Split Jamb vs Flat Jamb: What Are The Main Differences?

Many people don’t pay much attention to the selection of the door jamb. However, the quality, design, and durability of this element play a crucial role in the overall security of your home, as they ensure that the doors are stable and reliable. 

However, it isn’t only the quality of the door jamb that makes a difference in this selection. There is also an important distinction between a split jamb and a flat jamb for your door. What is the difference between these two types of door jambs, and which one is better?

You’ll find answers to all these questions if you keep on reading!

What is a door jamb?

Before we introduce either of these types, it is necessary to define the role of a door jamb and explain what it is. While many people confuse this door element with the door casing, there is a significant difference between the two.

While the door casing is the visible, decorative part of the door, the door jamb isn’t entirely visible, so it is more of a structural element. The door jamb consists of vertical frame elements which secure the door on both sides.

Some people also refer to door jamb as door leg, and it consists of a latch recess and strike plates on one side and hinges for mounting on the other side. It is mostly made of wood, but it can also be made of fiberglass in some cases.

The quality and endurance of your door jamb make all the difference when installing a door. This structural element is supposed to be durable and reliable enough to hold the weight of the door and ensure that the locking system is working impeccably.

Therefore, it is necessary to look for durable, quality materials in your door jamb, but also to ensure it is properly installed. Otherwise, both the durability and the security of your door may be seriously compromised.

When mounting the door jamb, make sure it fits properly, and there are no gaps. If all the panels are adequately positioned, there should be no gaps or cracks. It is safe to say that the door jamb is the most important part of the door frame, so you should invest in it.

What is a split jamb?

Now that we fully understand the role and the importance of a door jamb, we can move on to its different types, and the pros and cons of each. This way, you can make the right decision for your home and make the selection process much easier

You will choose a split jamb door if the rough frame of the interior wall is significantly thicker than a standard opening. In this case, it is impossible to install a regular size flat jamb pre-hung door, as it is impossible for it to fit.

Therefore, the solution is a split jamb, since it consists of two split parts, as the name itself implies. The two parts are separated along the length of the jamb, allowing the wall to fit in between the two jamb parts.

How to install a split jamb door

The first step to installing a split jamb door frame is to determine the direction of the door swing, either left or right, depending on your personal preferences or other factors in your home. If your right hand is your dominant hand, you usually want the door to swing to the right.

Also, when installing split jamb interior doors, it is important to align the unhinged side and the hinge-stop side in the opening track. 

Since the casing comes with a split jamb door, all you need to do is secure it with a nail at the top. Ensure that the threshold of the door is entirely narrow before aligning the opening.

The side of the opening with the hinge is supposed to be plumb, whereas the opening corners are supposed to be square in order for the split jamb door to function properly. It is recommended to have some assistance during this process as the door can be heavy:

  1. It is important to position the door directly into the center of the opening. The easiest way to do so is to position the bottom of the door first, and then align the top part accordingly. If you want to paint the door after installing it, it is necessary to prepare it using some moisture penetrant.
  2. Next, you want to proceed to install the shims to the jamb, positioning them where the hinges are. For this task, it is best to use 8D finishing nails, but before securing them, it is necessary to make sure that the door is still perfectly aligned and positioned. 
  3. The following step would be to secure the door frame, which you’ll also need 8D finishing nails for. Place the nails through both the jam and the shim, starting six inches below the top part of the frame. Of course, as you’re doing this, make sure the door is square.
  4. The nails should go all the way around the frame, reaching the point of about six inches above the bottom of the door frame. This step should also be repeated on the lock side. Also, remember to secure each nail you’ve added with a shim. 
  5. The final step would be to position the second part of the frame, adding nails through the jamb stop, and securing it to the wall using side-by-side nails all the way from the top to the bottom. 

What is a flat jamb?

The main difference between a split door jamb and a flat door jamb is that a flat jamb is considered to be standard. Therefore, a flat jamb will be 4 9/16″ wide, and it will include a separate door stop.

Also, in this case, the hinge side of the jamb will actually be the pre-cased side, and the casing will come with corrugated fasteners securing the miters. While split jambs are meant for more complicated situations, flat jambs are made for standard door openings and frames.

If the door opening is matching the door frame, and the dimensions are standard, there will usually be no need for a split jamb and a flat jamb will do just fine. Also, flat jambs are considered to be of higher quality, more durable, and secure than split jambs. 

How to install a flat jamb door

Before starting this project, it is necessary to measure everything: the door opening you’re working with, the door frame, and the door jamb. It is necessary for all of these to align before you can start installing the door.

Now that you’ve planned everything in detail, it is time to cut the plinth blocks, any decorative door moldings you may have, as well as the door jambs. When cutting these pieces, make sure you’re as precise as possible and the cuts are straight. 

If you don’t cut the pieces straight, there will definitely be some problems in installing them, and even if you succeed, it is probable that the door won’t function and lock properly, which is the last thing you want.

While the door jambs and the plinth blocks can be cut with a table saw, for any decorative moldings, you may need to use a miter to get the cleanest cut. Once you’ve cut and prepared everything, you can proceed to the door installation phase:

  1. In order for everything to fit correctly, ensure that the framing is stable and level. Now it is time to add the side jamb on the right side of the doorway vertically. Secure the jamb into the frame using 2-inch finishing nails, then repeat the same steps on the other side. 
  2. Next, you will position the plinth block, which is an important structural element of the door jamb. Secure the plinth blocks to the bottom part of the casing trim using nails, and then use the nails to secure the casing trim on the outside. 
  3. Ensure that the doorway and the top casing are aligned before using nails to secure everything. Of course, as you’re completing all of these steps, it is necessary to check that everything is level and aligned. 
  4. Finally, the last step would be to position the decorative pieces and secure them around the door using nails. In case there are some gaps or cracks, it is necessary to use caulk or wood filler, depending on the material of the decorative pieces.
  5. Once you’ve hung up the decorative pieces and secured them using nails, caulk, and/or wood filler, you can check for any imperfections and finish off by sanding and painting over the pieces to get a clean, polished final look.

What is the difference between a split jamb and a flat jamb?

The main difference between a split jamb door and a flat jamb door would be that they’re simply used in different situations. You will usually resort to split jamb in more complicated situations when the door opening and/or the door frame isn’t standard.

Flat jamb, on the other hand, has standard dimensions, and it is meant for regular door openings and frames and matches these standards. This is the first choice that comes to mind if there isn’t a noticeable problem with the door opening or frame. 

Also, flat jambs are considered to be a quality, durable, reliable option as opposed to split jambs, offering more security. Flat jambs can support more weight and are generally considered to be a premium choice.

Having that in mind, it is no wonder flat jambs are usually more expensive than split jambs. Therefore, if you’re installing a heavy-duty, massive door, you will certainly gravitate more towards the more expensive, but also the more reliable option which is a flat jamb.

Nevertheless, if you have to use a split jamb, or you simply choose to do so, there are some good ones on the market – you just need to look for durable, high-quality materials and ensure that the installation process is flawless. 

As we’ve explained, split jambs can work great in tricky situations, especially when the rough frame of the door is wider than the standard opening, which is the case with many homes. 

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