Composite Trim for Decks and Porches

Mamie writes: I am in need of direction. I have a totally wrapped front porch. The finish wood is pine and is definitely rotting on the outer face and edges. I need to use a product that will not rot, such as a composite. Which do you recommend. Also, how do you attach a composite material to the existing porch framing? I do have the porch wood sealed every year, but the wood has begun to rot despite my maintenance. Thanks.

Steve Bliss of BuildingAdvisor responds: Unfortunately, rotting of exterior wood is an increasing problem since the quality of wood has deteriorated over the years due primarily to the use of fast-growth plantation lumber. Also, many protective finishes are not as good as the old formulas, although they are safer and better for the environment.

All of which has lead to the increased use of synthetics and composites for exterior trim. There are a number of options, each with pros and cons. The main products used today are Hardiboard, which is a wood/cement composite, and expanded PVC products such as AZEK. Most manufacturers of composite decking also no make trim boards of the same material as the decking, which can be used to trim out decks and porches. A wood composite called Miratek is popular in some areas. And a  new product call Boral TruExterior, made primarily from recycled fly ash (a manufacturing waste product), is also becoming  popular in some areas.  Any of these can be used as trim around a porch, but you have to carefully follow installation instructions.

AZEK. Cellular PVC trim boards, such as AZEK, expand and contract a lot in length, but not in width (the opposite of wood) with temperature changes. So these require special detailing to avoid gaps at joints and/or buckling. AZEK is impervious to insects and decay, and does not need to be painted. It is white and has a slight sheen, making it look like plastic, which it is. It will never rot but needs occasional cleaning to remove dirt and, in moist areas, mold and mildew. It can be painted and holds paint well if done properly, but dark colors should be avoided because of  thermal expansion. A light sanding is a good idea and a primer is required. Make sure you use a paint recommended for use on vinyl. AZEK is probably the most expensive of the options discussed.

Decking Composites. Many of the wood-plastic composite decking products such as Trex have broadened their lines to include trim boards of the same or similar material as the decking. Some also offer railings and other deck and porch trim pieces. Because of the wood content, wood-plastic composites have less thermal movement than than 100% plastic materials such as AZEK.  Still composites will expand and contract along their length with temperature change and need to be installed to allow for movement. There are many different formulations, so it’s hard to generalize. Most hold up well and tend to weather over time, looking more-or-less like weathered wood. Some, such as Trex, have been on the market for decades and have established a good track record, despite some problems with earlier formulations.

Since most composites use wood pulp in the mix (along with plastic resins), it is possible for composites to rot in extreme cases where they are exposed to a lot of moisture and are unable to dry out. However, this is uncommon. In most cases, you just need to clean the surface occasionally to prevent mold and mildew growth. You can use a commercial deck cleaner made for composite decking or mix your own (follow the manufacturer’s instructions).  If you’re happy with a synthetic decking product,  you’ll probably be  happy with trim boards made of the same material.

With any synthetic product, it’s critical to understand and follow the manufacturer’s instructions for fastening, expansion gaps, clearance to other materials, treatment of cut ends, and finishes. While all these products are tolerant of moisture, details that keep them wet much of the time will lead to problems.

Fiber-Cement. Hardibacker and similar fiber-cement boards have a very good track record. They are very stable dimensionally and are impervious to just about everything. They are somewhat difficult to cut and install (and the dust is toxic). They can be purchased preprimed or prefinished and hold paint extremely well due to their dimensional stability. Fiber-cement is not rated for ground contact, and may degrade if subjected frequent and sustained wetting. For good performance, it’s important to pay attention to manufacturer’s installation details.

Boral. A relatively new product, Boral TruExterior Trim, is also dimensionally stable and impervious to insects and decay, and is rated for ground-contact. It is a little easier to work with than fiber-cement products (less brittle), but also dusty, hard on tools and the dust is probably not good to breathe. It comes preprimed and can be painted dark colors. I think it is a promising product and has good “green” credentials, as it is made out of 70% recycled materials.

Miratek. This is a wood composite that has been around for several years and comes with an exceptional (50 year, transferable) warranty. Unlike most wood composites, which have a very mixed track record, I’ve heard very few complaints about Miratek. It is easier to work with than fiber-cement, but will expand and contract a little bit, so corners and butt joints need to be caulked. It holds paint well, but probably not as well as fiber-cement.

LP Smartside. This is an engineered wood product used for siding and exterior trim. Many similar products introduced over the years have performed poorly in the field, but they may have got it right this time. Reports from users are very positive. As with all these products, pay attention to the installation details.

None of these products are structural and will need plywood or other backing to provide stiffness, even over pretty short spans. Again, it is important to carefully follow the manufacturer’s instructions for good performance and warranty protection.

I have used  most of these products to some extent except Boral and Miratec. For example, I’ve used composite decking to replace corner boards that were being attacked by woodpeckers. The boards have weathered nicely, need no maintenance, and seem unappealing to woodpeckers! I am a big fan of fiber-cement siding because of its long-term performance, but haven’t used the trim products. It’s no fun to work with, but seems to last forever. I’ve used AZEK brickmold (around windows) and for porch trim. If you don’t mind the cost and plastic appearance, it’s a great low-maintenance product.

I’ve played around a bit with Boral and Miratec, but not enough to make a recommendation. I would expect Boral  to perform similarly to fiber-cement in terms of durability and paint performance. I have heard good reports about Miratec, but it will probably need repainting more than fiber-cement — but less than wood.

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