Exterior building trim guide to product choices, installation methods, details: this article explains best construction practices in the choice, installation, painting, and maintenance of exterior trim on buildings. We discuss solid wood trim, finger-jointed wood trim, hardboard trim, LVL trim, fiber cement building trim, polymer trim products, and cellular PVC building trim products and details.
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This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.
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The original sketch (at left) showing how to place wood boards & trim when exposed to weather outdoors is not correct - details are at BARK SIDE UP on DECKS & STEPS. Our photograph (at page above) is of the Justin Morrill Smith Homestead, VT, 1840 - more examples of architectural styles including building trim styles can be found at Building Age & Architectural Style.
As costs rise and quality levels fall for solid wood board stock, builders have become more receptive to a wide range of alternative products introduced over the past 10 to 15 years.
Some of the products are variations on the material used in hardboard siding, a product that has been largely discontinued due to widespread problems with moisture absorption and buckling.
Others are fiber cement- based and offer the same durability and longevity as the siding.
Still others make use of PVC, urethane, or other types of plastics, which promise longevity and low maintenance but may cost significantly more than the solid wood they replace (see Table 1-10 below)
Our photograph above illustrates Azek® trim from a building exterior along with fragments showing that while this material won't rot when exposed to the elements, it is not completely damage-proof against flexing, bending or impact. Azek® trimboards are produced by Vycom Corporation and are a vinyl polymer material. (Also see COMPOSITE LUMBER DURABILITY)
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Solid wood is still the first choice of many builders for highly visible trim such as porch columns that require tight miters and smooth edges and need to tolerate a certain amount of wear and tear. Softwoods have served well in this capacity for many years, since they were traditionally inexpensive, dimensionally stable, and held paint well. Decay Resistance of Modern Wood Trim
As smaller, faster-growing trees replace older virgin timber stands, high-quality wood has become more expensive and harder to find. Even when using decay-resistant species, the smaller trees harvested today have less heartwood, which is where the extractives are found in sufficient quantities to be effective against decay (Table 1-11). With any wood species, the sapwood is more prone to decay.
Solid wood has virtually no shrinkage along the grain and, if finished on all sides, limited seasonal movement across the grain. In general, the denser a wood species is, the more it shrinks and swells with changes in moisture and the worse it is as a substrate for paint on a building’s exterior (Table 1-12). [Click to enlarge any image]
Vertical-grain, or “edge-grain,” softwoods, such as vertical-grain cedar or redwood, are the most stable and hold paint the best. The flat-grained woods more commonly used as trim are more prone to cupping and other moisture movement and do not hold paint as well.
To compensate for the cupping and to keep corners tight and edges from lifting, always install trim “bark-side down,” since the annual rings try to straighten as the wood shrinks (see Figure 1-31).
To improve paintability, it may be necessary to rough up the surface of flat-grained trim with 80 to 100 grit sandpaper before priming. s with siding, it is best to prime all surfaces and to prime cut ends to minimize water absorption through end grain.
Also, hold trim pieces back at least 1/2-inch short of flashings or other surfaces where water may collect and soak the end grain.
Watch out: On wooden decks and stair treads a defect "shelling" can occur with the bark side down method. Shelling is a term used by some builders to refer to the loss of portions of a board surface as late wood growth (the outer surface of the tree and thus the "bark side" of the board) separates from early wood growth (the inner portion of the tree or the tree-center side of the board).
Shelling is reported to occur more often in lumber made from Douglas Fir and Southern Pine. When building an outdoor deck wooden stair tread, it is a good idea to avoid using boards with questionable, likely to separate, portions of their upper surface. Shelling may be less of a problem with wood trim that is to be placed vertically and kept painted or sealed. -Ed.
See BARK SIDE UP on DECKS & STEPS for more details about cupping of flat sawn lumber.
Many manufacturers now offer solid wood trim made up of short lengths of high-quality lumber that is finger jointed and, in some cases, edge-glued to make boards as long as 24 feet.
