Roof and Siding Clearance
Smart Inspector Science
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?
First, here 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.
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.
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.
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.
Remove the trim and we can see the siding damage (Photo 4). Is the step flashing is really up/under the water barrier? And where is the kick-out flashing?
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?
During your inspection, you should always look for clearance between siding and adjacent asphalt shingles and all roofing materials. Identify lack of clearance and any visible damage. Siding manufacturers set their own clearance requirements, so be aware of products used in your area.
Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com (htoyh.com), he provides high-quality books and marketing materials that help professional home inspectors educate their customers. Copyright © by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.
To learn more, attend Tom’s technical presentations at educational sessions for ASHI chapters and local groups. Tom will present [topic to be determined] at ASHI Inspection World 2023. Tom can also provide his knowledge for your educational event - contact him at Tom@htoyh.com.
Photos, Illustrations, Captions
Photo 1. Siding Damage – No Clearance
Photo 2. Rotted Siding
Photo 3. Just Cover It Up
Photo 4. Under the Trim
Illustration R075 Roof Siding Clearance