Plinth block Whether you’re renovating an old house like I am or installing trim in your new dream home, plinth blocks can add an element of sophistication as well as provide a transition point for mouldings to meet cleanly.

Plinths can be a plain block or they can have a profile – usually on the upper part of the block. They’re easy to make and install.

Design

There are a few design considerations to take into account if you’re going to incorporate plinth blocks into your moulding mix. If you’re installing new casings and baseboards you have a bit more flexibility than renovating without removing existing trim as I did.

  • plinth edges should be thick enough to capture full depth of the casings and baseboard mouldings that meet it
  • plinths should be taller than baseboards
  • can be wider or same width as door casings
  • can have an angled face if the casing gets thinner at the door jamb
  • profiles are commonly routed on the face only

In my situation the plinths are just slightly wider than the casings since the baseboards had originally butted to the side of the casings. I wanted to add a touch of detail so I routed two simple coves near the top. Since both the casing and baseboard were about 3/4" thick I went with 1" thick plinths.I would have liked to have made them thick enough to capture the shoe moulding that covers the flooring gap but I was concerned that the door might bind on the block if it projected out to far and an angled face didn’t suit the casing profile.

Material

MDF blanks cut for laminatingI used 1/2" MDF for the plinths – laminating two layers to get the 1" thickness I wanted. I needed 6 plinths that would be 5 1/4" wide x 9" high. I cut the 1/2" MDF slightly oversize so I could trim them to the right size after laminating the blanks. I could get 5 blocks out of the long piece and the smaller pieces were for the sixth.

Cutting & laminating the MDF

  • Laminated MDF blanks clampedrip the 1/2" MDF into strips about 1/4" wider than your finished blocks. If you glue up long pieces you’ll have fewer cuts to make now and later
  • gather everything you’ll need (lots of clamps, damp cloth, glue spreader) before you apply glue to the pieces
  • spread a thin layer of glue over the entire face of both pieces – especially at the edges to avoid gaps
  • clamp the pieces and wipe off the excess glue. Let the glue dry as required

Trimming to size

  • set your table saw fence to trim half the extra width off the laminated blanks and run all the pieces through
  • reset the fence to the finished width you want, flip the blanks over and trim off the other edge
  • trim the end of the blank on the table saw or mitre saw to ensure it’s square to the edges
  • cut the blanks into the final lengths your blocks will be

I test-fitted the cut plinths and found I had one place where the floor was sloped at the casing so I pre-cut the bottom at a slight angle and clearly marked that end so I could install it in the right location later.

Routing the profile

Routing top edge profileI routed a small cove at the top edge of the blocks as well as a shallow groove about 1" from the top. Both profiles were cut with a 3/8" core box bit in a hand held router using a jig. You could use a table-mounted router for these operations as well. It’s a good idea to test your profile cuts on a scrap piece to get the right depth and positioning.

Routing groove For the top edge cut I just wanted to nip the front edge enough to be visible and still leave 1/8" of space in front of the 3/4" casing. The top cove on mine is about 1/8" high by about 3/32" deep.I set up my jig and fence so I could position the block, cut the profile, replace it with the next block and go again.

Once all the top profiles were cut I reset the fence to cut the groove using the same depth setting for the core box bit. Follow the same procedure if you’re using a table-mounted router.

Finishing

Painted plinths with nail in back Once the profiles are cut, sand the pieces to remove saw marks and ease the edges slightly with 220 sandpaper. Before painting, I always like to set a nail or screw eye in a place that won’t be visible to use as a ‘handle’ so I can paint the whole piece and set it down to dry. I also like to pre-finish all parts before installing and then touch up the nail holes after installation. Prime all the plinth blocks and apply two coats of the finish colour.

Installation

Installing plinth block If you’re installing all new trim you need to determine the reveal you’ll leave at the edge of the door jamb – about 1/8" to 1/4" is typical. In my case the plinth blocks needed to be as tight to the existing baseboard and the casing as possible.

The layers of old paint and rough plaster made it difficult to get a perfect fit in some places. I installed the blocks with construction adhesive and 2" finish nails.

  • pre-drill the plinth blocks for the size of finish nails you’re using
  • use cedar shims to hold the block in position for nailing
  • countersink the nail head with a nail set
  • fill the holes with spackle and a putty knife and smooth the surface
  • touch up the face with the finish colour

Plinth blocks and baseboard

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