Antique chestnut interior trim © Daniel Friedman Home Interior Trim: Choices, Common Trim Problems

  • TRIM, INTERIOR - home CONTENTS: Interior trim types, baseboard, quarter-round, door and window casings. Definitions of types of interior building trim: Cornice molding, door & window casing, picture rail, plate rail, baseboard trim, quarter-round trim. Properties & purposes of interior trim in buildings. Common problems with interior trim: diagnosis, cause. Tips for installing trim against uneven floor or ceiling surfaces. How to preserve and re-use valuable antique or historic wood trim in buildings.
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Interior trim in building:

This article describes interior trim used in homes and cites common problems with interior trim.

We include suggestions for installing trim over uneven surfaces, avoiding water damage to floor/wall trim, and re-using antique wood trim boards.

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Interior Building Trim: Types, Terms, Definitions

Types and terms for interior building trim (C) Carson Dunlop AssociatesThe building interior trim types and interior trim defects or problems described here are adapted and expanded from original citations provided courtesy of Carson Dunlop Associates, Home Reference Book, with permission.

[Click to enlarge any image]

The Functions of Interior House Trim

Most houses have interior trim including baseboard, quarter round and door and window casings. These trim details protect and conceal joints, corners and changes in material.

Interior trims conceal rough cuts and edges in plaster and drywall around windows and doors and at the floor/wall or ceiling/wall juncture. They add architectural appeal to a home, and better quality moldings and trim may indicate better quality construction.

Carson Dunlop's sketch (above left) illustrates the location of the principal types of interior trim:

Interior Trim: Tips, and Tricks of the Trade for Installing Interior Trim in buildings

Ornate plaster trim Mexico City © Daniel Friedman

Our photo (left) shows ornate plaster wall, cornice, and ceiling trim details photographed in a home in Mexico City.

Trim Fitting Tips:

Our mentor, Bernie Campbalik, taught his student carpenters that if the building walls or floors are not dead flat, fitting some wood trim boards perfectly can be difficult or impossible. But a few tricks of the trade can still produce beautifully-fitted interior building trim. We describe a few basic trim fitting ideas here.

Fitting Trim to an Uneven Floors or Ceilings:

If the wall is flat but the floor sags or humps, it is often possible to push more narrow boards trim down flush with the floor or up flush with the ceiling surface or close to flush with the floor or ceiling during nailing. Where the trim boards are wider or heavier stock that bending is impossible.

To fit heavier trim boards to an uneven floor surface we used two alternatives: the trim board can be scribed and custom cut or planed to meet the floor surface variations, or a smaller-gauge quarter-round additional trim strip can be nailed in place at the juncture of the larger trim baseboard and the floor. It's easy to bend quarter-round trim to accommodate wall/floor irregularities.

Fitting Trim to an Uneven Wall:

Sometimes we [DJF] found that the building walls were scalloped or bulged, perhaps because of uneven drywall installation over wall studs that were not in a flat plane, or in some older homes that were insulated using foamed-in wet urethane during the 1970's we found that the moisture of the insulation process caused a scalloping effect in the wall drywall after the insulation dried.

To fit floor/wall baseboard trim against a slightly-scalloped wall it may be possible to simply push the trim board tightly against the wall during nailing. But where the wall scallops are deeper, we use a flexible caulk to seal the otherwise dark gap that appears between the inner edge of the trim board and the wall surface.

Baseboard and Quarter-Round Interior Trim

Figure 5-18: (C) J Wiley, S Bliss

Baseboard and quarter round are usually wood (or wood fiber) components installed at wall/floor intersections.

Baseboard protects the bottom of the walls from things like feet, brooms and vacuum cleaners, and provides a clean joint at walls and floors. Baseboard can be anything from a two-inch high piece of plain lumber to an intricate two or three piece architectural molding, 10 or 12 inches high.

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As we detail at TRIM, INTERIOR INSTALLATION, most wide, flat interior trim moldings are recessed reduce cupping. The figure at left is provided courtesy of Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

Quarter round [not shown in the sketch] is usually relatively small (approximately 3/4 inch radius) and covers the joint between the floor and the baseboard. It may be the same material as the baseboard. Some architectural treatments omit quarter round, and occasionally baseboard is omitted as well.

Solid wood trim boards that are painted or that are intended to be painted are commonly constructed of finger-jointed wood, permitting lower-cost construction of long, straight interior (or exterior) trim boards. Finger-jointed exterior trim, unlike interior finger-jointed trim boards, is exposed to weather and can be less durable - illustrated at TRIM, EXTERIOR CHOICES, INSTALLATION.