As with solid lumber, finger-jointed lumber is available in a number of grades and species.
Finger-jointed exterior trim, unlike its interior trim cousin, is exposed to weather and has proven less durable than hoped at some homes, as illustrated by Roger Hankey's photograph of a Nov. 2000 inspection in Burnsville, MN.
The rotted window trim shown was found on southeast facing windows. The home was built in 1989 and was found to have wood windows and trim in very poor condition, having decayed in only 11 years.
In general, finger-jointed stock is durable and dimensionally stable since short pieces of wood are less likely to warp and twist. The main concern is whether the glue joints will telegraph through the paint as the material swells and shrinks in response to changes in relative humidity.
Our photo shows a close-up of finger-jointed (or "finger spliced) wood before painting, so that you can see how these joints are constructed. -DF
Because no two pieces of wood swell and shrink at exactly the same rate, the joints often do show through.
The best protection is to keep excess moisture out of the wood by starting with quality pre primed stock or using a high-quality water-resistant prime coat and two top coats of paint.
When purchasing finger-jointed trim, look for long term warranties against any delamination or glue lines telegraphing through the paint. As with any wood-based product, minimizing exposure to water and maintaining the finish are important for long-term performance.
Use of finger-jointed trim boards in building interiors is discussed separately and includes additional photos of finger jointed wood at TRIM, INTERIOR INSTALLATION.
The leading alternative to solid wood trim, and the oldest in the marketplace, is hardboard, essentially the same material used in hardboard siding.
Hardboard consists primarily of ground wood fibers and phenolic resin, the same adhesive used in exterior-grade plywood, along with additives to improve weather resistance.
Hardboard trim is typically available with either a smooth finish or a wood-grain texture and is sold in 16-foot lengths. Hardboard weighs about 4 pounds per square foot for 1-inch stock, roughly twice as much as softwood.
Our photo of badly deteriorated hardboard siding, representative of failures with this type of product, is provided courtesy of Carson Dunlop Associates.
To its credit, hardboard trim is very uniform in consistency and holds paint well. However, because it has no grain, it shrinks and swells equally in all dimensions—up to twice as much as wood along its length. In very dry or very humid climates this can lead to gaps or buckling over long sections. Most manufacturers recommend leaving an 1/8-inch gap at butt joints and caulking (CAULKS & SEALANTS, EXTERIOR) with a high-quality paintable sealant to avoid problems.
While hardboard is relatively easy to nail, it does not hold nails well and is prone to split if edge nailed. Drilling pilot holes will help. Compared to solid wood, it is more difficult to set nails and fill the holes in hardboard. Finish nails and pneumatic nails tend to pucker the surface, which must be sanded smooth before filling the holes.
Round-headed nails driven straight in at a 90° angle leave a clean hole for filling. Most manufacturers recommend face nailing flush to the surface of the board to avoid these problems. If nails do penetrate the surface, sand the material smooth and fill the hole before painting.
The biggest problem with hardboard exterior building trim is swelling and delamination where water has penetrated the material at unpainted cut edges, nail holes, or other penetrations (see Figure 1-32).
Even if primed, the material is vulnerable in wet locations, for example, at the bottom of an exterior door casing or in direct contact with a concrete slab or foundation.
Sharp edges are also vulnerable to chipping, making this not the best choice where wear and tear is expected.
To avoid problems, corners constructed using hardboard trim boards should be butted, not mitered.
OPINION-DJF: details about what to expect when using hardboard trim on buildings are at the following articles:
Widely used in beams and headers, LVL has also been put to good use as a trim material with some minor modifications such as water-resistant edge sealing and adding a couple of cross-laminated layers to minimize cupping.
Like LVL beams, LVL trim is dimensionally stable and is easy to cut, nail, and install, similar to a piece of plywood. Its weight falls in between solid wood and hardboard. It can be used for fascia, casings, corner boards, and most other exterior trim, and is available in lengths from 8 to 24 feet.
