A plinth block is installed on the corners of door and window casings to give a classic architectural and elegant look to the trims. This is achieved more by blocks that have ornate decorations. A casing is also easier to install with plinth blocks since there is no need to deal with the angles. However, these blocks are not immune to damage and rot. Hence they need to be replaced once their aesthetic value begins to lessen.
Removing the Old Plinth Blocks
Removing the old plinth block is simple. Just insert the thin-bladed knife behind the plinth and gently move it until the plinth starts to loosen. Vigorous actions may damage the wall so do not use force. If putty has been used in the previous installation, sand it away with a medium fine-grit sandpaper. You may have to remove the trims as well to have an all-new casing or to prevent them from being damaged.
Marking the Spot
Set your level along the inner edge of the casing vertically where you are going to put the trim. Extend the marking a few inches above the door or window casing with a pencil. Do this with the horizontal edge too. Once you are done, you would have drawn the two sides of a square on the upper corner of the casing. Repeat the steps on all corners of a window casing and on the other upper corner of a door casing.
Setting the Plinth Block
Set a plinth at the upper corner of a casing within the two lines you have drawn perpendicular to each other. Drill pilot holes about an inch deep into each corner of the plinth. Secure it in place with trim nails and set the heads with the nail set. Repeat on the other plinth blocks for all corners of a window or door casing. You can also attach the blocks with adhesive if you do not want to nail them in place. This will harden in a few minutes so you will have time to adjust the plinth block should you misplace it the first time.
Installing the Trim
If you are going to replace the trim as well, measure the space between blocks and saw across the board straight. Set the trim between the blocks, making sure to line up the edges of the trim with the edges of the plinth blocks. If the plinth block is wider than the trim, let the blocks rise above the trim on the outside but keep the inside level. Drill pilot holes about every six inches and nail them with the hammer or nail set.
To ensure a perfect finish, fill all nail holes with the wood putty as well as any marks and dents you have made in the installation. You can apply stain or paint to cover flaws and to match the door and window casing with the rest of the house theme.