"Water Infiltration Part I: Some Causes and Ways to Prevent It" by Marv Shetler, Blazer Industries, Inc.

Our company is located in the Northwest where it rains quite a bit. To make matters worse, we have long periods of time without much sunshine to dry things out. Consequently, we are very conscientious about keeping water out of our buildings. The combination of rain, wind, and damp weather for prolonged periods of time makes this more difficult.

We know from personal experience and from reports we have heard from dealers that water infiltration is a universal problem. The following information and ideas should be helpful if you find you have leaks around windows, doors, corners, or other places. Over the 25 years we have been in business, we have developed several methods that we feel work very effectively in regards to keeping water out of our buildings.

First let's look at some general principles in good building design--things to do and things to avoid. Possibly the most important design feature that will help prevent building leaks is a roof overhang (the more, the better). Ironically, the majority of temporary modular buildings do not have roof overhangs, therefore it is more important than ever to use good construction techniques to prevent water leaks.

Siding materials are either permeable (water can pass through it) such as cement board, or non-permeable (water can not pass through it) such as wood T1-11, OSB, and metal. All sidings are subject to water getting through at joints, both horizontal and vertical. Because of wicking, water can get to the back of siding at Z-flashed horizontal joints on T1-11 or similar siding products.

This leads us to caulking and building papers to help keep water out. Historically, we have not used building papers except for lap siding. We did not think it was necessary. We discovered the hard way (experience) that you have to use it behind permeable sidings such as cement board. We have also learned through experience that caulking has its pluses and minuses. Sometimes it helps and sometimes it does not.

There are many different types of caulk, some being better than others. Application is important. It is easy to end up with voids and/or pinholes. Bottom line is: caulking should not be used as the first line of defense in water resistive construction. It should be a secondary defense only.

As for building papers, there are house wraps and there is 15 lb. felt, which has been around a long time. There is quite a bit of controversy as to which is better. "The Journal of Light Construction" has had some excellent articles on this subject over the past several years. We will not get into that except to say we believe Tyvek and R-wrap are two of the better building wraps. Again, proper installation is important. Lapping of seams and taping around windows are two critical areas.

In summary, proper selection and use of materials (to provide layering as gravity pulls water downward, or as water passes through joints in siding and trims via wicking or the force of the wind) is crucial to keeping water away from the structural and internal parts of a building.

Now let's talk about how to prevent window leaks, particularly when the siding is T1-11 or similar sheet board. First you have a choice as to whether you install the window under or over the T1-11. The traditional site installation method is to install it under. That, in effect, flashes the top of the window but does not give much protection to the sides and bottom. If water gets between the siding and the side of the window it will end up on the back of the siding. Building paper or felt may help, but the odds are against you.

For years, we have installed the window on top of the T1-11. There are several pluses to this method. First you can install the siding quicker because you do not have to cut around the windows. You just come back with a router and following the framed opening, you end up with a nice clear precise fit. The secret to preventing leaks is to remove a narrow strip of siding at the top so you can insert a piece of galvanized flashing up under the siding, reinstall the marrow strip of siding that was removed, then bend the flashing down over the top window flange after the window is installed. Caulk the vertical edges of the window for extra protection, then trim the entire window with your choice of trims. This method really works and it is not hard to do. Any water that gets between the trim and the edge of the window just runs down the window flange then exits on top of the siding, rather than the backside.

In Part II of this series, we will discuss corner board installation and belt board installation. Both of these areas are prone to water infiltration and can result in rotted corners and floor rims.
 

Marv Shetler heads Blazer Industries, Inc., a manufacturer of factory-built buildings in Aumsville, Oregon.  He serves the Modular Building Institute as Treasurer and a board member.

Copyright © Modular Building Institute, July 2000.