It drives me crazy whenever I have to install a pre-hung door with a threshold in a garage with a sloped floor. I am working on one now that is a 32″ door and the floor slopes 5/8″ in the opening. In the past I’ve installed the door cockeyed in the opening when the slop was not so severe. Other times I’ve shimmed up the threshold to get the door square. I’m trying to find out the best way to shim the door and still get a solid foundation for the threshold. I would like some ideas on how others tackle this beast.
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We've used a mortar bed with 6 mil plastic on top to prevent the threshold from bonding to the fresh mortar. We mixed in a bit of glass fiber to hold things together on the thinner edge.
You could either lay the bed and then ease the door into it or hang the door and then pack it from the underside. I prefer to nest the door down on it as this insures that the thin edge actually has material under it. You'll find that either approach works easiest if you tape the plastic to the threshold and then cut off the excess after the fact.
Inside a garage I'm assuming that water isn't as much of an issue. Like the mortar idea, but haven't done it. Usually to adjust for a taper I've taken pressure treated and cut it at a taper to fit. So here, you'd have zero to 5/8 over 3 ft. But try to punch a hilti into the 0 end. If there's room, add a little so maybe you start at 3/8 and go to an inch. Caulk under and on top of, anchor securely.