What is a drip edge on a roof?

What does it actually do?

Water leaving a roof does not simply fall. Surface tension pulls it around the edge of the shingle and back along the underside — the same effect that runs a drip down the outside of a teapot.

Without an edge metal, that water tracks back onto the fascia board and the exposed edge of the decking. Both are wood, neither is designed to be wet, and the damage happens somewhere you cannot see until the fascia is soft.

The drip edge gives the water a metal lip to leave from and a kick-out that carries it past the fascia into the gutter.

Where does it go, and in what order?

Two locations, two different layerings, and the order is the thing that gets wrong.

At the eave, the drip edge goes under the underlayment. Water on top of the underlayment then runs over the metal and out.

At the rake — the sloped gable edge — it goes over the underlayment, so wind-driven rain running down the rake is carried on top of the metal rather than behind it.

Getting either backwards routes water precisely where the metal was supposed to keep it out of. It is invisible once shingles are on, which is why it is worth asking about rather than assuming.

Is it required by code?

In most jurisdictions, yes. Modern model code requires drip edge at eaves and rake edges for asphalt shingle roofs, with specified overlap at joints and fastening spacing.

Adoption varies by state and often by county, and older houses were frequently built before it was required — so a roof without one is not necessarily a code violation at the time it was built, but replacing that roof triggers the current code. Confirm what your jurisdiction has adopted with your building department.

Where an ice barrier is required, the two interact: the ice barrier is normally installed over the eave drip edge and under the rake drip edge.

How do I tell whether mine has one?

From the ground, with binoculars or a zoom photograph. Look along the eave line just above the gutter for a thin metal lip projecting past the fascia, usually painted white or brown.

If the shingle edge sits directly on the fascia with nothing between them, or the shingles overhang into space with no metal visible, there is probably none.

Soft or discoloured fascia, peeling paint along the eave line, or staining on the fascia directly under the shingle edge are the symptoms of it being absent or wrongly lapped.

What does it cost, and can it be added later?

As part of a re-roof it is a minor line item — inexpensive metal and modest labour, and it should be itemized in any bid.

Retrofitting to an existing roof is a different job. Doing it properly means lifting the first course of shingles and the underlayment edge to slide the metal underneath, which on a brittle older roof breaks shingles. Some contractors will slide it in without lifting the underlayment, which puts the metal in the wrong plane and achieves very little.

If your roof is near the end of its life, this is a reason to plan the replacement rather than a retrofit.

What should a bid say about it?

It should say drip edge, eaves and rakes, as its own line, and specify the metal and the colour.

A bid that does not mention it is a bid to check. Along with reusing old step flashing, omitting drip edge is one of the standard ways a low number is reached, and both are invisible once the job is done. See hiring a roofer.

Common mistakes

• Rake and eave lapped the same way. Under the underlayment at the eave, over it at the rake. Reversing either sends water behind the metal.
• Assuming the gutter does the job. A gutter catches water that has already left the roof. The drip edge is what makes it leave cleanly.
• Omitting it on a re-roof because the old roof did not have one. Replacing the roof triggers current code.
• Retrofitting by sliding metal under the shingles only. Without lifting the underlayment the metal is in the wrong plane.
• Not seeing it in the bid and assuming it is included. Ask.