Replacing a Threshhold

Pity the poor wooden threshold. That stomped-on, scuffed-up, taken-for-granted plank bakes in the sun, gets soaked by the rain, and endures the grit from every shoe that tromps in or out of the house. Without regular attention—an occasional lick of paint and some caulking—sooner or later that wood's a goner, and the underlying framework isn't far behind.

At the current This Old House TV project, now airing on PBS, the back-porch threshold had clearly suffered from years of abuse and neg­lect. You could tell at a glance that the wood's splintery, ­un­painted surface and deep length­wise cracks were beyond the help of a coat of paint or even epoxy. Worse, it flexed when stepped on, a clear sign it had to go. "A threshold should feel solid," says TOH general contractor Tom Silva. "But sometimes it's just resting on rot."

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. It took Tom a few hours, but when he was done and the new threshold was in, the handsome oak strip brightened up the entire doorway, looking not at all like something to take for granted. Here's how you can do the same at your house.

1. Before you start
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.

2. Cut out the old one
Fit a wood-cutting blade, at least as long as the old threshold is deep, into a reciprocating saw. Then 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.

3. Remove the pieces
Stick a flatbar 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.

4. Install sill flashing
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.

5. Lay out the notch
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.

TOH Tip
When replacing a threshold, 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.

6. Cut out the notch
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 above. Trim the horns flush with the edges of the casing. Sand the threshold with 120-grit paper and spray it with borate.

7. Coax it in
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.

8. Foam It
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.

Buying a Threshold
Originally, most entry thresh­olds on old houses were milled from a solid plank of oak. These days, the stock replacement thresholds in lumberyards and home centers are usually laminated from oak strips. That's fine with Tom Silva. "Laminated stock is often stronger than solid stock," he says. "And if the wood color and grain of the laminations are closely matched, you can use a clear finish." If you can't find the profile you need at a home center, check with a shop that does custom millwork

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