Engineered Core Doors

In a way that makes good use of resources, engineered wood products are manufactured by binding together wood strands or fibers, or even waste material such as saw dust and wood chips, with adhesives for a composite material. Most mass-produced doors are made with an engineered-wood core faced with a relatively thick veneer, a construction method that minimizes warping and movement and makes a door more affordable to build. In fact, industry experts consider this construction to be the most secure method, surpassing even solid wood doors which are prone to warping and bowing - providing the veneer is at least 1/16-inch thick, as thinner veneers can be easily damaged.

 

 

Up to 90 percent of door production today is engineered as opposed to solid wood. Manufacturers claim the engineered version has several advantages. Not only are engineered doors more environmentally sound - using veneers and waste wood for the core allows better use of resources - but they are virtually non-combustible, easy to glue and more dimensionally stable, all of which add up to a product that lasts longer and requires less maintenance.

Stave Core Doors

With an engineered core door, the core is plywood or some composite generally made up of different wood species. In a stave core, slats of the same wood are stacked on top of each other or small pieces are butcher blocked, with best practice keeping the wood in the core the same species as the veneer covering the core. Mixing woods can result in serious consequences. "Different woods move at different rates. With marginal door construction techniques and climate changes, the wood may separate, causing the veneer to de-laminate from the core or crack," says Art Reeves of Marvin Entry Doors. "If the core and veneer species are the same, you get a natural bonding, and the wood is going to move at the same rate, making for a more stable construction." A stave core is considered solid wood and the more solid the core, the more the door will have the look and feel of a solid wood door while being more dimensionally stable and durable.

Stile and Rail Doors

Long ago, while recognizing the fact that wood moves with changes in temperature and humidity, woodworkers devised a door construction method known as "stile and rail" or "frame and panel." Essentially, a panel "floats" within a sturdy frame, expanding and contracting with seasonal variations in temperature and humidity without moving the frame. Dating back to the 1700s, stile and rail doors are built with separate components designed to fit together. Today most stile and rail doors are constructed with solid veneers affixed to an engineered core. The simplest floating panel or stile and rail door consists of five members - a panel surrounded by a frame that consists of two horizontal rails and two vertical stiles. Panels can be divided into smaller sections via midrails added to the frame.

Because stile and rail doors are built with separate components, door parts naturally contract and expand without warping or bending - a distinct advantage over doors made of a solid piece of wood.

The single most important consideration in the stability of a stile and rail door is how the stiles, rails and panels are constructed. Laminated or stave core stiles and rails and two-ply laminated panels produce the best results. In other words, multiple pieces of wood are glued together in reverse grain for added stability and ease of hand carving. "Reversing the direction of the grain in the two-ply panels helps to reduce wood's natural tendency to cup," says Reeves. Cupping occurs when the edges of a board are higher than its center. The culprit is often high humidity and an imbalance in the moisture throughout the wood. If the wood expands significantly, the boards are crushed together, causing them to deform at the edges.

Considered the strongest method of joinery, mortise and tenon joints connect stiles and rail by a simple interlocking method: a projection in one member fits into a cavity in another, with the projection called a tenon and the cavity the mortise. The mortise and tenons may be shaped in different styles. As opposed to a straight tenon which only has two cut sides, a haunched tenon has a "stair step" cut into it - a scenario that provides a good, strong joint.

Joints of this type are secured in various ways. Glue is the preferred method. "A haunched tenon mortise and tenon gives a 2 ¾-inch by 3- to 6-inch gluing surface," says Reeves. "With such a substantial gluing surface, there is a high comfort level that the joint won't separate." The haunched tenon also acts to resist twisting in the joint and helps keep the flanks of the mortise from separating at glue up. Because of its tight fit, over the long run, the mortise and tenon joint will provide superior mechanical strength without taxing the glue bond.

Other methods include a dowel/rod construction and biscuit joinery which uses small football-shaped pieces of plywood or pine as a spline in connecting the stiles and rails. Both methods are considered inferior to mortise and tenon because there is less gluing surface and because often the dowels and biscuits are made of a structurally weaker species than the substrate.

Panels

Panels are available in a wide variety of profiles with bumps, raises, contours and the like. Panels are intended to float, so there is a small space, at least 1/16 of an inch, between panel and frame. To prevent moisture intrusion, it is best that the tiny crack be filled with special silicones or closed-cell butyl foams to decrease the ability of water to penetrate, while still allowing the panel to move.

For greater depth and a more sophisticated look, moldings can be used to hold the panels in place. To further counteract the seasonal movement of the wood and keep panels in place, special bowtie-shaped fasteners can be affixed to the backside of the molding at the mitered joints without the need for clamps, presses, staples or screws. "Wood swells during wet seasons in the summer and contracts in dry seasons," says Reeves. "If the molding opens up during dry seasons water can infiltrate the panel and cause the wood to turn black. Fasteners eliminate the possibility of movement in all seasons."

Standards

Applicable standards for exterior entry doors include those set by the Architectural Woodwork Institute (AWI) for quality, materials, workmanship and installation in three grades: premium, custom and economy. Most architectural woodwork is custom grade, with the higher premium grade reserved for exceptional projects, and economy used to describe the minimum acceptable levels. AWI also sets standards for the clarity of wood in finished projects, rated by Grade I, Grade II and Grade III limits that refer to clear surface area and vary according to species.

Wood has many favorable inherent qualities from strength and durability to natural warmth and beauty. Understanding the qualities of various species, the certification rating systems and the longest-lasting door construction methods can enable architects to act in sync with sustainable goals when specifying wood for windows and doors.

 

Marvin Windows and Doors brings its Built Around Youâ„¢ philosophy to life with every customer and every solution. A premier manufacturer of made-to-order wood and clad wood windows and doors, Marvin offers the industry's most extensive selection of shapes, styles, sizes and options to fit architects' diverse needs. www.marvin.com