Door Jambs & Casings. What is the difference?

What is a door jamb? This is a very simple, innocuous question with a more complicated answer than expected. The simple answer is that the door jamb is the backbone of your door; without a jamb it is merely a motionless slab of wood, fiberglass or metal. Your door jamb, made up of two legs and a head, gives the door its structural integrity. When installed correctly and level, the door frame ensures that the door opens and closes smoothly.

That seemed like a pretty simple answer, what is so complicated about that? As all door jambs play important roles in security and functionality of the door, it is also important to understand the parts of the jamb, materials, interior vs. exterior differences, even design integration customization with drywall. Learn about these below as we explore differences between jambs and casing, how to install a door, interior vs. exterior jambs, and customizing jambs for interior design.

What is the difference between door jamb and door casing?

That is a great question! As stated above, the door jamb is the structural backbone of the door system. While the door jamb has a functional purpose, the casing is purely decorative. When the door is installed between the studs of a wall, there can be a small gap between the stud and jamb. Also, drywall and other wall coverings can have rough cut edges that are hard to keep looking clean, which causes this small gap to look messy. The casing decoratively covers that gap so that it looks seamless, enhancing the door and surrounding wall. Where the jamb is what keeps the door upright and closed with its connections through hinges, strike plate and screws, the casing is merely applied with thin nails, touched up and sealed with paint or stain.

What are the common types and sizes of a door jamb?

The two most common types of door jambs are interior and exterior. Exterior jambs are generally larger than interior jambs because exterior walls usually include more items such as insulation and weatherproofing that interior walls do not. Jamb sizes can vary greatly; on average they range from 4” to 7"

The most common size and style for an interior jamb is around 4-1/2” double rabbeted jamb (rabbet is the recess that allows for a door stop) that allows to cover a 2x4 stud with basic drywall on either side of the stud. The most common size and style for an exterior jamb is around 6-1/2” single rabbet with a kerf (kerf is a slit for the weather strip to attach) that allows to cover a 2x6 stud drywall and sheathing and siding.

Can you customize a jamb to accommodate design specifications? Absolutely! We see creative design specifications where casing is eliminated, which requires more labor and craftsmanship to cover the gap between the jamb to the stud and drywall. For standard jambs and millwork, we are happy to provide from local vendors, like Gascoigne, and national manufacturers, like Metrie. However, at Interbay Door, we also have relationships with custom lumber and woodworking companies in Washington that can craft custom jambs for most any job.

Why do I need casing?

Shown below is an example of gaps between a door jamb and the surrounding drywall. Similar to base trim that covers the small gaps between the floor and the wall materials, casing helps to cover gaps between the jamb and the wall.

A door needs to be level to operate well, however it is much harder than expected to have perfectly leveled studs and headers in a wall. To make a door level, installers will use shims on the sides and the top between the jamb and the studs. This will cause the gaps between the jamb and drywall to vary greatly depending on how straight and level the rough opening is to begin with.

Once the jamb frame is level, they will connect the jamb to the studs with screws and shave down the shims. The screws are often hidden behind the weather stripping for an exterior door or covered with spackle for an interior door.

Can you do without casing? Yes! It usually takes a custom jamb with a kerf (or slit) where you can slide in a metal edge piece that with help to cover the gap. This creates the illusion of a clean edge that can be plastered and painted like the drywall. However, as mentioned in jamb styles, this process is much more labor intensive.

What materials do jambs and casing come in?

For inside a home, a jamb is recommended to be made of either primed finger jointed wood, or “PFJ”, or it can come in a variety of different types of hardwood PFJ needs to be painted for a uniform look, but hardwoods could be painted or stained. Hardwood jambs, as they are made from a single piece of wood, can be limited in size due to the size of the tree itself. However, finger jointed wood, as seen below, has more range in size because it can be made from multiple pieces that are fitted together. Is hardwood stronger than PFJ? That is a common misconception, and the answer is not necessarily. Although there are indeed stronger wood species that can be made into jambs, like white oak, in comparison to what they use for PFJ, cracks and splits can happen naturally in wood which is not a common problem found in PFJ. While putting together PFJ jambs, those cracked portions, knots, and other irregularities are eliminated and only use strong wood.

While hardwood and PFJ jambs can be used for exterior doors as well, in the case that an exterior door has a short overhang or not much cover it is recommended to use something called Weather Guard instead. Weather Guard is a poly-fiber doorframe system that has resistance to moisture, rot and insects.  

While casing, on the other hand, does not need structural integrity or protection from insects, it does not need to be made of a poly-fiber. Although, it is recommended to use wood or PFJ casing and trim in bathrooms due to moisture, casing is also available in the more economical MDF that often offers more decorative, endless options.

If you are in the Greater Seattle area, we want to know how we can help you make the right decision when it comes to jamb, casing and trim needs. We have been providing quality doors, hardware and millwork to the Greater Seattle area since 1990 and are ready to help you start your next project. Give us a call at Interbay Door today!

425-485-6404

info@interbaydoor.com

The Finished Space, “What is a door jamb? Understanding door jamb basics.” Metrie, June 11, 2024. What is a Door Jamb? Understanding Door Jamb Basics

Bolivar, Marisa. “What is a Door Jamb? Different types and extending tips.” Designingideas.com, September 9, 2024. What Is A Door Jamb? Different Types & Extending Tips