According to Laminated Veneer Lumber, Overview [Quoting:]
Being a homogenous and dimensionally stable building material, LVL can be used in both bearing and non-bearing walls, as planks, as beams, as headers, in door and window frames, sliding door rails, steps, as well as in the roof trusses of single-family houses and engineered building constructions such as swimming halls, sport halls, etc.
The example photograph of LVL boards used to form board-and-batten trim on a building exterior is from the above document.
One manufacturer, Pacific Wood Laminates, makes a pre primed Douglas-fir LVL trim faced either with textured wood veneer (Socomi Lam®) or medium-density overlay (Clear Lam®). Medium-density overlay (MDO) is a highly durable resin-impregnated paper that resists surface checking and holds paint well. It has a 20-year track record as a durable facing in concrete forms, outdoor signs, and other exterior applications.
Georgia Pacific Wood’s LVL trim is sealed on all edges with a water-based elastomeric coating that hides the end grain and resists moisture penetration. It is then primed on all faces, with a second prime coat applied to the finish face and edges.
As with other engineered trim products, all cut ends of LVL should be primed in the field and butt joints gapped 1/8-inch and caulked (CAULKS & SEALANTS, EXTERIOR). Miters and scarf joints can be used. Cut ends should be kept 6 inches off the ground, concrete, or other wet materials. Resistance to swelling and delaminating will be similar to plywood siding panels such as T-111.
The manufacturer recommends that the nails be set below the surface either by hand or pneumatic nailer and caulked. Use a small-headed finish nail or siding nail so as not to make too large a hole in the MDO facing.
LVL is defined and explained at DEFINITIONS of ENGINEERED WOOD OSB LVL etc
Conventional plywood with MDO facing also makes an attractive and economical smooth soffit material.
Combined with LVL fascia, this should produce a durable and attractive eaves detail.
"MDO" plywood is constructed of a core of overlapping veneers of wood, but with a surface layer of medium density fiber. The resulting product has a smooth surface. MDO plywood can be used once as a concrete form material, but should not be re-used for that purpose.
MDO plywood is further defined and explained at DEFINITIONS of ENGINEERED WOOD OSB LVL etc
APA (The Engineered Wood Association) provides an HDO/MDO Plywood Product Guide that offers details about these products.
Contractors who install fiber-cement siding are looking for equally durable materials to trim out their houses. Some have found their answer in fiber-cement trim, which boasts the same dimensional stability, paint holding ability, and resistance to rot, insects, and fire as the siding.
Standard 1 7/16 -inch fiber-cement trim is comparable in price to mid grade softwoods and is available with either a smooth face or a wood-grain texture.
Like the siding, fiber-cement trim is heavy, requires special tools and procedures to cut and machine, and presents a dust hazard to those cutting the material. Although special diamond-tipped tools make it easier to cut and form, standard fiber-cement trim is not easy to miter or to make complex cuts in, and it is too hard to hand-nail. Since it is not a nail base, all pieces need solid wood backing. Like the siding, fiber-cement trim must be face-nailed with the nails set flush to the surface.
Because of its durability and resistance to moisture, fiber-cement is a good choice for soffits, using 1/4 -inch-thick panels. For other trim, most manufacturers sell 1 7/16-inch planks, which contractors typically build out with 1/2 -inch plywood or oriented-strand board (OSB) to create a thicker profile. In some markets 3/4 -inch fiber cement- board stock is also available, eliminating the need to fur out the trim but adding considerable weight.
Also see SIDING, FIBER CEMENT.
To simplify installation, James Hardie Building Products has introduced a low-density fiber-cement trim board called HardiTrim XLD that handles and installs more like wood trim.
The new material can be installed with pneumatic finish nails set below the surface and puttied. Unlike standard fiber-cement, XLD holds nails and can be mitered and edge-nailed, simplifying details like corner boards.
The 1-inch-thick material weighs about 4 1/2 pounds per square foot versus about 2 pounds for an equal sized piece of white pine.
