Engineered Exterior Trim
Wood composite, plastic, and fiber-cement trim
products promise to hold paint better than solid wood.
More and more customers are insisting on low-maintenance exteriors, while
builders are facing rising prices for quality lumber. So when it comes time to
choose exterior trim, many builders are considering new engineered alternatives
to solid wood. Some builders may be looking for a lower-cost alternative to
clear cedar or redwood. Others may be willing to pay extra for a premium
product that promises to outlast wood. In all cases, builders are looking for
something that won’t need to be painted frequently.
There are a bewildering number of products available. These products can be
grouped into three general categories: engineered wood, fiber-cement, and
plastics. In general, engineered wood and fiber-cement are less expensive than
solid wood, and much less expensive than the plastics. Most of the plastic
trims cost 20 to 60 percent more than cedar, except for polyurethane trim,
which costs about four times as much.
Fiber-cement trim, like this James Hardie product, can be used with almost
Engineered wood products are variations on the basic recipe of wood
fiber and glue. Evaluating the claims of composite trim manufacturers can be
difficult, since most are unwilling to reveal the details of their
manufacturing processes. After the widely reported failures of some hardboard
and OSB siding products in the early 1990s, builders may be reluctant to accept
manufacturers’ claims that their trim products are weather-resistant. When
asked how their engineered trims differ from the materials involved in the
class-action siding lawsuits, manufacturers speak vaguely of “optimized resin
content,” “a proprietary edge seal,” or “chemical treatment for rot
resistance.” Only long-term field experience will provide adequate assurance
that these products can withstand weather exposure.
Swollen when wet. Most engineered wood trims, like wood, will
expand when wet. With solid wood, the expansion occurs in thickness and width,
but not significantly in length. In contrast, most engineered woods expand in
all directions, including length, as they take on moisture. When installing a
40-foot fascia of engineered wood, the installer must allow for expansion, or
the material may buckle.
Louisiana-Pacific, the manufacturer of SmartStart trim, requires a 3/16-inch
gap at end-to-end butt joints. The same gap is required when installing Max
Trim particleboard. Masonite, manufacturer of Miratec, requires a 1/8-inch gap
at butt joints. Some, but not all, manufacturers permit the use of scarf
Installation instructions. Installers may
be put off by installation instructions that warn against penetrating the face
of a piece of trim. For example, Temple-Inland, which makes TrimCraft hardboard
trim, specifically forbids countersinking nails: “Drive nail heads flush with
the trim surface, and do not set or countersink.” Similarly, the instructions
for ABTco’s TrimBoard advise, “Nails should be driven flush with or slightly
above the surface.” To some eyes, leaving prominent fastener heads makes the
trim look riveted to the building. Although most engineered trim manufacturers
discourage countersinking, some products – including SmartStart, Max Trim, and
Miratec – permit countersinking as long as the nail holes are caulked.
Because the interiors of engineered trim products are more vulnerable to
moisture than their faces, several manufacturers recommend that their trim not
be ripped. The installation instructions for Clear Lam MDO trim advise, “Clear
Lam fascia/trim is factory sealed and we do not recommend that the product be
ripped in the field.” SmartStart warns, “Ripping of the trim and fascia is not
recommended.” Since installation of fascia and soffit boards often requires
ripping, these warnings are worrisome. After making it clear that ripping is
not recommended, both manufacturers proceed to specify how to paint ripped
edges should the installer choose to violate the manufacturer’s
Hardboard trim. To manufacture hardboard,
green wood chips are heated with steam and hot water, then mechanically ground.
Resins (chiefly phenol formaldehyde) are added and the fibers are consolidated
under pressure to form hardboard (see Figure 1).
|Figure 1. Most hardboard trim, like Georgia-Pacific’s Prime Trim
(left), is sold preprimed. Many hardboard trim manufacturers emboss their
products with a surface texture that resembles wood grain, as on this TrimCraft
There are at least five manufacturers of hardboard exterior trim (ABTco,
Collins Products, Georgia-Pacific, Masonite, and Temple-Inland). All of them
sell their hardboard trim in only one length, 16 feet. This limitation may
result in high levels of waste when installing 9-foot corner boards, for
Anton Tenwolde, a research physicist at the U.S. Forest Products Lab in
Madison, Wisc., has been conducting ongoing tests of hardboard siding. “With
hardboard, you can get a wide variety of properties,” says Tenwolde. “We’re
performing swell tests, which involve cyclical wetting and drying. There’s
quite a range in the water absorption properties. All of the samples meet the
minimum AHA [American Hardboard Association] standard, but you still get quite
a bit of variation from one material to the next even in samples from
the same mill.”
