Why Roof and Siding Clearance Is Essential

Siding manufacturers specify a clearance
to asphalt shingles and flat roofs to avoid moisture damage. Why did they establish
this requirement?

Minimum Clearance

First, there are basics to look for with roof/siding clearance (Illustration R075). In all cases, there must be some clearance between asphalt shingles and the siding and trim on a vertical wall abutting the shingles. The clearance allows water to drain out of the siding assembly and freely run down the roof. The clearance requirement varies with the type of material and manufacturer.

Illustration R075

Normally, vinyl requires a minimum of ½ inch from the “J” channel to the shingles. Most manufactured wood composite siding and trim requires a minimum of 1 inch, and fiber-cement products require 2 inches. Most adhered stone and synthetic stucco finishes also require 2 inches.

Water is No Friend of Exterior Materials

Over time, water buildup eventually damages all materials. Clearance allows drainage of water and helps the material assembly to dry.

Capillary action can also draw water up into materials. The minimum gap/clearance to stop capillary action is 3/8 inch. Manufacturers want a larger gap and they know they can’t rely on contractors to strictly adhere to a bare minimum, so they increase the necessary clearance.

Photo 1. Siding damage, no clearance.

Lack of Clearance Results in Dramatic Damage

Take a look at the fiber-cement siding and wood trim in Photo 1. This siding is about 10 years old. Edges near the shingles are falling apart, the finish has failed and mold is growing at the tight joint.

Photo 2. Rotted siding.

The fiber-cement siding in Photo 2 lacks clearance to the shingles. The paint finish is failing and the siding is crumbling. The wood trim with no clearance also traps water against the wall and the siding.

Just Cover It Up?

In this case (Photo 3), the home inspector noted the lack of flashing over the trim and no clearance. The trim should be 1 inch above the shingles, and there should be cap flashing over the trim and up/behind the siding and water barrier.

Photo 3. Just cover it up.

Remove the trim and we can see the siding damage (Photo 4). Is the step flashing really up/under the water barrier? And where is the kick-out flashing?

Photo 4. Under the trim.

The Takeaway

It all makes you wonder: Why do contractors install siding and trim tight to asphalt shingles? Because it looks better? Because they don’t read instructions?