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Extension > Environment > Housing Technology > Moisture management > Durability and water management

Durability and water management

By Richard Stone, Extension Educator in Housing Technology

Building in durability is the "old fashioned way" to achieve Resource Efficiency. While we may complain a little about the comfort and energy costs of older homes, we also speak with reverence of their durability. There are a number of reasons why older homes have endured the test of time. One reason is the difference between the dimension lumber used to construct older homes and the growing number of composite materials used in newer frame construction. While some composite materials are more durable than traditional framing and sheathing materials, others are less durable and are not able to survive wetting and drying cycles as well as dimension lumber.

Another factor that contributes to the durability of homes is their ability to resist Minnesota weather. On the exterior of homes, building materials that cannot stand up to repeated exposure to water must be protected by another layer. This protective layer is often referred to as the Drainage Plane. It should cover the entire house from bottom to top and includes the foundation (footings and walls) drainage system, Weather Resistive Barrier (housewrap or building paper), flashings, and roofing components. The outside surfaces of windows, doors and skylights are also part of the outer surface of a building and must be carefully integrated with drainage plane components. Starting at the foundation, every element of the drainage plane must be installed so it will direct water out and over the layer below it in the same fashion as the overlapping of shingles on a roof. If they are not overlapped properly, or reversed, water may be directed into the building and result in a failure.

incomplete-apron-flashing

Photo by Richard Stone, University of Minnesota

We are seeing newer homes with roofs that are more complex than the roofs on homes built here a few decades ago. The more complex roofs require flashing in more locations. Skilled onsite fabrication of flashing demonstrates the classic combination of form and function. Some of the more interesting details can be like origami with coil stock. Most are much simpler. Two commonly seen flashing details are apron flashings and step flashings. Because they are used so often, they are also seen incorrect or incomplete the most often. Here are some suggestions on how to get the best results with these important drainage plane details.

complete-apron-flashing

Photo by Richard Stone, University of Minnesota

step-flashing

Photo by Richard Stone, University of Minnesota

Apron flashings are seen where the top edge of a shed roof meets the side wall of a building. In the photo showing the incomplete apron flashing assembly, the roofing felt and apron flashing were both applied after the housewrap and are open at the top. This will direct any water that gets past the siding and runs down the outside of the housewrap into the building instead of down and to the outside. To avoid damage to the home, this can be corrected before the siding is installed by cutting the housewrap horizontally above the roofing felt and slipping another strip of housewrap up under the existing housewrap on the wall (shingle fashion). The new strip of housewrap should extend over the roofing felt and flashing as shown in the photo of the completed apron flashing. Contractors tape is used to fasten the strip of housewrap where it tucks under the existing housewrap and also to hold it against the metal apron flashing until the siding has been installed over it. Now, any water that gets past the siding and runs down the surface of the drainage plane (housewrap) will be directed to the outside of the apron flashing and then onto the roof. Contractors tape is the fastener that holds the drainage plane layers in place and is not a substitute for properly lapping the layers shingle fashion. If tape is applied at the top of a reverse-flashed detail to "seal" the connection, it is only postponing failure and usually not for very long. Some builders prefer to install a wide piece of housewrap or building paper between roof and wall connections with about a foot extending above the roof and about a foot extending below. They can then lap the upper wall housewrap over it from above, following completion of the roofing details, and tuck the housewrap or building paper from the walls below up underneath it. Other builders cover the entire wall with housewrap before adding roof framing against the walls and then cut and hold the housewrap up and out of the way while installing the roofing paper, shingles, and apron flashing. After the roofing is completed, they can fold it back down to cover the apron flashing and fasten it in place. Each method has its advantages.

reversed-step-flashing

Photo by Richard Stone, University of Minnesota

Step flashing is used where a roof gable meets a side wall. In both pictures illustrating step flashing, this occurs on the side of a dormer. In this situation, the step flashing extends over the apron flashing that is used where the lowest wall of the dormer meets the roof. The same principles apply when installing step flashing. Again, some builders prefer to complete the roofing details before installing housewrap while others prefer to install the housewrap but leave it held back for the step flashing to be installed by the roofers as shown in the picture. If the roofing crew installs the roof underlayment and flashings against the outside of the housewrap as shown in the picture of reversed step flashing, it can still be corrected by cutting the housewrap above the flashing and inserting a strip of housewrap following the same steps as described and shown above for the apron flashing.

The details may vary slightly depending on the location of the flashing but the concept is always the same. To achieve durability, water must be kept from entering building assemblies from the outside. To keep the water on the outside, drainage plane layers must be applied shingle fashion to direct the water out and away which allows gravity to work for you instead of against you.

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Reviewed 2010

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