, rather the flashing extends behind the joists. 


Adhered Masonry Manufacturers’ Association

Detail to provide guidance on adhered cladding, such as stucco and synthetic stone.


Three rules for Flashing (intent and purpose)
  • Flashing must not corrode
  • Flashing must be installed like shingles — flashing relies on physics to do its job properly
  • Flashing must do its job and keep water out of the building cavity


—Glenn Mathewson is an inspector/plans examiner in Westminster, CO. This video is excerpted from his Building Code College, a free ICC-approved online school for building codes.