Designing architectural elegance with cellular PVC

by Elaina Adams | October 1, 2011 2:32 pm

[1]

By Bill Hutt and John Pace, PE
With respect to architectural trim, columns, pergolas, trellises, and other ornaments, the last decade has seen growing interest in alternatives to traditional wood products for residential projects. With the costs of construction and materials steadily climbing, and the long-term performance of components made from increasingly inferior wood continually being called into question, the attention of a growing number of design professionals has steadily shifted to cellular polyvinyl chloride (PVC).

Cellular PVC offers many of wood’s performance and handling characteristics, and couples them with an essentially infinite lifespan. It is impervious to moisture, will not rot, and is also resistant to mould, insects, and most chemicals. Cellular PVC will not burn without a flame source to keep it ignited. Further, it will not degrade under normal exposure to sun and other environmental elements. Combined, these attributes are convincing many builders and designers to abandon traditional materials and start specifying cellular PVC products for a wide variety of exterior architectural applications.

Understanding the differences with cellular
A baking analogy can be used to differentiate rigid PVC from cellular PVC. Yeast makes dough rise; one can consider rigid PVC (typically used to produce siding or windows) to be unleavened bread or flatbread, with no rising agents. When a foaming agent is added to the rigid PVC compound via a ‘free-foam’ process, it acts like yeast in dough, resulting in a ‘raised’ board—a cellular product.

Cellular PVC can be produced in thicknesses ranging from 6.4 to 31.75 mm (1⁄4 to 11⁄4 in.) in sizes ranging from 4x sheets as long as 6.1 m (20 ft) to dimensional lumber sizes such as 6⁄4x, 5⁄4x, and 1x in nominal widths from 51 to 406 mm (2 to 16 in.). Rigid PVC and cellular PVC are both produced on an extruder equipped with either a sheet or profile die. However, the extrusion process and downstream equipment used to produce and finish each product are totally different. In addition, compound formulations for each type of PVC have some subtle differences due to the processing method used to produce them, as well as the finished product you are trying to extrude.

[2]

There are many manufacturers of this material, with a wide range of product performance qualities. Some materials are essentially foam cores with laminated exterior finishes, while some are profoundly malleable or rigid. Still others have uneven surface qualities. Most cellular PVC is available in four thicknesses—9.5, 13, 19, and 25 mm (3⁄8, 1⁄2, 3⁄4, and 1 in.).

Some manufacturers’ products have achieved processes that alleviate yellowing on the board edges, while others still have this discolouration or scorching. With PVC as the starting point for creating products, it deeply widens the opportunities when laminating together multiple layers to form thicker components or to accomodate structural reinforcements.

In more desirable cases, cellular PVC products are designed to the individual project’s specifications and engineered to be packed flat, simplifying shipping to the worksite. Proprietary technology then allows fast and easy site assembly.

To work properly and to provide absolutely smooth, perfectly aligned joints during the chemical bonding process that permanently locks components in place, these must be made of a PVC material with enough flexibility to allow pieces to slide into the locked position, as well as possess enough rigidity to hold them in place. The products must also be capable of being run through a trimwork provider’s tongue-and-groove equipment without exposing an unfinished core for the absolute best-finished product. No sanding is required to create a perfectly smooth finished surface suitable for painting or attractive enough to be left in its natural white colour.

New angles on squared columns
Aluminum and fibreglass columns offer significant structural capabilities that vinyl and PVC simply do not. However, if these products are subjected to any structural impact or penetration—even something as minor as a screw being installed in the wrong place—the load capacity may be affected, the integrity of the column compromised, and the warranty impacted. When this happens, there may be no other choice than to pull the column out and find a replacement (which can be difficult, as it would need to tie in other architectural features). Additionally, aluminum and vinyl columns are prefinished, which restricts selection to three or four stock colours.

[3]

When rigid vinyl products are used to clad column cores, they are often prone to fracturing, particularly during the cold winter months. While vinyl is available as pre-coloured, one is restricted to the four or five hues offered by the three primary manufacturers. Since vinyl is notoriously difficult to paint, esthetic compromises usually need to be made. Further, like fibreglass and aluminum, vinyl often means being limited to the standard sizes offered by manufacturers—anything else needs to be cobbled together.

PVC columns are always intended to wrap around a load-bearing post. PVC can be drilled to accommodate architectural details or connections to other components without turning into scrap. (Since the vinyl itself is not load-bearing, drilling into it does not affect its structural capabilities.) This means columns with a significant amount of applied detail can be created without worry of structural compromise. One can also create column surrounds to accommodate a structural column or post of any height, which expands the creative potential.

For designers wanting to customize colour palettes to their clients’ requirements, cellular PVC can be easily painted. The free-foam cellular PVC process allows finished sheets or boards to be produced with extremely low surface gloss, enabling acrylic latex paints (with or without urethane) to adhere to them. (Latex paints stretch and move with the boards, whereas oil-based paints are more brittle and lack flexibility.) It is important to be aware it can take days or weeks for paint to fully cure after it is applied to cellular PVC—unlike wood, the plastic material is impervious to moisture. Based on testing performed by several national paint manufacturers, paint on cellular PVC will last five times longer than on wood, due primarily to the fact that paint blisters on wood as a result of the moisture content of the wood. Since there is no moisture in cellular PVC, there is no catalyst to cause the paint to blister.

