As usual, back in the day things were simpler. When it came to “running trim” or “trimming out” the exterior of a house, we used wood. We installed it with tried-and-true methods handed down from previous generations, and for the most part it stayed in place and provided years of trouble-free service.
But today we need much more than carpentry skills in order to achieve the same results. We must understand moisture movement and thermal expansion. We must also have knowledge of new methods and trim materials and how they interact with other construction materials. In short, for our seams to stay tight and our joints to remain true, we need to alter our methods to account for things that our predecessors rarely had to consider.
Account for the framing’s moisture content
Even though a concrete sidewalk may be thick and strong, if the ground beneath it moves enough, the sidewalk will shift and crack. The same is true for the trim on a house. If the frame to which it is attached moves enough, it takes the trim with it no matter what type of material it is or how well it has been installed. And with it goes your reputation.
When thinking about the stability of the framing, remember that all wood contains moisture and all wood moves depending on the change of its moisture content (MC). The more the moisture content changes, the more the wood moves. Typically, dimensional lumber is available in varying degrees of moisture content depending on the species, drying practices, and storage conditions. Once it has been built into a structure and protected from the elements, it begins to dry out and shrink over a relatively short amount of time.
If you’re working on an existing house that is at least a couple of years old, then the framing lumber has more or less stabilized and movement isn’t a consideration. However, if the house or addition is new, then there will almost certainly be movement (shrinkage) of the dimensional lumber; how much movement depends on its initial moisture content.
Ideally, if you are in charge of the construction and material selection, a good option is to substitute engineered lumber for dimensional lumber wherever you can (such as using “I” joists for the floor system). Engineered lumber has low moisture content and is much less prone to movement.
For any remaining dimensional lumber, order stock that has been dried. The lower the moisture content, the less it will shrink. Green lumber has a moisture content of 19% or more—30% and even 50% MC isn’t unusual for green lumber! Next is surface-dried (SD) and kiln-dried (KD), which run between 16% to 19%, and finally, MC-15 which has a moisture content of 15% or less.
The rule of thumb is that for every 4% change in moisture content, wood moves 1% in size across its width. So if you framed an addition with lumber that had 22% moisture content and then dried to 12% (which is about average), you could count on the 2 x 6 wall studs to shrink about 1/8 in. Likewise, 2 x 10 rafters will shrink about 1/4 in. This will create a noticeable gap in the eaves as your frieze boards get pulled in towards the house while the soffit moves up toward the roof. Of course, it will occur after the painting has been completed. (Click here for more information on moisture content and wood movement.)
No matter what lumber you buy, proper storage on-site is important to reduce moisture absorption and subsequent shrinkage problems. Make sure to stack materials up off the ground and keep them covered. It’s also important to not simply drape a plastic tarp over the materials, especially if the tarp reaches to the ground. This can easily trap moisture from the ground, which the wood will absorb. You will quickly undo any advantage of buying dried lumber by leaving it on the job site unprotected.
|The bottom cords in this patio roof dried out and lifted off the joist hangers, pulling the fasteners out of the ceiling boards.|
If you are installing trim on a project that you haven’t built yourself, you need to know the lumber’s moisture content in order to anticipate its movement. If you don’t have a moisture meter, it’s time to buy one. It’s become a necessary tool for all carpenters and you can get a decent, digital meter for under $100. Take readings from a sample of the framing lumber and plan accordingly. For example, if the studs, joists, and rafters all have a moisture content of about 15%, then the movement will be negligible. But if the moisture content readings are high, you know the frame will shrink.
Trim selection is crucial
Separate from the issue of the framing-lumber movement, the other key element for a great looking, long lasting trim job is the selection of the trim material.
Today there are a lot more choices of trim material than there used to be. We have more liberty to match material characteristics to the job requirements, such as allowing plastic trim to come into contact with a driveway—something you don’t want to do with wood!
We also have more liberty to create new problems that cause our trim jobs to fail in one way or the other, often before the project is complete. Many of us are handling, storing, installing and finishing all of the different types of trim material with old, one-size-fits-all methods. That’s where the trouble starts.
The best way to keep the trim in place and performing well is to understand its characteristics and limitations. This understanding will help you avoid choosing a trim type that is inappropriate for the project. Once you’ve made the selection, make sure you use up-to-date installation methods that match your specific choice.
