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Expansion / Contration Properties

People generally don't think of thermal expansion and contraction when selecting what type of exterior trim to use. But one thing is certain - all materials expand and contract with changes in temperature based on inherent coefficients of thermal expansion.

Relative Expansion / Contraction Properties of Building Materials
Wood moves very little with changes in temperature which makes it an excellent building material. Unfortunately though, it reacts poorly to moisture changes which often results in it rotting and/or warping. Metals and plastics, on the other hand, react more to changes in temperature, but have great durability properties. The degree of change that occurs with plastics is dependant on the type of plastic it is, and the impact of any materials that are combined with it.

Plastics commonly used in building materials include polyvinyl chloride (PVC), olefins (polyethylene, polypropylene) and engineered plastics (ABS, acrylic, polycarbonate). In general, the olefin family will have the highest amount of thermal movement, followed by PVC. Typically, engineered plastics have the least amount of movement with temperature changes. (The table below shows relative movement expected from these materials).

Plastic Type Relative Thermal Movement
Engineered Plastics
(ABS, Acrylic, Polycarbonate)
Polyvinyl Chloride (PVC) 1.5
Polyethylene 1.8
Ponderosa Pine .3

Royal Wood Expansion / Contraction Properties
Royal Wood is manufactured with wood fiber and engineered plastics. These materials were selected because both provide improved dimensional stability over alternative materials. Furthermore, the core of the product is foamed which adds to its thermal stability. It is this combination of materials (wood and engineered plastics) with a formed core that results in one of the most stable composites available.

Through Royal Wood Exterior Trim is inherently more stable than most other composites and plastics, it will still react to changes in temperature. The amount of movement is dependant on a number of variables, some of which can be controlled during the installation of the products and some that cannot. These variables include:
  1. The temperature at the time of installation
  2. 1. The degree of temperature change incurred
  3. 3. The "tightness" and "fit" of the joints of material at the time of installation
  4. 4. The way the trim is fastened or attached to the building structure
Since weather can't be controlled, there's not much that can be done in respect to the first tow variable listed. However, the third and fourth variables lend themselves to workmanship, and will have a significant influence on the amount of thermal movement that may occur.

Important Installation Considerations to Minimize Expansion / Contraction
In general, the more secure Royal Wood is attached to a building structure, the less it will expand and contract. The most pertinent considerations are:
  • The size and strength of the fasteners used. Bigger and stronger fasteners are better, smaller and weaker fasteners are worse.
  • The length of the fasteners used. The further it penetrates the substrate, the better.
  • The nailing substrate. The better the substrate holds the fastener, the less the movement of the trim.
  • The number and spacing of fasteners. More fasteners, spaced at regular intervals and spaced closer together rather than further apart, yield better results.
  • The number of fasteners at the joints. In general, more fasteners should be used at the joints than in the body.
Refer to Royal Wood Installation Instructions for minimum requirements for each of the above factors. However, remember that going with a minimum usually yields minimum results. In other words, about an 1/8" to " of movement can be expected in a 16' piece over a 50 degree temperature change when the product has been applied in accordance with the application minimums. Using application techniques above the minimum requirement will yield progressively better results.


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