Cellular PVC is produced in sheet form, and can be cut into boards in just about any width. Rigid PVC, on the other hand, is limited because it is produced on much smaller extruders with smaller profile or sheet dies, and much shorter and narrower downstream systems. As cellular PVC can come in custom sizes up to 1219 mm (48 in.) wide, it has fewer seams and a cleaner appearance when column faces are milled and installed. The authors have been part of a team that has custom-designed a wide variety of column components, including some up to 12.8 m (42 ft) in length.

[4]

Pergola possibilities
Beyond columns, compact cellular PVC is being employed to create pergola structures. The flexibility of cellular PVC allows creation of curved structural components that would normally have required lamination of multiple pieces of wood. By substituting cellular PVC in place of traditional forest products, one can fabricate structural members with an 11- or 12-degree arc to form the top section of a 3 x 3-m (10 x 10-ft) pergola. Additionally, custom-ordered cellular PVC beams have enough stiffness to make the structure stable, once they have been interlocked together and the joints have been chemically bonded.

For example, in larger (e.g. 3.7 x 3.7-m [12 x 12-ft]) custom-fabricated models, the cellular PVC material can be laminated around an aluminum core to form beam structures that meet the required structural performance standards. Electrical conduit can be incorporated within these beam structures to allow for the installation of lighting.

In one particularly challenging application, the authors designed a pergola that would hold a 113-kg (250-lb) lighting fixture. For structural support, 25 x 102-mm (1 x 4-in.) aluminum tubing, which formed the core of the 51-mm (2-in.) thick and 254-mm (10-in.) high beams, was concealed, along with a 19-mm (¾-in.) diameter cellular PVC electrical conduit within the beam.

[5]

When it was assembled onsite, the pergola components were tied down to structural steel posts, and the steel posts wrapped in PVC column surrounds. All electrical wiring ran within the beams and columns, and ultimately to the fixture’s electrical box. When complete, the lighting fixture was successfully hung, and the pergola, which was built on a lakefront site subject to significant winds, met all provincial requirements pertaining to load and uplift.

Trellises and gables
Trellises can be extremely labour-intensive to create; in their traditional form, they require milling and handling of wood, which impacts the final product’s cost. When cellular PVC was first used for this application, it meant spending a great deal of time laminating two pieces of trim together to arrive at a 38 mm (1 ½ in.) nominal, which resulted in a finished size of 31.75 mm (1 ¼ in.).

All this lamination required priming and painting before shipping (adding yet more expense). With customization capabilities, newer cellular PVC products can be ordered in the correct sizes to begin with, eliminating milling and lamination, and making it nearly impossible to identify seams and joints.

A Canadian big box store provides blueprints and lumber kits to builders across the country. About 5000 houses are built from these kits each year, and many of them feature at least some type of gable detail. In years past, these architectural elements were made at the building site, using a drawing supplied in the kit as the production guide. The elements were typically made out of plywood or pine, and often did not weather well over time due to the porous nature of the materials.

[6]

The company knew of PVC products that could be substituted to avoid decay and onsite fabrication problems, but decided one-piece units were costly and difficult to ship. However, a solution was found in a cellular PVC gable detail with a tenon joint that could be easily assembled onsite. Each gable detail was drilled with predetermined holes that were already pre-cored. This allowed the gable to be mechanically fastened from the back, without any mounting hardware being visible. The best features of these gable details were their finish and appearance. Unlike their wood predecessors, there was no variation in dimensions or composition. As a result, there was no need to use drum sanders or to fill open edges. The finish was pristine right out of the box, so the gable details could be left in PVC white or painted to match a selected colour scheme.

From the gable details, a wide variety of other architectural elements made from cellular PVC—many of which are intricately tooled—were offered, including louvres, shutters, brackets, pilasters, dock boxes, window surrounds, and planter boxes. Since many of these elements are installed in high, difficult-to-reach locations, eliminating the prospect of rot and decay—not to mention painting —helps owners reduce maintenance costs and preserve the architectural detail.

Conclusion
Cellular PVC is making the cost of adding high-end architectural details attainable for more projects that face small budgets. With traditional materials, the time spent on sanding and finishing was incredible when it came to fashioning brackets. Now, architectural pieces are essentially ready to go within minutes after they have been shaped and finish-sanded.

Bill Hutt is president of Decor Innovations and Distinctive Rail, and has more than 25 years of construction and renovation experience. Décor Innovations is a supplier of trimboard and a producer of millwork products, while Distinctive Rail is a provider of railings and columns. He can be reached at billhutt@decorinnovations.ca[7].

John Pace, PE, is president and chief operating officer of Versatex Trimboard. He has more than 20 years of experience in the design, development, production, and installation of rigid and cellular vinyl building products for residential and light commercial applications. Pace is also a founder of Wolfpac Technologies, an extruder of cellular PVC sheet and board materials that has been serving the building products industry since 2003. He can be reached at jpace@versatex.com.

Source URL: https://www.constructioncanada.net/designing-architectural-elegance-with-cellular-pvc/

---