2020 Boise Idaho Real Estate Blog
Exterior trim details Main The Durable Green Home
I visited a project the other day that had used MDF as the exterior trim. This particular house had an exterior porch detail that was very nice, however the trim was tight to the concrete, and actually extended to the grade on the exterior of the porch. Are there durability problems with todays techniques? unfortunately this is the standard detail I see on allot of homes. what are the consequences? what should I be doing different?

A good place to start would be the "Journal of light construction" or the Fine Home building magazine and resources. In todays construction we are seeing more and more MDF or WasWood is what I call it. It is very easy to work with, mold, patch, paint and finish. However wood or WasWood does not do well when exposed to the elements. No exterior trim ( other than plastic materials ) should be in contact with cement or the grade (dirt), and they should be back primed as well. How do I know back priming was not done? look at any cut siding, trim, etc and you  will find that although the manufacturer says in the installation instruction to back prime, hardly anyone does. Also the trim product needs a separation from the elements, or other building components. If the trim is in contact with the concrete or dirt in this case, it will wick water into the trim material  and lead to decay, paint and product failure. When we build exterior trim details like columns we should be incorporating an air space to let the product dry out, and a capillary break to eliminate moisture transfer into the building component. Most capillary breaks need to be at least 3/8 of an inch, because water can span smaller gaps, although this is different depending on application. Cladding material on buildings that meet a roof for instance are specified at 1 1/2 inches or more depending on material and manufacture. Look around your home, do you have durability problems? If it is a new home or remodel, I would venture to say you do. Research the correct details at www.jlconline.com or www.taunton.com/finehomebuilding these web sites should give you some good guidelines and details to incorporate into your next project. There are also building products that are made of materials that are not acceptable  to moisture, maybe there is a need for a different product in some locations?


Posted by Tad Duby at 2/26/2007 12:31:00 PM
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