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Everything Moves – Toolbox

I’ve heard carpenters and trim installation contractors complain that PVC trim expands and contracts too much. My comment to them is: Yes, cellular PVC trim does move, but so do all other exterior building products, and many of them just as much as, if not more than, cellular PVC.

A Note from the Publisher:


If you are sensitive about articles that seem to favor a particular manufacturer, then DON’T read this one!! It’s written by the President of Versatex! But after years of experience, I’ve found that some manufacturers know more about their products than anyone else; if a carpenter wants to learn the best way to install a product, sometimes the best source of information is the manufacturer. In the future, look for more carefully-screened articles from manufacturers.


The fact is, movement can be caused by different forces of nature. For wood, wood composites, and fiber cement, movement is all about the moisture content of the product. Wood expands and contracts with changes in the surrounding humidity and, to a lesser degree, the temperature. More humid air will cause wood to expand, while drier air will cause wood to contract.

Wood does not move in all directions equally. In fact, the greatest movement will always be across the grain. If you read the installation instructions for fiber cement, or composite wood siding and trims, you’ll see the manufacturers recommend gapping between boards. Why would you need to gap something unless it’s going to move?

Coefficient of Thermal Expansion

For building products made from cellular PVC, aluminum, steel, or other polymer-based materials, it’s all about the temperature at the time of installation compared to the temperature swings the product will experience throughout the year.

You need to learn how to deal with this phenomenon, and I’d like to tell you how without getting too deep in the weeds.

First of all, let’s discuss product movement. Just about every material—be it a natural resource, or man-made product—has a coefficient of thermal expansion. Wow! Those are some big technical words. So, what does it mean?

The coefficient of thermal expansion describes how the size of an object changes with a change in temperature. Specifically, it measures the fractional change in size per degree change in temperature at a constant pressure.

There are several types of thermal coefficients: volumetric, area, and linear. Which one is used depends on the particular application, and which dimensions are considered important or critical to the material. For solids, like cellular PVC, one might only be concerned with the change along a length, or over some area. Some common coefficients of thermal expansion for some standard building product materials are:

For cellular PVC trim, as well as most exterior building products, the focus is on linear movement, since movement along the product’s length is what needs to be controlled—especially where there are long runs of trim. If your cellular PVC trim is going to move, it will be most noticeable in the fascia, frieze, or rake boards on a house.

Determining Amount of Movement

Before getting into how to best control movement, let’s look at how to determine the amount of movement for a given set of conditions. Let’s say we’re installing cellular PVC trim when the outside temperature is 50° F. The boards are 18 feet long, and the house is in Maryland, where the temperature can reach 100° F.

To determine the maximum amount of linear movement, we need the coefficient of linear thermal expansion for cellular PVC, which is 0.000032 in/in-F, the length of the board in inches (216), and the maximum temperature swing the product will be exposed to during the year—in this case, 50° F. The formula to determine movement is as follows:

The Change in Product Length (unrestricted) = The coefficient of linear thermal expansion for cellular PVC x the length of the trim x the maximum change in temperature or (Temperature at time of installation – Maximum Temperature product can reach on any day during the summer).

Δ Length = 0.000032 in/in-F x 216 in x (100° F – 50° F)

Δ Length = 0.3456” (unrestricted) or 0.1728” (when properly nailed) which is between 5/32 in. and 3/16 in.

When I say “properly nailed,” I’m not talking about an 18 gauge or even a 16 gauge trim nail. We recommend an 8d, 12 gauge trim nail.

Now, you’re probably saying: I can’t get 8d, 12 gauge nails that I can gun. Yes, you can. Swan Secure (now part of Simpson Strong-Tie) makes such a nail that is branded their “TRIfecta” nail. These nails come in a strip and are collated, so there is no problem gunning the trim to the framing members of the home.

Further movement can be reduced by using an adhesive in combination with the fasteners. For instance, gluing the cellular PVC fascia board to the sub-fascia with Liquid Nails sub-floor or heavy duty construction adhesive can reduce the board from expanding or contracting.

Best Installation Practices

Here are some “best installation practices,” given to us by contractors and remodelers with years of experience putting up long runs of cellular PVC trim (i.e. fascia, rakes, frieze boards):

  • Screws restrict movement more than nails
  • If you can bend the fasteners you plan to use to secure your trim between two fingers, they are too thin.
  • If practical, you can further restrict movement on long runs by reducing the on-center fastener spacing to 12 in. A good example here is a fascia board where there is a wooden sub-fascia allowing a tighter on-center fastener spacing.

  • Shiplap joints offer a superior joint to scarf or miter cut joints. They increase the adhesive surface area while also aligning the face of the boards, thereby preventing any offset.

  • Allow the cellular PVC trim to acclimate to the outside temperature before installing. If possible, install any long runs on a house when the outside temperature, and the temperature of the cellular PVC trimboard, is between 60° and 65° F.
  • Double-fasten on both sides of any board-to-board joint using the recommended number of fasteners based upon the width of the board (see image, right).
  • Pick inconspicuous spots away from sight lines for expansion joints that will compensate for any movement in the cellular PVC trim.
  • Southern exposures, or areas where the product is in direct sunlight, can result in slightly greater product movement due to the heat gain potential for the trim in these areas.

So, there you have it. Everything you wanted to know about why cellular PVC trim moves, but were afraid to ask. I hope the information and recommendations provided here help you with your cellular PVC trim applications, thereby making installation easier, and providing a finished project that meets or exceeds the homeowners’ expectations.


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