Molded from high-density polyurethane, polymer moldings have been in use for over 20 years and have proven their durability in both interior and exterior applications. Because of its high cost, almost four times the price of premium-grade softwood, urethane is used almost exclusively for complex profiles, such as dentils, window pediments, and decorative brackets (see Figure 1-33).
Most material is sold preprimed and can be cut, planed, and sanded like wood, only more easily because of the lighter weight. Polymer moldings are installed with adhesive rather than nails, although a few finish nails are often used to hold it in place while the glue dries.
Polymer molding trim butt joints and miters are bonded with the same adhesive used to hold the molding to walls or soffits. Most manufacturers provide a proprietary adhesive for installation of polymer molding trim and building details.
Because polyurethane foam expands and contracts with changes in temperature, installers should cut long runs of molding 1/8 to 1/4-inch long and “spring-fit” the material into place. If installed slightly compressed, the molding will not leave gaps when the temperature drops and the material shrinks.
When joining two pieces of polymer molding outdoors, apply a generous amount of adhesive on both surfaces and clean the squeeze-out with a putty knife. Solvent may be needed to clean adhesive from the joint after it dries.
Before painting, fill any holes or dents with an exterior spackling compound and paint. Avoid leaving cut edges exposed, since without its hard skin, the material has a rough, irregular surface, even after painting.
Since polyurethane will degrade from prolonged exposure to UV radiation, it should be painted soon after installation. If the surface is undamaged, it holds paint well.
Also called expanded PVC, or cellular vinyl, this relatively new material is a form of PVC that has been expanded with foaming agent and extruded into boards and a wide range of exterior molding profiles.
Like other plastics and fiber cement, cellular PVC is impervious to moisture and insects and is approved for contact with ground or masonry, making it well suited to moisture-prone applications such as garage-door trim.
Warranties for vinyl building trim typically run to 25 years. Since cellular PVC is cost-competitive with premium wood, it is an attractive option for those seeking a more durable alternative.
Trim stock is available in thickness from 5/8 to 1 inch and in lengths up to 20 feet. Sheet stock, which can be used for soffits and other panel applications, is available in 3/8- to 1-inch thicknesses, with a smooth or beaded face.
Manufacturer Marley Moldings makes a wide array of molding profiles, while Azek offers a wide range of trim materials, including a 1-inchthick prefabricated corner board for use with any type of siding (Figure 1-34 above.) Our photos (below, from a Home Depot® building supply store) show examples of Veranda™ Cellular Vinyl mouldings that the manufacturer describes as "Specially formulated to withstand termites, moisture, and rot."
Similar in density to pine, expanded PVC can be cut, drilled, sanded, and even routed like wood. It has moderately good nail holding ability and is installed similarly to wood trim, although with allowances for thermal expansion and contraction.
Although it has less thermal expansion than polymer (polyurethane) foam, cellular PVC expands and contracts considerably more than wood, so it requires special detailing to avoid gaps, buckling, and other movement problems.
Manufacturers recommend leaving a 1/8-inch gap for movement for every 18 feet of length. This space is required where trim terminates into an inside corner or against an intersecting piece of trim, such as where corner boards meet the frieze board. A small back cut at the end of the board can help conceal the gap.
This amount of movement assumes that the PVC trim board is secured with two nails every 16 inches on-center to a solid substrate (three nails for boards 12 inches or wider). Nailing along the length of the board restricts its overall movement. You can further restrict movement in PVC trim by gluing it to a wood substrate with construction adhesive.
Because cellular PVC has less strength and stiffness than wood, it must be installed over a solid substrate, and material 1/2 inch thick or less should be glued to a solid substrate. It will tend to conform to an uneven substrate, showing any waviness.
Using PVC cement to weld one piece to the next, contractors can fashion corner boards, window surrounds, or other trim assemblies, using screws or nails to hold the pieces in position while the glue dries (Figure 1-35). The assembled sections are then nailed or glued in place.
For long runs like fascia, scarf joints are best since they provide the greatest gluing area. Manufacturers recommend nailing on each side of the scarf joint to hold the trim in place while the glue dries.
