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Dry rot is not inevitable but it is common on many homes, apartments and other structures with wood siding and trim.

The first key to avoiding dry rot is to select quality wood. But a second vital component is proper priming, sealing, painting and flashing when applicable.

Unfortunately, the quest to maximize profits on the part of builders, especially spec builders, often tempts them to use materials (such as exterior wood trim) that are vulnerable to dry rot.

Not to mention eventual twisting, warping and cupping — that also enhances the potential for dry rot.

An all too commonly used, inexpensive exterior wood trim material is called Hem-Fir, and the attraction is its low cost.

For example, a 1x4, 8-foot-long piece of Hem-Fir costs around $2.99, whereas a quality, treated and pre-primed board costs around $8.25.

The total cost savings just by using Hem-Fir on a new home project can add more than a $1,000 dollars to a builder’s pocketbook.

Unfortunately, it will be five to 10 years before you, the homeowner will pay the price — a second time — to repair and rectify the problem.

Whether you are commencing a new project, or repairing the wood trim around windows, doors and vertical corners, the following recommendations will serve you well.

Back priming new trim

The problem with Hem-Fir is not that the wood is inherently bad. Even though Hem-Fir is vulnerable to dry rot, this propensity can be limited and it can be a serviceable material if properly prepped, and equally important, maintained.

But I personally never recommend or use Hem-Fir on any of my jobs.

A common oversight in the installation process of any exterior wood-trim is when the painter fails to back prime (in addition to the edges and front surfaces) before installation of the trim.

Back priming limits the intrusion of moisture through the wood, which lessons the progression of dry rot and serves to equalize the penetration of moisture, which will subsequently lesson the wood’s propensity to warp or cup.

More on back priming

Wood is most susceptible to dry rot when a moist condition continues for extended periods.

Since the inside face of the wood trim is sandwiched firmly against the wood siding, the space between the two surfaces remains moist long after a rainstorm has passed — becoming the perfect breeding ground for not only dry rot, but mold and fungus.

In addition to wood trim, the same potential happens anytime two exterior wood surfaces are tightly abutted.

This is the reason why dry rot and fungus sometimes develop on exterior wood deck-railings at the point where horizontal 2x4 rails are attached to 4x4 posts, and vertical pickets abut the rails.

Re-staining or repainting your deck railings will slow the intrusion of moisture. Soaking the intersecting joints with a copper-Napthanate liquid treatment or similar material will lengthen your deck railing’s life.

The treatment can be beneficial to your wood deck boards’ life as well if you apply a treatment to the butt joints.

Salvaging and restoring existing wood trim

If your exterior trim has not yet developed dry rot, you can preserve and protect the existing wood trim by the application of an elastomeric caulking along all vertical and horizontal edges, followed by repainting with a high-quality exterior paint.

Be sure to first clean the old painted surfaces to ensure maximum adhesion. This also is the time to scrape all loose and flaking paint and to patch nail holes with exterior filler/putty.

Be sure to apply an exterior primer over the patches and scraped areas before repainting.

Replacing dry-rotted exterior trim

Selecting a quality, rot resistant material might cost substantially more than Hem-Fir, but over the serviceable life of the material, it will actually be more cost effective due to the future labor cost to replace the inexpensive wood.

For all exterior applications, I personally use and recommend the Kelleher brand Advantage boards that come pre-treated to resist decay and pre-primed on all four surfaces with Alkyd paint.

Advantage boards are available at most Sonoma County lumber suppliers.

If you live in a coastal environment or your home is exposed to direct blasts from prevailing storms, and if cost is not an issue, than consider using the AZEK brand cellular-PVC trim. It comes pre-finished white, with a wood-grain texture and is ready to install without primer or paint.

It will not warp, twist, crack or shrink and is maintenance free.

You will pay a premium for AZEK: A one-by-four 10 feet long costs approximately $23.00 vs. $9 for Hem-Fir. But it’s money well spent over the long term.

The critical factor in installing and maintaining any wood siding trim, including AZEK, is minimizing or ideally eliminating water intrusion on the backside, including priming and painting the wood siding behind the trim.

Tom Wilmer has been a licensed general contractor since 1986. Contact him with questions or comments at tomwilmer@aol.com