In spite of these reported variations, many builders feel that as long as
hardboard trim is carefully installed and well painted, it can provide good
service (see “On Site with Prime Trim,” JLC, 6/95).
OSB trim. There is only one oriented-strand board product
marketed for use as exterior trim, Louisiana-Pacific’s SmartStart. SmartStart’s
OSB core has a resin-impregnated paper overlay. It is usually sold preprimed,
although an unprimed version, called Fiberstrate, is available. Like the
hardboard trims, SmartStart is available only in 16-foot lengths.
SmartStart’s installation instructions warn that the product must be
“applied in a manner that will not allow water to intrude.” This statement is
not reassuring, since virtually no siding or trim can be expected to exclude
wind-driven rain. When asked whether this advice is a disclaimer devised by
lawyers, Tom Reierson, a technical representative from Louisiana-Pacific,
answered, “I don’t think that statement was put there by someone with
construction details in mind.”
Although SmartStart can be mitered, the manufacturer recommends leaving a
3/16-inch gap at miter joints, which would tend to spoil the effect.
Particleboard trim. In general, most
particleboards are suitable for interior use only. However, one company,
Smurfit Newsprint Corp., sells a particleboard trim product, Max Trim, for
exterior use. Particleboard is manufactured from sawdust or planer shavings.
Like hardboard, it is bonded together by resins under heat and pressure.
Both faces of Smurfitís Max Trim are covered with a resin-impregnated
paper overlay made from recycled newsprint. Max Trim is available only in
The manufacturer claims that the material can be mitered. If an adhesive is
desired, construction adhesive can be used. The material can be ripped, but if
the ripped edge is exposed, it should be painted.
MDO trim. MDO, or medium-density overlay, is plywood faced
with a layer of smooth resin-impregnated paper. Since MDO is assembled with
exterior glues, it can be exposed to the weather; in fact, MDO is sometimes
used for siding. The paper facing, which takes paint well, solves the problem
of veneer checking. MDO is commonly available in sheets up to 10 feet long.
“Generally, the durability of MDO and other exterior grades of plywood are
equal,” says Merrit Kline, product support specialist at APA. MDO is an
excellent soffit material.
Most MDO is sold in 4-foot-wide panels not the ideal shape for fascia
boards and corner trim. To respond to the need for MDO configured for use as
exterior trim, Pacific Wood Laminates developed Clear Lam, which is available
in dimensions similar to sawn lumber, in lengths up to 24 feet.
Unlike MDO panels, in which adjacent veneer plies are oriented at 90 degrees
to each other, Clear Lam consists of an MDO face over laminated-veneer lumber,
in which most, but not all, of the plies run lengthwise (Figure 2). The
crossband plies help reduce cupping. Clear Lam has an impressive ten-year
non-prorated warranty against delamination.
|Figure 2. Clear Lam trim consists of an MDO paper
face over a laminated-veneer lumber core.
Clear Lam comes preprimed. Voids on the edges are filled and sealed, and the
material can be mitered. Because Clear Lam has good dimensional stability,
there is no need to leave a gap for expansion at butt joints.
Although Clear Lam costs almost twice as much as the least expensive
hardboard trims, it has a proven record of weather resistance, and is less
likely than hardboard to swell in length.
Fiber-cement siding has gained a good reputation for its resistance to fire
and rot, and its use is growing rapidly. Until recently, however, fiber-cement
trim products have been limited to thin materials generally between 1/4
and 1/2 inch thick. The main problem with developing full-thickness
fiber-cement trim is the high density of the material, which is made of
Portland cement, sand, and wood fiber (See “”). When conventional fiber-cement
is manufactured as thick as 3/4 or 7/8 inch, the trim becomes awkward to handle
and hard to nail. “If fiber-cement of the same density as our siding was made
7/8 inch thick, it would be difficult to get a nail through,” says Todd
Griesemer, an engineer at Cemplank.
Cemplank took on the technical challenge of developing a low-density
fiber-cement. In the fall of 1998, they began selling a 7/8-inch-thick
fiber-cement trim called Cemtrim, which is less dense than Cemplank siding.
