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I installed prehung exterior doors today and I realized there was no pre-mortised latch for the door knob.

Not sure if I'm supposed to chisel out the strike side given there's this white air-seal band (the hole was a screw from the manufacturer that had a door plug) or screw the plate over this.

On the door side nothing is mortised either. Do I need to chisel that side too? Can I get away with chiselling only one side?

I'm interested in knowing what would be the usual best practice.

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Most latch sets are convertible--they'll come with the rectangular bolt plate installed, but it's easily removable and a ring is provided for simple drive-in fitment. That's the preferred installation these days.

With a Schlage, simply pry in the small gap between the trim plate and its backing plate and pop them apart. Then you can slip on the ring or switch to the plate with rounded corners, as needed.

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It's very odd that the strike wasn't bored and mortised. You'll usually see a rounded rectangle mortise and a bore for the bolt at both the knob and deadbolt positions. (It doesn't appear that you have a deadbolt, which is a security concern in my opinion. I'd only install an exterior door that has one.)

Go ahead and mortise for the strike. The latch plate will fit neatly between the weatherstripping and the jamb edge. Here's a typical install. Note that I've placed shims behind the latch bore for better appearance and stability.

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My strategy looks like this:

  1. Close the door and apply light pressure against the seal.

  2. Mark the interior vertical face of the latch bolt along its full height where it contacts the jamb.

  3. Place the strike plate on the jamb inside out (with the back face showing, but otherwise in the correct orientation) so that the marked line just shows in the opening. Be sure it's parallel with the jamb, then mark the screw hole positions.

  4. Pilot for the screw holes, a bit smaller than the screw shank. Jamb wood is very soft, so it can be undersized somewhat for a tight fit.

  5. Install the strike plate temporarily with the screws, again inside out. This allows it to lay flat to the wood.

  6. Carefully score the jamb with a utility knife around the strike plate. This will prevent the wood from splitting outside the mortise area and make it easier to get the right shape. Cut as deeply as the plate is thick, using multiple passes if necessary. Be sure to not slip and scar the jamb.

  7. Remove the plate, then mortise with a sharp chisel and light taps. The wood will split easily along the grain.

  8. Use a countersink bit or about a 3/8" standard drill bit to create dimples at the screw locations for the strike plate screw hole bumps on the back side.

  9. Place the plate and check fitment. Mortise and countersink further as needed.

  10. Mount the plate. Don't overtighten in the soft wood. Consider using 2" gold flathead screws instead of the short ones provided, especially if you don't have a deadbolt. These will penetrate framing and provide at least a little more break-in security.

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If you use a latch with a press-in fitting, you won't need to mortise the door for an edge plate. Though personally I think the edge plate solution is mechanically stronger

Yes, you need to drill and chisel to mount the strike. It should be possible to cut the mortise for the strike entirely inside the weather stripping, I'm told; I've only dealt with older doors where that was iffy.

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