"Help, my new house is rotting away!"

I receive this cry for help quite often. Living here in Ipswich near the shore, I spend a considerable amount of time repairing rotted exteriors. I’m not talking about the 350-year old houses for which this historic community is famous. It’s the new ones that begin deteriorating almost as soon as they go up.

The fault lies in the materials being used. My early 50’s ranch is nestled into the landscape surrounded by oak and pine. The front side sees sunlight only in the morning. Yet I have had to repair only a couple of spots where leaking gutters kept the wood consistently wet. The framing is Douglas fir, the beveled siding is red cedar with a solid stain, and it looks almost as good as the day it was built.The exterior trim is pine with lots of resin, and does occasionally need paint but has not rotted.

Jump forward to the 21st century.

Cup fungi love wet wood

The farm-grown spruce used for framing is lightweight and cannot tolerate moisture.For exterior trim, builders are using primed finger-jointed radiata pine,a non-durable timber that will deteriorate very rapidly when wet if not treated with a fungicide. It looks great at first, but the finger joints begin to open up, unprimed cuts and edges absorb moisture, and within a year black streaks start radiating up the corners of the house. Fungi and mold move in, and much of the exterior trim will need to be replaced in just a few years.

The zealous use of moisture barrier house wraps may have even increased the problem.In order to prevent moisture from saturating wall insulation, most houses today are built with plastic sheathing beneath the drywall on the inside and Tyvek or other moisture barriers applied to the exterior sheathing. In a perfect world this prevents water from getting in. The world is not perfect, water does get in, and it has no way to escape. Often when I remove a rotted exterior trim board I find soaking wet Tyvek beneath.Walls become sponges. While the building code in many areas specifies modern house wrap, I’ve never encountered a seriously rotted soaking wet wall underneath old-fashioned tar paper.

What trim to use?

PVC trim board
PVC trim board

For wood purists, the ideal solution is cedar or fir for your exterior trim. All surfaces, cuts and edges should be painted with a coat of oil primer before installation. The best finger-jointed primed pine is treated with boric acid and guaranteed for 50 years. My preference is PVC trim which comes in standard board dimensions and is almost indistinguishable from wood. Some brands come with all four edges smooth, while other brands finish only the flat surfaces. Ripped edges should be planed, the edges sealed with acetone,and painted with a couple of coats of exterior latex paint so that the rough edges don’t create a home for algae. Joints should be glued or caulked.

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