How To Install A Door On An Unlevel Floor 

If you’ve installed more than a handful of exterior doors, you’ve probably run into a situation where the wood subfloor or concrete is way out of level. What do you do? Creating a bunch different sized shims is not a good idea because that creates voids under the threshold. Forming one large shim that spans the whole opening is doable if you have a band saw on hand, but that’s not often the case.

Here’s the best solution I’ve come across. Eric Klein, the window and door installation instructor’s instructor, passed along this ingenious nugget. Cut down strips of self-adhering roofing underlayment (aka ice and water). Eric prefers WinterGuard made by CertainTeed. Add layers of the underlayment to the floor, each one smaller than the next, until the low side has enough layers to level things out.

Keep the lengths of the strips spaced evenly to achieve a straight incline. The number of layers you need will ultimately depend on how far the floor is out of whack. The underlayment is pliable enough that the door should press into it and create a tight seal. Of course, you will still need to add sealant in the pattern that the manufacturer requires. Check out the video below.

Thanks for the tip, Eric!