Jamb Saw - Undercut Doors, Casing & Trim
A jamb saw is a mechanical gizmo designed specifically for removing the bottom of door casings so flooring can slide under, thus providing a nice clean appearance after the floor is installed. It is used predominantly for laminate, wood and cork flooring installations. Depending on the skill of the installer you can also find the work done with stone and tile. Jamb saws also come in a variety of shapes. The particular one we are using works ideally for larger 
sized plinth blocks, extremely thick casings or casings that may be squeezed into a corner requiring a deeper than usual cut.
How They Cut Through Door Trim Moldings
For those unfamiliar, door casings (seen below) are the decorative trim molding that wraps around the drywall or the woodwork the door is hinged to. Powered jamb saws have a depth setting where you can adjust for any flooring thickness. In our example (right) we're handling a thinner floor type whose thickness is 3/8 of one inch. If the flooring requires an underlayment (common with floating floors in this case) it is vital to place that under a scrap piece of flooring reflecting the actual vertical height of the finished floor.
Dull Blades Splinter Woodwork
Next step is to remove all materials and have at it with the door casing. Slow down Millie, don't take it too fast! This is a very important part of flooring installations. Cosmetics such as this are likely to be seen by many people. First check to see if the blade is sharp. A dull blade will splinter door casings and possibly throw off heat/friction that can show minor burn or burr marks on the woodwork. You're not ready yet. How about that eye protection? Depending on how the trim was installed you never know where the trim carpenter placed his nails.
For thinner flooring 3/8" and under I have rarely run into any nails but getting into 3/4" hardwood there will be times a nail or two that cannot be avoided. On occasion small bits of metal may fly if you hit them dead on. Don't be concerned; a sharp blade and slow cutting will slice through it well, but a firm grasp on the tool is suggested. I have found cheaper grades of pine trim will have a tendency to split or chunks may come loose when a nail is hit.
Check Cutting Height
It's best to cut left to right with a jamb saw if your right handed and the reverse for lefties. Prior to this procedure I would take a small nip from the baseboard (it will get covered anyway) just to double check that your blade setting is correct. It's a bummer to find you may have cut 25 door casings in the entire house only to find out you've cut it too high, leaving an unsightly gap. Worse yet is cutting it too low if you happen to be using a hand jamb saw.
 |
 |
Many may be wondering; why did he cut that far into the baseboard? This allows for extra space when installing the flooring under the casing. Basically it frees up areas for expansion and contraction all floating floors require. It also allows for an easier installation of quarter round trim once the excess is chiseled away. You'll get the idea with the pictures below.
Getting the idea? The way we've cut the flooring around the door casing leaves unobstructed room for the floor to expand and contract while making it easier for the quarter round trim to cover the gap up to the outside of the actual casing.
 |
 |
Locating Jamb Saws?
Nowadays powered jamb saws can be rented for about twenty dollars per day from most major tool rental centers. If you’re faced with just a few door casings one could always buy the hand saw that sells for around $15.00 online. Shown is a QEP product many professionals use for smaller jobs.
Why Not Use A Multifunction Tool?
Some guys swear by them, but my view is it doesn’t get the fine cut a balanced power jamb saw is capable of. And if you should happen to make a cut that’s not high enough, it’s murder (without butchering the casing) trying to shave off a sixteenth of an inch or so with the multifunction.