Although the density has been reduced, the material is still heavy: A 10-foot
length of 1×12 Cemtrim weighs 53 pounds, making it about twice as dense as pine
Recently, James Hardie introduced their own low-density fiber-cement trim,
which they call Hardie Low Density, or HLD trim. It is 3/4 inch thick (Figure
3), and differs from conventional fiber cement in ways that the company is
reluctant to reveal. “It’s a new type of animal,” says Hardie’s John Molder.
“Comparing it to fiber-cement is like comparing apples to oranges.” The new HLD
trim is about 30 percent less dense than conventional fiber-cement, making it
easier to handle, cut, and nail. For the time being, the HLD trim is available
only in the southern half of the United States reportedly, because there
is still some uncertainty about how the material will hold up to freeze/thaw
|Figure 3. This Harditrim HLD, like Cemtrim, is a
low-density fiber-cement product. Low-density fiber-cement trims can be
installed proud of the siding without the shimming required for thinner
fiber-cement trim boards.
Since neither Cemplank nor James Hardie will reveal what they use to fluff
up the fiber-cement used in their full-thickness trim (more wood fiber? air?),
some questions remain as to whether these low-density trims are as durable as
Several manufacturers make conventional fiber-cement trim products in
thicknesses ranging from 1/4 inch to 1/2 inch. These thinner materials can be
used for soffits. They can also be used for fascias, as long as they are backed
up by a 2-by subfascia. If used for corner boards or window trim, these thinner
materials generally need to be packed out with a continuous OSB or plywood
Fiber-cement trim, like fiber-cement siding, should be cut with a carbide-
or diamond-tipped blade. Fiber-cement does not expand or contract significantly
with changes in humidity or temperature, so the material holds paint well.
Plastic Trim Products
There are several types of plastic materials being sold as exterior trim,
including polyurethane, plain PVC (vinyl), cellular PVC, foamed polystyrene,
polymer composite resin, and polyethylene lumber.
Temperature sensitive. Unlike engineered
wood trims, which expand and contract with changes in humidity, plastic trims
expand and contract with changes in temperature. In the case of cellular PVC,
for instance, a 55 deg. F increase in temperature will cause an 18-foot piece
of trim to expand in length by about 3/8 inch. “It is important to remember
that this stuff shrinks in cold temperatures,” says Steve Roth, director of
marketing at Style-Mark, a manufacturer of polyurethane trim. “That’s why you
need to spring-fit the butt joints, and always use adhesive. Otherwise there
will be a gap when it shrinks.” Thermal movement in fascias can be disguised
somewhat by the use of scarf joints.
Polyurethane. Polyurethane (also called urethane, polymer, or
copolymer) trim has been around a long time for about 30 years in the
U.S., and even longer in Europe. Because of this long experience, the ability
of painted polyurethane trim to survive exposure to sun and moisture is well
Polyurethane trim is manufactured by pouring liquid polyurethane into a
rubber mold. Because the trim is molded rather than extruded, complicated
profiles like dentil moldings are possible (Figure 4). The material can be
mitered, ripped, planed, and sanded like wood.
|Figure 4. Because polyurethane trim, also called polymer trim, is
molded, not extruded, complicated profiles like dentils are possible. It can be
cut and machined like wood (left), and is installed with adhesive
Polyurethane trim is secured to a building with adhesive, not fasteners. A
few nails are used to hold the trim in place until the adhesive sets up (see
“On Site with Polymer Moldings,” JLC, 3/97). When used at a miter or a
coped joint, the adhesive chemically welds the pieces together. Since the
adhesive is tenacious, it is difficult to remove a piece of installed
polyurethane trim without destroying it.
Unpainted polyurethane can become crumbly when exposed to ultraviolet light
for extended periods. All polyurethane trim is sold with a factory primer, and
manufacturers recommend a topcoat of paint after the trim is installed. Because
polyurethane does not absorb moisture, the paint on polyurethane trim will last
much longer than on wood. “The paint should last five to ten years,” said Don
Stitch, director of marketing at RAS Industries, a polyurethane trim
Several polyurethane manufacturers make flat stock in sizes comparable to
dimensional trim lumber, from nominal 1x4s to 1x12s, for use as window casing,
corner boards, and fascia. Since polyurethane flat stock generally costs two or
three times as much as cellular PVC, and about four times as much as cedar, it
is rarely chosen for trim unless the customer is willing to pay a premium for
the promise of lower maintenance. Polyurethane trim becomes more cost-effective
for complicated moldings that canít be extruded, since the only
available alternative is often carved wood.