Construction adhesive behind the scarf joint—for example, between the PVC fascia and the wood subfascia—will also help reinforce the joint. As with other PVC joints, the plastic surfaces must be in direct contact to bond properly since the glue will not fill any voids.
Predrilling is generally not necessary as long as you use small diameter, blunt-head nails. Either galvanized or stainless-steel box nails are recommended, but contractors have used pneumatic nailers with ring-shank nails successfully. In general, the nailing pattern is the same as for wood trim except nails should be kept 2 inches from the ends of boards. Typically nails are set and puttied before painting.
Long runs of fascia require expansion space at each end or in the middle. Where a long run turns a corner, for example on a hip roof, one option is to glue the outside corners and leave an 1/ 8-inch space along the center of each run where it will be less conspicuous than at the corner.
To hide the joint, either leave an unglued scarf joint or butt the two pieces of fascia and fill the gap with a polyurethane caulk (CAULKS & SEALANTS, EXTERIOR). If building in hot weather, it is best to construct tight joints, since they will open in colder weather as the material contracts.
As Jim Blahut comments, special methods are needed to avoid open butt joints on long trim sections, including gluing, back gluing, 45 degree bevel cuts, and extra nails at the butt joints, and in very cold weather installations, Blahut staggers nails 3/4" away from board edges to avoid cracking :
One of the drawbacks of plastic trim like Azek is its greater expansion and contraction compared to wood. On cold days, gaps can develop at unglued butt joints and miters. On long lengths of trim like corner boards or fascia, those gaps can be obvious, so we overlap joints with opposing 45-degree bevels, glue them with PVC cement, and put some construction adhesive on the back side ... Intersecting trim (corner boards terminating in a frieze, for example) should have a gap of 1/4 inch for every 20 feet of length, so we leave 1/8 inch at each end to make the gap less noticeable. - Jim Blahut, Journal of Light Construction 2002 
Like PVC windows and vinyl siding, PVC trim has UV inhibitors and does not need painting.
However, if you want a color other than white, use 100% acrylic latex paint and avoid dark colors, which can cause the vinyl to overheat in direct sunlight. PVC trim requires no special preparation to paint and reportedly holds paint well.
Azek indicates that AZEK Trim™ does not require painting but that it can be painted with dark colors using Sherwin-Williams VinylSafe™ paints, Pittsburgh Paints - PPG Architectural Coatings, Duron Paints, ICI Paints, on AZEK trim products.
What type of glue is used to glue(weld) lap/ butt joint using this vinyl molding? Thank you for any help you can offer. - R. Whitney
Reply - courtesy of Steven Bliss
If you’re talking about expanded PVC products like Azek, these vinyl or PVC trim and molding products are fastened with PVC cement. In addition, special detailing is required at PVC trim and moulding joints, and allowances must be made for expansion and contraction.
Details about PVC trim installation are in this article at Cellular PVC Moldings - Plastic or Cellular Vinyl Building Trim Installation.
Check the Azekwebsite for details or see this vinyl trim AZEK installation guide: www.azek.com/pics/installation_document/AZEK-Trim-and-Moulding-Complete-Installation-Guidelines-2011.pdf
Also see "PVC trim holds paint well, resists rot, and looks just like wood", Jim Blahut, The Journal of Light Construction, September 2002. [copy on file].
Details for this topic are now found at
The best exterior trim details are designed to keep water out but to provide easy drainage for any water that penetrates the exterior.
This is particularly important when using trim materials that are vulnerable to decay or moisture damage, such as non decay-resistant softwoods or hardboard.
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(Dec 20, 2014) Joebob said:
can I use gorilla glue to attach the framing for outside flood lights to hardie plank
I'd be worried that weather, moisture, temperature variations will ultimately let that attachment fall off, damaging the siding. Floods require a weatherproof box that is mechanically secured to the structure, right? Be sure to seal around the box against weather so that water does not run behind it and into the wall.
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