Moisture absorption and thermal movement are the two most important factors that can negatively impact a job if ignored. This is where you start to distinguish one trim material from another for the right choice.
Dimensional wood, engineered wood, and fiber cement are considered reservoir materials. This means that they have the ability to absorb moisture that leads to issues such as movement, paint failure, and rot.
Among the best practices for these types of materials are:
- Store the materials well up off the ground and keep them protected from rain.
- Prime all sides, especially the end grain edges.
- Install materials at least 6 in. to 8 in. off the ground.
- Keep it at least 2 in. off decks and roofs.
- Check the moisture content of dimensional wood and plan accordingly.
- Provide proper gapping for engineered wood and fiber-cement trim.
Poly-ash, and PVC trim are unaffected by moisture. Both can be in direct contact with wet surfaces (roofing, decking, the ground) without any adverse effects.
|Wood trim must be 2 in. above the shingles, not the sheathing—which is why this siding is rotting.|
|Poly-ash and PVC trim are not affected by moisture but a gap is still required so that the roofing can dry out.|
PVC moves due to temperature change. It expands and contracts along its length as the temperature warms and cools. A temperature difference of 50 degrees can cause an 18-ft. long piece to move about 3/16 in. or more if it isn’t properly fastened.
Also, applying a medium-to-dark colored finish on PVC trim will cause it to absorb too much heat and increase the risk of failure and most likely void the warrantee. Only a paint with a light reflective value (LRV) of 55 or higher should be used. There are some newer paint formulas that are referred to as ‘vinyl safe’ which may allow you to use a coating with an LRV lower than 55. Be sure to check with the manufacturer before you use any coating with an LRV lower than 55.
Best practices for PVC trim include:
- Provide proper gapping based on ambient temperature and length of trim.
- Use screws for fastening.
- Finish with only light colored paints.
- Glue mitered and beveled joints.
Poly-ash, dimensional wood, engineered wood, and fiber-cement trim have negligible expansion and contraction movement due to temperature changes, even with extreme temperature differences.
The following handling and installation attributes have an effect on the selection of a trim type.
Poly-ash, dimensional wood, engineered wood, and PVC can be cut and routed with basic carpentry skills and tools, however carbide blades may be recommended. More specialized tools are needed for fiber cement.
Poly-ash and fiber cement can be fastened with a pneumatic finish nailer. Dimensional wood, engineered wood, and PVC require a fastener with about a 3/16-in. head.
Poly-ash and engineered wood weigh about the same (more than most wood and PVC, less than FC). Engineered wood is relatively stiff and must be stored in the same manner as dimensional wood. Poly-ash flexes more when it’s carried on the flat and is more brittle than wood. PVC weighs a little less than fiber cement and is very flexible. In fact, long sections are difficult to carry by one person on a warm day and by anyone under 6-ft. tall because the ends will drag on the ground even if carried overhead. Moisture is not a consideration for storage but it should not be stored on a potentially hot surface such as a paved driveway.
Fiber cement is the heaviest and is also brittle. Heat is not an issue with storage, but it must be stored up off the ground and kept dry like wood.
Priming, gapping and joints
Dimensional wood, engineered wood, and fiber cement must be primed on all sides, particularly on the ends when field cut. Engineered wood must be installed with 3/16-in. gap between details and then caulked, and fiber cement must also be caulked to seal out water.
Beveled joints are not permitted with engineered wood or fiber cement. Neither beveled joints nor mitered joints are a good idea for dimensional wood.
PVC must be installed with gaps depending on the length of the piece and the ambient temperature. Beveled joints should be glued both together and to the substrate to insure it stays closed.
Poly-ash requires no priming, gapping, gluing, or caulking. Beveled and mitered joints are permitted.
Quick View Table
In the end, if it comes down to a tie between two material types with similar characteristics, the deciding factors are: cost, available profiles and sizes, product availability, and warrantees.
The ultimate “best practice”
The best practice that you can follow, which will produce the longest lasting, most durable exterior trim job no matter what type of trim you use, is to read the manufacturer’s instructions. Let’s repeat that a little louder, READ THE MANUFACTURER’S INSTRUCTIONS.
The bottom line for all of us, whether installers, builders, manufacturers or homeowners, is that we want our trim to stay exactly where it belongs and hold the finish for a long, long time. These are two things that really shouldn’t be difficult as long as you take basic precautions with the frame of the house, and select and install the trim properly.