Top Plates and Bottom Plates

Plates are the long boards along the top and bottom of a wall framing assembly that holds all the studs together. Sometimes the bottom plate is called the “mud sill” or “sole plate”, while the top plate is sometimes called the “wall header”.

Materials: Most exterior walls and load bearing interior walls require two top plates and one base plate. The top plates are of the same material as the studs, usually #2 SPF. The bottom plates are ground-contact pressure treated lumber since they will be in contact with the foundation and may have to resist moisture due to wicking and condensation.

The Four Foot Rule: For walls with studs on 16″ centers, the lumber for top and bottom plates should be ordered in lengths that are multiples of 4 feet to ensure joints between plate boards line up with a stud. The stud then can serve as a cleat to nail the ends of two plates together. (You’ll get a chance to see how this works later.)

Overlapping Top Plates: Top plates add strength and connect walls at building corners by overlapping. This means you need to plan out pieces so that the seam between the ends of the bottom course (layer) of top plate won’t line up on the same stud as the seams of top course of top plate. Think of it this way: if someone would try to tear this wall down the middle, having seams line up in the plates would make a real weak point. Instead, the seams should be offset at least three stud spaces (4 feet).

How To Figure

Base Plates

1. Look at the floor plan (in our case Sheet A3).
2. Plot what boards you’ll need to create a continuous base plate for each wall. Remember you’re going to use boards whose length is a multiple of 4′, so your options are 8′, 12′, 16′, and 20′. (Some lumber supplier won’t be able to get 20′ boards, or will charge a lot for them.) Generally, you should use the largest boards to create the fewest seams in each plate.
3. Enter these boards on the “Materials” sheet of the Cabin Project Spreadsheet. You’ll see and entry for “Base Plates | #2 PT | 2″x6″x12′”. Adjust the dimension to suit what you figured, adding rows to the sheet if you’re using boards of different lengths.

Top Plates

1. You can use the same boards as you just figured to the bottom plate for one course (layer) of your top plate, however you’ll have to change the material to “#2 SPF”.
2. For the second course, you may need to draw out the boards to plan the proper overlaps. If the boards of a layer are two different lengths, try reversing the direction. For example, the bottom course might be a 12′, then an 8′ board to make 20′ with a seam on a stud. The top course can then be an 8′ board followed by a 12′ board: reversing the direction.
3. Enter these boards on the “Materials” sheet of the Cabin Project Spreadsheet. You’ll see three rows of different board lengths entitled “Top Headers”.

Here’s what I came up with after drawing out the top plates.