How To Replace A Door Threshold In 9 Steps

It’s best to start replacement work early in the day, so there’s time to fix any problems that come to light after the old threshold is out. A rotten subsill, punky joist ends, or a termite-riddled rim joist can stretch this two-hour job into a full day’s labor. Here’s a step-by-step tutorial on how to install a door threshold.

How to Replace a Door Threshold in 9 Steps

Step 1: Know the Parts That Make Up the Exterior Door Threshold

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Step 2: Prep With Replacement at the Ready

Measure the length, width, and thickness of the old threshold, and have a replacement with a similar profile on hand. It should be an inch or two longer than the distance between the door casing’s outside edges so you can trim it for an exact fit later on. Remove the storm door and any weatherstripping attached to the threshold. Tip: You’ll spend a lot of time kneeling on the outside of the house. Place rubber mats on the porch, as Tom did, or invest in a good pair of knee pads.

Step 3: Remove the Old Threshold

Fit a wood-cutting blade, at least as long as the old threshold is deep, into a reciprocating saw. Slice across the wood in two places, about 10 inches from each jamb. Hold the blade so its teeth are nearly parallel to the surface, and watch its tip so you don’t cut the flooring. Stop sawing as soon as you feel the blade cut through the threshold. Wait until the blade stops moving before lifting it out of the kerf.

Step 4: Pry Out Exterior Board & Treat

Stick a flat bar (aka pry bar) between the threshold and the subsill and pry up the center section. Wiggle the ends away from the jambs, if you can, or split them with a hammer and chisel and take them out piece by piece. Use patience and finesse, not brute force; you don’t want to damage the ends of the door stops. Pry off the toekick (the exterior board below the threshold), and probe the wood underneath for signs of decay. Replace any rotten wood you find, and spray it all with a borate solution like Bora-Care to prevent rot and repel termites.

Step 5: Install the Rubber Seal

A layer of flashing on the subsill prevents water from reaching the framing below. Tom uses Vycor Plus, a rubbery, self-adhering membrane that he cuts to fit between the jambs and a few inches wider than the subsill. The flashing goes over the subsill, adhesive side down, with just enough overhang in front to cover the top edge of the toekick. After smoothing out the membrane, Tom grabs the edge closest to the inside of the house and rolls it over onto itself, forming a small dam against water infiltration.

Step 6: Lay Out the Notch for the New Threshold

The ends of the new threshold have to be notched to fit around both door jambs. This creates a “horn” that extends under the casing. First, measure the width of the right-hand casing, then hook the tape on the threshold’s back right-hand corner and transfer that measurement to the threshold’s back edge. Next, measure the width of the jamb and stop, hook the tape on the same corner, and transfer the measurement to the end of the stock. Use a square to draw a perpendicular line from each mark until both lines intersect.

Step 7: Cut Out the Notch By Measuring Jamb to Jamb

Cut along the layout lines with a jigsaw or circular saw; finish circular-saw cuts with a handsaw. To mark the cuts for the left-hand notch, measure the width of the opening from jamb to jamb, then hook the tape on the cut-out notch and transfer that measurement to the threshold’s back edge. Mark the width of the jamb and stop on the threshold’s left end, join the marks with a square, and cut out the notch, as shown. Trim the horns flush with the edges of the casing. Sand the threshold with 120-grit paper and spray it with borate.

Step 8: Install the Threshold

Slip the notched threshold into position under the door stops, then nudge it into place by tapping a hammer on a wood block against the threshold’s edge. Change the block’s position so neither end of the threshold gets too far ahead of the other. When it’s tight to the subfloor, slide pairs of shims—ones cut from cedar shingles are best—under the center and both ends of the threshold, but not under the horns. Arrange the shims so the thin end of one rests on the fat end of the other. That way, as you tap on the fat end, the threshold will lift without tilting. When the threshold is tight against the door stops and casing, snap the shim ends off flush with the subsill.

Step 9: Seal with Foam

Squirt expanding foam sealant under the threshold to hold it in place and plug any air gaps. Immediately put the toekick up under the flashing and tight against the underside of the threshold, and screw it to the subsill. Run a bead of sealant between the threshold and each door stop. Finally, protect the wood with deck paint or several coats of spar varnish.