How to Determine the Jamb Depth of a Hollow Metal Frame

The Jamb Depth of a frame is one of the most important dimensions needed when specifying and ordering a hollow metal frame. 

Jamb depth is the dimension measured from between the outside faces of the frame profile.

 

Jamb Depth should not be confused with the Throat Opening dimension. The Throat Opening will typically match the wall thickness or butt up against the wall depending on the wall type. Anchors will be attached to the wall using various methods. The most common wall applications are steel stud, wood stud, or masonry. 

Most often, in stud wall applications, the jamb depth will be determined by taking the wall thickness and adding the backbend dimension to come to the jamb depth. Typically, the backbend dimension is 1/2 inch.

So, for example, a 6" metal stud wall with 5/8" gypsum board on either side will bring the total wall thickness to 7-1/4" (6" + 5/8" +5/8" = 7-1/4").

If we add the typical 1/2" backbends to that wall thickness, we come to a jamb depth of 8-1/4" (1/2" + 1/2" + 7-1/4" = 8-1/4").

This rule applies for most jamb depths expect for the 5-3/4" jamb depth, which we will now explain. In many interior wood stud walls, a 2x4 wood stud is used. The true dimension of that 2x4 is 1-1/2" x 3-1/2" wood stud. With 5/8" gypsum board on either side, our wall thickness comes to 4-3/4" (3-1/2" wood stud + 5/8" + 5/8" = 4-3/4").

In many interior steel stud walls, a 3-5/8" steel stud channel is used. With 5/8" gypsum board on either side, our wall thickness comes to 4-7/8" (3-5/8" steel stud+ 5/8" +5/8" = 4-7/8").

In the hollow metal frame industry, instead of stocking hundreds of jambs with the small 1/8" difference in jamb depth, the 5-3/4" jamb depth is formed so that the backbends are 7/16" instead of the typical 1/2". This allows the throat to have an additional 1/8" which will then allow either the common wood stud walls or the common steel stud walls to be wrapped. For a 4-3/4" wood stud wall, the 4-7/8" throat provides an extra 1/8" in space and is not a problem when installing the frames. For a 4-7/8" steel stud wall, the 4-7/8" throat is a perfect fit. This makes the 5-3/4" stock jamb depth more versatile to meet customer needs.

For masonry walls, we need to know if we are butting the frame up against the wall or if the frames will be wrapping the masonry wall. Often, when butting frames up against the masonry walls, a stock size jamb depth of 5-3/4" is recommended since it is more readily available.

When wrapping a masonry wall such as brick, it is recommended to add 1-1/8" to the wall thickness instead of the typical 1" to arrive at our jamb depth. This allows for the slight variation in layered brick that can make the wall thickness slightly wider than the exact brick thickness. So, for a nominal 6" masonry block thickness, we would add 1-1/8" to the wall thickness to arrive at a jamb depth of 6-3/4" (5-5/8" true block dimension + 1-1/8" = 6-3/4").

Similarly, when matching the jamb depth to the masonry block and butting up against the masonry block instead of wrapping it, we would add 1/8" to the jamb depth to allow for the variation in layered brick. So, for a nominal 8" block that we want to match the wall thickness and butt up against the wall, the frame jamb depth would be 7-3/4" (7-5/8" true block dimension + 1/8" = 7-3/4").

Jamb depth is an important dimension to get right the first time. Coordinate with your project manager to ensure your wall details are read correctly and frames are produced accurately to meet the needs of each opening on your project.