Exterior Trim Molding Secrets and Tips

DEAR TIM: I have some rot happening on the flat trim boards next to my windows, doors and all around my house. Some of the rot is severe and some is minor. The actual wood boards appear to be glued together with funny V-shaped joints. I kept the trim painted. What can I do to repair it, and if I decide to replace the trim boards, what are some of my options? I'm really disappointed in the quality of the wood trim as it's less than ten years old. Eliza Q., Franconia, NH

DEAR ELIZA: Just last week, I spoke with another person who has the exact same problem with the exterior trim boards on his home. I know it's a widespread problem that's keeping many carpenters busy with repair work. The problem, in my opinion, can be traced to both material and installation defects.

Based on your description the lumber is fingerjointed trim that's glued up using many small pieces of scrap lumber. The glue that's used is waterproof, but the actual lumber often comes from trees that grow quickly. This causes the lighter colored spring wood in the lumber to be very porous and plentiful. As such, it easily absorbs water which is the source of the wood rot.

This exterior trim may look plain, but one things for sure it will never rot. Photo Credit: Tim Carter

Trim lumber on many old homes was taken from trees that had tiny growth rings of both spring and summer wood. Summer wood is the darker band of wood you see at the end of a log or on a flat piece of lumber. It's very dense and does not readily absorb water. The more summer wood you have in a given piece of lumber, the less likely it is to rot.

If your trim lumber had been painted on all sides and edges before it was installed and all cut ends painted, it's possible it could have lasted much longer. My guess is you'll discover only the parts of the trim lumber you can see are actually painted. It probably came from the factory with a thin coat of paint primer on it.

Where the rot is not too severe, you can repair this with chemicals that soak into the wood and revitalize the wood. You then remove any loose wood and add a special epoxy that's formulated to stick to the fortified wood. Once the epoxy hardens, you can sand it like wood. It accepts paint very well.

If you're faced with replacing entire trim boards, you have a multitude of options. Let's say you want to stay with real wood. There are certain species of trim lumber that have natural preservatives in them. Redwood is one. I've used redwood for years as trim lumber, and it can last generations if cared for properly.

The trick with redwood, and any real wood for that matter, is to prepaint it on all surfaces and edges before it's installed. When you cut it, you must paint the cut ends to stop water from soaking into the end grain.

I recently had the opportunity to work with some plastic exterior trim. It should appeal to you because it will never rot. It cuts cleanly and easily with a standard miter saw. It's easy to nail with standard exterior fasteners.

But plastic exterior trim has one minor flaw that you need to know about. It has a significant expansion/contraction coefficient. This means it grows and shrinks in response to temperature changes. You can paint this plastic trim, but it's best to stay away from darker colors that absorb heat from sunlight.

The manufacturers of the plastic trim have clear instructions on what kind of paint to use, how to paint it, the acceptable colors, etc. Be sure to read these instructions before you purchase the trim to ensure that you can work within the restraints of the material.

There are composite trim boards that are a mix of plastic and wood fibers. You can also purchase fiber cement trim boards that are very durable and accept paint quite well, even dark colors.

Whatever exterior trim boards you decide to purchase, be absolutely sure you read the installation instructions. Don't make a mistake and use the wrong fasteners. All too often homeowners don't use nails that are long enough, or they use ones that corrode and cause staining.

It's not hard using the wood epoxy repair compounds, but I would absolutely practice on some small areas first. Be careful about using too much epoxy at one time as it could droop on vertical applications. It may look great as you apply it, but as you walk away to take a break or put away tools, it could droop creating a massive mistake.

You can watch a video that shows painting exterior wood trim. Simply type "exterior painting video" into the search engine here at www.AsktheBuilder.com.

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