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Figure 5-20: (C) J Wiley, S BlissBest Practices Guide to Choosing & Installing Interior Doors

Interior building doors: how to buy, install, troubleshoot, or repair: guide to best practices.

This article series discusses and provides a best construction practices guide to the selection and installation of building interior surface materials, carpeting, doors, drywall, trim, flooring, lighting, plaster, materials, finishes, and sound control materials.

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Best Practices Guide to Choosing & Installing Interior Doors

Steel fire door between garage and living area © D Friedman at InspectApedia.com This article includes excerpts or adaptations from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , by Steven Bliss, courtesy of Wiley & Sons.

Over 90% of interior doors today are either flush or molded. In either case, a facing of wood veneer or hardboard is glued to a core, providing the door with its strength.

Traditional rail-and-style construction is still used, primarily for stain grade work, although composites and veneered construction are widely used with this type of door as well.

Residential steel doors are used both at exterior entrances and in some interior passage installations such as the one shown here.

Our steel door photo is a fire rated steel door used indoors, installed between the garage and living area of a home in New York.

Fire-rated doors for this application must also include a self-closing mechanism. Photo courtesy Eric Galow, Galow Homes.

Frame-and-Panel Door Properties & Uses in Interiors

These are the most expensive doors and are used mostly for stain-grade work.

Frame and panel interior doors gain their stability by allowing the flat or raised panels to float in the frame without increasing the door’s overall width (see Figure 5-20 also shown at the top of this page).

Photo below, illustrating a 6-panel solid pine interior door is provided courtesy of Eric Galow, Galow Homes.

Rails and stiles on solid paneled wood interior doors are typically dowelled and glued to make a rigid connection at corners.

Six panel solid pine inteior door (C) D Friedman Eric GalowFor both cost saving and increased dimensional stability, many doors now use composite materials.

Rails and stiles may have a wood veneer over a core of finger-jointed wood, particleboard, or MDF.

Other doors build each rail and stile from a solid strip of appearance-grade solid wood sliced in half, reversed upon itself, and reglued so the opposing grain patterns help resist warping.

On paint-grade doors, the raised or flat panels are often MDF, which does not move with humidity changes or leave an unpainted strip when the panels shrink. If painting doors with solid wood panels, order them preprimed to help reduce problems with the paint line.

On stain-grade doors, the panels are either solid wood or veneered MDF, which offers greater dimensional stability and the appearance of solid wood to all but the most discerning eye.

Flush and Molded Doors Properties & Uses in Interiors

The standard choice for modern homes in the 1950s and 1960s, flush doors have a  1 8-inch wood or composite veneer glued to either a solid or hollow-core frame. Molded doors are constructed the same way, but with a hardboard facing molded to simulate a frame-and-panel wood door.

All flush and molded doors have solid rails and stiles and a solid area (the lock block) where the lockset is installed. The rails and stiles are either solid wood, fingerjointed stock, or MDF in lower-end doors.

Wood stiles may be combined with MDF or particleboard rails to save money. MDF stiles may not perform well in bathrooms or wet areas due to their tendency to absorb moisture.

Hollow Core Door Properties & Uses in Interiors

A corrugated cardboard grid fills in between the rails and stiles and keeps the facings rigid on a hollowcore door. The lock block where the lock set is drilled may be solid wood, particleboard, or MDF. The rails and stiles are often wider than on solid-core doors to provide structural stability.

Despite their light weight, hollow-core doors are dimensionally stable and problem-free as long as the installer does not remove too much material during installation.

Solid Core Door Properties & Uses in Interiors

In residential doors, the rails and stiles are typically not fastened to one another or to the core material, but are held together by the wood or hardboard facing. The core is typically particleboard, MDF, or low-density fiberboard, which reduces the weight by about 25%.

A standard 2'6"x6'8" hollow-core flush door weighs about 30 pounds versus 75 to 80 pounds for a solid-core version. The price difference is modest, but most homeowners prefer the solid feel and better sound blocking of a solid-core door.

However, the extra weight can put a strain on MDF jambs, which are now finding their way onto job sites. Driving one long hinge-screw into the framing at each hinge will help avoid problems.

Molded Doors: Properties & Uses in Interiors

A molded door is built like a flush door, except the hardboard facing is molded to simulate the appearance of a traditional frame-and-panel door. Most are available with an embossed wood grain. As with a flush door, the core may be either hollow or solid.

How well the molded surface simulates a wood panel door varies from one manufacturer to another. Look for a product with crisp, well-defined details at the panels and molded edges around them, called sticking. The solid-core version also feels like a solid wood door when operated and provides better sound blocking than a hollow model.

Procedures for Cutting and Trimming Doors to Fit

Table 5-12: (C) J Wiley, S Bliss

With flush or molded doors, how much can be trimmed depends upon the width of the rails and stiles and, with solid-core doors, whether they are glued to the core material.

With doors that comply with WDMA (Window and Door Manufacturers Association) specs, follow the minimum widths in Table 5-12.

Trimming too deep into a door’s stiles or outer rails can destroy its structural integrity. How much material can be safely trimmed depends on the specific door, so pay attention to the manufacturers’ recommendations.

As a general rule, do not cut more than 3/4-inch off the top or bottom rails of traditional frame and panel doors, although some doors can be trimmed by 2 or more inches on the bottom rail.

Interior Door Standards and Warranties

With laminated doors, look for products in compliance with the WDMA Standard I.S.1-87.

Under this standard, door samples must withstand multiple wetting and drying cycles without significant delamination.

Products in compliance typically carry a one- to five-year warranty against delamination. Most warranties also cover any warping and twisting in excess of 1/4 inch across the length or width of the door but require that the door be sealed on all six edges.

Oversized doors may have more limited protection against warping.

- - Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .

Industry & Trade Associations for Carpeting, Lighting, Finishes, Wood Products, Flooring, Painting & Decorating

- - Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .

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Reader Comments, Questions & Answers About The Article Above

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Question:

(July 25, 2012) Vlad said:
Recently my family moved to a new house (colonial style of 1958, good condition). Between garage and Living space I have a hollow core door of not standard size 24"x77". I know that it must be a fire rated door. Can I attach a steel plate to it to make it so? If I can, of what thickness this plate should be? Do I have to use one solid piece of steel or can I overlap couple or so smaller sheets? Do I have to have a locking mechanism and fire rated frame? What is the cheapest solution can you suggest (door opening can not be different)?
Thank you for your great site, it has been very informative to me.

Reply:

Vladimir, I think you'll find that your local building supplier stores sell a sheet metal cover in several standard door sizes and intended to be tacked onto an existing passage door between house and garage to improve fire resistance between those areas. You'd buy one of those and cut it to size. But first You should ask yourn local building department if they still accept that remedy as they may not - the fire rating of the solution younpropose is probably less than of a fire rated door. Also the door should be self closing. Cheap won't lookmso good if you buybit and later have to tossmout your solution anyway when it is disallowed, or ... After the fire.


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