Plain PVC. One simple alternative to
painted wood trim is good old-fashioned vinyl. Vinyl soffits, of course, have
been available for years. Tamlyn & Sons offers a variety of vinyl trim
pieces intended for use with fiber-cement siding. Many of their trim pieces,
including ventilated soffits and outside corner trim, could be used with a wide
variety of siding types.
Cellular PVC. Cellular PVC (also called PVC foam or expanded
PVC) is a form of polyvinyl chloride that has been extruded with a foaming
agent. Cellular PVC trim has not been on the market as long as polyurethane
trim, but interest in cellular PVC appears to be increasing.
PVC has proven its weather resistance in such products as vinyl windows and
vinyl siding, and cellular PVC appears to be at least as durable as
polyurethane. Cellular PVC trim boards generally cost about half as much as
polyurethane boards with comparable profiles. Another advantage: Cellular PVC
has a lower coefficient of thermal expansion than polyurethane (). Unlike
polyurethane, there is no need to paint cellular PVC trim as long as the
customer likes white (Figure 5). If another color is desired, the trim can be
painted if it is first wiped with alcohol.
|Figure 5. Cellular PVC is a type of vinyl that has
been foamed and extruded. It is weather-resistant, even if left
In general, cellular PVC is said to machine like wood, and can be mitered.
If it is ripped, the cut edges will not be as smooth as the factory edge, since
the small entrapped bubbles will be visible. Although ripped edges are just as
weather-resistant as factory edges, they are best concealed for reasons of
appearance. Cellular PVC can be glued, if necessary, using the same PVC cement
used for plumbing pipe.
Most manufacturers of cellular PVC trim offer basic flat stock in typical
board dimensions, as well as brickmold. Most trims are sold in 16- or 18-foot
Foamed polystyrene. Foamed polystyrene has been used for years
to manufacture interior moldings. Recently ABTco has introduced a line of
foamed polystyrene products, Prime Molding, intended for exterior use. Other
manufacturers of foamed polystyrene, including Marley Mouldings, have been
reluctant to recommend the use of polystyrene outdoors: “The problem is UV
exposure,” says Marshall Quina, marketing director at Marley Mouldings. “It can
cause the product to degrade.” ABTco is selling their foamed polystyrene trim
primed, and recommends that it be kept painted. Since this product is brand
new, there is little field experience available to back up claims that foamed
polystyrene will hold up when used outdoors.
Marty Fajerman, product manager for polystyrene trim at ABTco, was asked if
foamed polystyrene has any advantages over cellular PVC. “Styrene is cheaper,”
said Fajerman. “It costs less to make it.” Prime Molding is available as 1×4
nominal flat stock, as a one-piece corner trim, or as brickmold (Figure 6).
Wider dimensions are not available. Prime Molding is sold in 10-, 16-, and
|Figure 6. ABTco’s foamed polystyrene trim, called
Prime Molding, is available as brickmold.
Polymer composite resin. Flex Trim Industries manufactures
exterior trim products from a material they describe as a polymer composite
resin. Flex Trim pieces are flexible enough to be used as casing on a round-top
window, or as a fascia on a round tower. Because of its high cost for
profiles wider than 3 inches, $19 to $44 per linear foot the material is
generally chosen only when flexibility is the most important factor.
Polyethylene lumber. Most polyethy-lene lumber is used for
deck boards and picnic tables. However, some builders have experimented with
using polyethylene lumber as exterior trim. “We do have a customer who used it
to trim windows,” says Jim Kerstein, president of Polywood, a manufacturer of
polyethylene lumber. Christopher Neville, customer service representative for
Renew Products, a manufacturer of polyethylene lumber, says, “We’ve seen it
used as a fascia board.”
One employee of a polyethylene lumber manufacturer who asked not to
be identified doesn’t recommend using polyethylene lumber to trim
windows and doors. “It isn’t suitable, because it expands and contracts too
much. It will open up on you.”
by Martin Holladay
Martin Holladay is an assistant editor at the Journal of Light Construction.
This article has been provided by www.jlconline.com. JLC-Online is produced by the editors and publishers of The Journal of Light Construction, a monthly magazine serving residential and light-commercial builders, remodelers, designers, and other trade professionals.
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