Foundations (of a metal roof system)

First Published On: PRSRoofing

What are you talking about?  Roofs don’t have foundations.  Things stuck in the ground do.  Building do.  But roofs?  They’re up there, talking to the sky.  What need do they have for foundations?

And yet, they do.

Because roofs are structures, and structures need a base, a foundation.

For metal roof systems, the foundation is the system of flashings.  Flashings are purpose-built pieces of folded metal that perform specialized functions. These functions include:

  • anchorage/support
  • drainage
  • shielding
  • locking/security
  • appearance

House schema Trouble spots

A well-designed system pays particular attention to the flashings, as the flashings are used in the places where there are the greatest weaknesses or risks.  The flashing provides a specific way to diminish the risk, to provide additional protection at the point of danger, and to work cooperatively with the other elements of the roof system.

For example, in the above schema, the various elements are described as follows:

A.  Chimney flashing (resist water, snow, ice, uneven chimney surface)

B.  Ridge (usually overlap, flashing and/or cap)

C.  Hip (similar to ridge, but with own issues)

D.   Gable or rake (water flow, wind pressure, exposure)

E.  Valley (concentration of water, snow, ice)

F.  End-wall or Sidewall (interface to siding, water flow)

G.  Eave (water flow, ice & snow movement, wind forces, connect to fascia, gutters)

H.  Base-of-Dormer (transition from roof to vertical surface, interface to siding)

Despite the many types of systems on the market, the flashings generally group themselves into specific families.  For instance, the front edge of the roof, the eave, is usually covered with a starter flashing, which takes a variety of forms, but has common elements:  the fascia portion, the drip edge, the edge lip (or attachment point), and a nailing or attachment flange.  The gable/rake detail is usually built from one or more flashings.

When it comes to flashings, having the right design and installation are the difference between “kinda works when the sun is shining” and “performs well under all conditions”.  Paying attention to how the flashings are attached, joined to each other, and how they interface to the other roof components is the basis of a good installation.

The roof covering is the pretty skin, the flashings are the bone and sinew that allow the roof covering to work.  Good installers know this, and spend the necessary amount of time preparing the roof base, identifying the potential weaknesses, and choosing and installing the right combination of flashings that will minimize or eliminate the weaknesses.  Once this foundation is in place, the installation of the roof covering is almost as straightforward as the filling in of the space between the flashings.

House all flashings

When things go wrong, they are often connected with one or more of the roof joints, and the way the roof covering was interfaced to the flashing system.

If you want to know how well the roofing rep or salesman/woman knows their products, ask them to describe how they handle the runout of a valley on top of a dormer, and what should be the sequence of layering the various protections that ensure the safety, reliability, waterproofness, and durability of that area.  Or ask them how they will interface to pre-existing siding in terms of the flashings they will use.  If they know what they are talking about, they should be able to draw you a picture or two showing the detail and their specific layering of the underlayments, membranes, flashings, and coverings to give the desired performance.  If they can’t explain, do you really trust them to understand what has to be done on your roof?

Depending on the feedback to this post (there’s a comment button to the right of the post headline), I could go into more detail about the considerations to take into account when choosing both product and associated flashing system.  So if you want to know more, please comment and either ask the questions, or add your commentary to the above.  I look forward to the discussion.


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