Alternatives to Wood Interior Trim

Tile or marble may be used for baseboard. This is an expensive treatment, of course. A commercial treatment occasionally found in homes is broadloom flooring turned up the wall a few inches to form a carpet baseboard.

Finger jointed interior trim photo © Daniel Friedman

At TRIM, INTERIOR INSTALLATION we describe other alternatives to solid wood trim including MDF (Medium Density Fiberboard composite trim shown in our photos below), urethane interior trim moldings, and other flexible plastic moldings.

Be sure to also read Definition & Characteristics of MDF Medium-Density Fiberboard where we include research about this material.

What is the Difference Between Door/Window Casings and Moldings?

Door and Window Casings or Trims

Door and window casings provide a finished look to the junction of a wall and door or window opening. Casings on homes built before 1985 [estimated] are most often wood.


Finger jointed interior trim photo © Daniel FriedmanMoldings at wall/ceiling intersections are referred to as cornice moldings. They may be made of wood, plaster or foamed plastic like the urethane molding product shown at left.

Ceiling Medallions

Ceiling medallions or rosettes are decorative plaster or foamed plastic details on ceilings around light fixtures. These details were common in principal rooms such as living rooms or dining rooms.

Traditionally ceiling medallions were cast from plaster or were carved from wood. In modern homes ceiling medallions are more often a foam or urethane plastic molding that is lightweight and can be glued to the ceiling surface.

Ceiling medallions also can be fabricated on site although most are pre-manufactured.

Common Problems with Interior Trim

Missing, Loose, or Damaged Interior Trim

Chestnut interior trim in a 1900 home © Daniel FriedmanTrim can be missing, damaged or loose. Replacement of decorative trim with a matching system may not be practical. Custom millwork is expensive.

Also, some of the woods used in the past are not available today. It may be more cost effective to replace the entire trim in a room.

Tips for Preserving U Re-Using Valuable Antique Wood Trim in Homes

During renovations of a home built in 1900 (photo at left) we had to remove all of the wood trim in order to run additional electrical wiring for receptacle installation.

Because the home was trimmed in beautiful red chestnut wood, we wanted to preserve and re-use the trim boards. Working very carefully using paired flat-bars each trim board was removed without splitting the wood.

Don't try removing trim nails by hammering them back out from the pointed-nail end. Doing so will split the surface of the trim board when the nail head begins to back out.

The finish nails were pulled out of the trim boards from the back side of the board.

When we were ready to nail these antique trim boards back in place we discovered that over the nearly 100 years it had been in place, the wood had become so hard it was like iron, and almost impossible to nail without causing splits. After the first split was observed we changed tactics. Every nail hole was pre-drilled through the trim boards, successfully avoiding any more splitting.

Quarter round is often removed and not replaced when wall-to-wall broadloom is installed. New quarter round is often provided when broadloom is removed.

Plaster trim such as cornice moldings, ceiling medallions, are difficult to repair. Rebuilding or repairing a damaged molding is time consuming and expensive. Replacement of damaged interior trim with a manufactured trim system is often more practical.

Water-Damaged, Stained, or Rotted Interior Trim

Our photos of water damaged interior trim below sow two different examples of visible and hidden results of the effects of wet floors on trim and on drywall located behind floor baseboard trim.

Water damaged interior trim © Daniel Friedman Water damaged interior trim © Daniel Friedman

Water damage to interior trim from leaks can stain, damage, and even rot wood trim products. In buildings that have been flooded, even from a brief single-event flood that soaked the building floor, we often find mold damage on the back and under-edge of wood floor baseboard trim boards as well as on drywall found behind the trim boards.

Our photo (left) shows water damaged door casing trim, and you can see that floor baseboard trim to the left of the door casing has been replaced with a plastic trim product.

On a floor exposed to water or dampness, plastic trims offer the advantage of rot and mold resistance.

Watch out: however if there is drywall or drywall behind paneling that is behind the baseboard trim, keep those wall coverings at least 1/2" off of the floor to reduce the chances of moisture or water damage from small spills.

Sources of Interior Building Trim Products

List adapted with permission from Best Practices Guide to Residential Construction.

MDF Producers & Sources

Polymer (Urethane), Vinyl Trim Producers & Sources

Flexible Trim Manufacturers & Sources

Our page top photo shows antique red chestnut wood trim that was preserved and re-installed in a Poughkeepsie New York Home.


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