Buy the Extreme How-To Book

Plinth Blocks

Construction How-To, Floors, Molding December 1, 2015 Sonia



 

 

 

 

 

 

 

By Rob Robillard

I love using plinth blocks in finish carpentry, especially when the door casings I’m installing have deep and ornate details.

Originally plinth blocks were masonry support bases, but have since evolved into more of a decorative add-on for many interior decors, adding a touch of style and class to a room. If you are in the process of upgrading your interior trim, consider adding plinth blocks to your doorframes.

Often referred to as “do-hickies” and “thingamajigs” by a customer of mine, plinth blocks act as a decorative base for door trim, pilasters or columns, and are often seen in higher end trim work and older homes where the trim and baseboard intersection do not visually meet well.

Plinth blocks stand proud at the bottom of the casing, adding a reveal and shadow-line depth between the vertical and base trim. Plinth blocks can be a wonderful visual element to virtually any room, adding a nice aesthetic detail to a molding, baseboard or door frame.

What Does Plinth Mean?

“Plinth” is an architectural term used to describe a support or base for a column. Plinth blocks are located on the floor and below door casing to create a transition to the baseboard. These molding pieces create a classic appearance to door surrounds and are commonly used with either rosettes or a door header, but can be used with mitered casing, too.

Base your plinth design on existing blocks throughout the home.

Base your plinth design on existing blocks throughout the home.

The plinth block should always be at least 2 to 3 inches taller than the baseboard, it must be thicker than both the casing and the base. It should be 1/4 inch wider than the casing.

When do you need a Plinth Block?

Usually when the door casing or architrave is thicker than the baseboard skirt, you do not need a plinth unless it is part of the design (or the lower part of the trim needs protection). If, on the other hand, the baseboard skirt is thicker than the casing, a plinth block is a good solution.

Sketch a template.

Sketch a template.

The Golden Rule for Sizing

For a plinth block to look right it needs to clear, or at least meet, the baseboard and base cap height. The other issue is proportion. Plinth blocks look best when the height is 1.5 to 2 times the width. Many designers follow the “golden rule” which is to make the height equal to 1.6 the width.

Store-bought Plinths

Many lumberyards and millwork companies sell or make plinth blocks that match the profiles of specific door trim molding they sell. This is an easy way to accomplish plinths in your design, as long as your baseboard trim work with the plinth you bought.

In my experience, there are not that many commercially available plinth block sizes available for all the different trim that I deal with.
Sometimes making your own plinth blocks gives you the flexibility and versatility of making the exact height and thickness for whatever door trim you’re installing.

DIY Plinth Blocks

I typically use 5/4 trim for my plinth blocks. In this particular job I used 5/4-by-4 and duplicated the existing plinths found elsewhere in the home.

Transfer the measurements to your trim stock.

Transfer the measurements to your trim stock.

These plinth blocks have a bevel that took the thickness from 1 inch, on one end, down to 3/4 inch. I’m guessing this was originally done to mimic the narrowing of the casing used.

Use an angle gauge to measure the bevel.

Use an angle gauge to measure the bevel.

Determining Bevels

To determine this bevel, I marked up a scrap block measuring inward 1 inch and then marking the 3/4-in. thick edge. I then used a bevel to connect the lines and measured that angle. This angle was transferred to my table saw where I cut the bevel first on the test piece and then on the plinth block, once I was satisfied

I had the bevel angle correct.

Set your table saw blade to a matching angle.

Set your table saw blade to a matching angle.

Making the Blocks

The first step is to rip your board the width of the plinth block. When ripping, leave enough material (usually 1/16 inch), so you can plane and sand the edges to clean up the cut and remove the saw marks.

Arrange an outfeed roller or other system to support the workpiece.

Arrange an outfeed roller or other system to support the workpiece.

Determine your bevel by mimicking an existing plinth block or making a test piece. Then cut bevels on the table saw.

Cut your beveled plinth block stock to the height you want.

Lock down the rip fence and make the bevel cut.

Lock down the rip fence and make the bevel cut.

Next, add 45-de. bevels to the edges. If applicable, you might want to ease the edges of your plinth blocks. This can be done with a hand plane, table saw or a router bit. Not all designs have this look.

Shown is the fresh bevel cut.

Shown is the fresh bevel cut.

Use a block plane to remove saw marks. Sand the end-grain smooth, and break all edges slightly to accept paint better.

Adding Plinth Blocks to Existing Trim

The easiest way to install plinth blocks is when there is no existing door or baseboard trim in place. If there is, you need to measure your plinth block location and use a multi-tool to cut back the trim in order to insert your plinth. It should fit tightly underneath the cut-off molding, and tightly against the side of the cut-off baseboard.

Slightly round the corners with a hand-plane.

Slightly round the corners with a hand-plane.

New Trim Installations

Measure and mark your reveal marks on your doorjamb. I use a small combination square set to 3/16 inch to make two small pencil marks, one at the top edge and one on the lower edge. The 3/16-in. setback is to provide the reveal, lip or edge between the plinth block and the side of the jamb.

A shop-made jig can speed up production when you need to cut several blocks of the same length.

A shop-made jig can speed up production when you need to cut several blocks of the same length.

Place the plinth block at the base of the doorjamb on the edge of your reveal mark. If your jamb is plumb, then your plinth will be, too.

If the plinth block does not fit flush, use your hammer to flatten in the plater or drywall a bit until you get your desired fit.

Fasten the jig in place to serve as a temporary stop when cutting the trim stock.

Fasten the jig in place to serve as a temporary stop when cutting the trim stock.

If hand-nailing, predrill four holes through the plinth block with a 3/32-inch drill bit. Drill two holes, one high and one low, into the door jamb side. One the other side, drill your low hole so you can hit the bottom 2×4 plate, if applicable. On the upper hole, if no wall stud or nailer is present, drill at an inward angle toward the door jamb. The angle helps lock in the plinth.

The size of the blocks will be determined by the prevailing trim design throughout the house.

The size of the blocks will be determined by the prevailing trim design throughout the house.

If using a pneumatic nailer simply install in the same locations. Install nails slightly recessed for wood filling.

Sand the surface. Here's the finished product.

Sand the surface. Here’s the finished product.

The presence of framing will dictate your fastening pattern. When possible, install fasteners into wall studs and use care placing fasteners on plinth bevel lines or other aesthetic details.

Plan your reveal to create a shadow line.

Plan your reveal to create a shadow line.

Try to nail 3/4 to 1-inch diagonally from the bottom corners, and diagonally down from the profiled edge of the plinth block.

Once all the plinth blocks are installed it’s time to install door casing and baseboard trim.

When possible, nail into solid framing. Finish by filling nail holes and caulking joints, then prime and paint.

When possible, nail into solid framing.
Finish by filling nail holes and caulking joints, then prime and paint.