Renovation & Design

Keeping the sheeting from rotting the shingles

QUESTION: My cousin built a house in a rural area of Manitoba about 35 years ago. His north and south walls are sheeted with Aspenite, and shingled with asphalt shingles. He has about a five- to 10-degree angle on the wall sloping in at the top. The shingles are in good shape, in fact they look brand-new. He has a problem with the Aspenite rotting away under the shingles, enough that they fall off in the area where the rot is. This is the second time this has happened to him. I feel he should strip all the shingles on both sides, repair the rotted portions, cover the wall with Tyvec and put on vinyl siding.

Do you agree this would be his best way to solve the problem? During the '40s and '50s, people would cover their walls with imitation brick panels, which looked like impregnated ground-up paper and tar. I heard a lot of walls rotted out because of this.

Thank you, Jack Aspin

ANSWER: I think you have a very good grasp on the problem and a very reasonable way to solve it. Your advice to your cousin is sound and I will elaborate on the reason for this and caution others about the reasons for this occurring.

Many homeowners will realize excessive moisture is the largest factor in deterioration to their homes. Not as readily understood is that most exterior siding appears to be impervious to water, but is actually not. Many types of siding materials are quite porous and will allow significant amounts of moisture to penetrate. While this seems odd on the surface, the explanation makes perfect sense. What we have to understand is many of the best building materials in our homes can absorb quite a bit of moisture from the environment. Proper release of this moisture is the true key to longevity.

To illustrate the previous point, we should look at two of the most common types of siding materials used historically: brick and stucco. Both of these products are made of natural materials that have been modified, mainly cement and clay. While they appear to shed water at the surface, both of these actually absorb water quite readily. That is why we may see a significant colour change when they are wet, as opposed to dry. What makes them both good siding materials is the ability to easily dry out after a significant wetting, with the help of sun and wind. This property is critically important in allowing the escape of moisture that gains entry to the wall cavity, behind the siding. This prevents trapping moisture in the wall system, which is the obvious cause of your cousin's dilemma.

Another unknown feature of many siding applications is there is a small gap behind the siding that helps in its drying properties. Both of the previous types of siding normally have a built-in cavity, which prevents any moisture that penetrates the full thickness absorb into the wall sheathing. With brick and stone veneers, a small void is installed by leaving the masonry finishes slightly away from the sheathing. With stucco, the wire reinforcement that holds it in place is crimped to naturally form a small space behind the initial "scratch coat" after it cures. Both of these small voids allow excess moisture that may develop in this area to run down to the bottom of the siding and harmlessly leak out to the exterior. For brick veneer walls, small weeping holes are installed in the bottom course, every few bricks, to allow this water to drain. For stucco, an often unnoticeable gap along the bottom serves the same purpose.

What is also critical is the building paper or housewrap installed between the sheathing and the siding. This material provided the dual purpose of being an air barrier, and moisture wicking from the wall assembly. While these paper-thin membranes allow water vapour to escape the wall cavity, they also prevent wind-driven rain from wetting the exterior sheathing. They also provide a moisture-resistant surface, for water that penetrates the siding, to flow down. Both of these properties prevent rot in the wooden sheathing on the outside of the wall. While it is slightly different in nature, natural wood or vinyl siding also should operate in the same manner in relation to moisture.

Older homes with typical horizontal siding, often referred to as lap siding, also have a gap behind to allow escape of moisture. This type of siding only has the top portion touching the building paper, with the bottom resting on the upper section of the lower piece of tapered wood. This creates a small gap behind a portion of each piece, so any water that penetrates the surface will drip on to the lower section. Because the bottom of the siding is not securely fastened, this moisture can work its way out to the exposed surface, where it harmlessly dries, preventing the wood from rotting. Vinyl siding, while not as breathable as wood, is also loosely connected to the lower sections and has small drain holes integral in the bottom of each piece. This also allows any rain or melted snow that penetrates the surface to escape.

You have properly identified the problem with using materials for siding that were not designed specifically for that purpose. Asphalt shingles may be well-designed for roofing, due to their properties of being completely impenetrable to water when sealed, but that does not suit a siding application. Proper siding materials should be somewhat permeable to moisture, and be designed with a method to eliminate any moisture that does penetrate the surface. They must also be able to allow the escape of water vapour that penetrates the wall assembly from the living space, not trap it like the asphalt roofing on the house in question. Use of proper vinyl siding and synthetic housewrap sheathing should prevent a recurrence of the problems, as you have suggested.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the president of the Canadian Association of Home & Property Inspectors -- Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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