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Community and Q&A

Threshold Extension for Exterior Door in Extra-Thick Wall

artisanfarms | Posted in General Questions on

The house I am renovating will end up with 11″ thick walls (1/2″drywall, 5 1/2″ studs, 1/2″ sheathing, 2 1/2″ polyiso, 1″ rain screen, 1″ board siding).

Are threshold extensions available that I can fit underneath an existing threshold on a pre-hung exterior door? Or, am I better off making my own threshold?

I looked in the archives and found a similar question from 2013 and it didn’t seem that at that time there was anything commercially available.

I have a press brake and can fabricate an extension if needed and also have a woodshop, so I can easily make a hardwood threshold, but would like to know what might be available commercially before going the homegrown route.

For appearance sake in the house, I need the doors to be in-swing and to mount on the inside edge of the door jambs.  From the interior, the appearance needs to be as conventional as possible.  I will probably add a storm door on the exterior to manage the appearance on that side.

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  1. Expert Member


    One other alternative, which does come with an energy penalty, is to reduce the depth of the walls at the door, and perhaps six inches each side. This removes the problems of sealing the difficult intersection between the sill-extension and whatever you are using to extend the jambs, and providing support for the deep sill.

    1. artisanfarms | | #2

      That is an idea worth considering for the front door as the door is on a porch and I can see some ways to integrate the transition into the facade. I'm not sure about the back though.

      1. Expert Member
        MALCOLM TAYLOR | | #3

        Would it work if you kept the same amount of thinner wall at the head too?

        1. artisanfarms | | #7


  2. Expert Member
    Michael Maines | | #4

    Andy, I wrote about some of my approaches here: The most straightforward is to simply order doors with thick jambs and sill extensions. Not all door shops/distributors will do them, but some will. If you are stuck working with 6 9/16" jambs, Pemko makes sill extensions. Depending on your details you might have to get creative, but it sounds like some minor metalworking is within your skill set.

    Here is a sill extender:

    Here are ramps, which works better in some cases:

    1. Jon_Lawrence | | #6


      Thanks for the link. I have a slider in the rear of my house that really needs a metal sill extension and it looks like this will work well.

  3. Jon_Lawrence | | #5


    I ran into the same issue and my door supplier did not have a solution. After doing a lot of research and some help from Malcolm, I came up with something that seems to have worked well. The door jambs were pretty deep, but we still set the face of the door jamb 1 ½” proud of the Zip sheathing. We added 2x4 furring around the door to give us additional nailers to secure the door. I have 3” of Roxul on the exterior, but I was only able to cover the nailers with an 1 1/2" layer of the Roxul which was a trade-off I was willing to make. The sill is ½” proud of the jamb so 2” proud of the Zip. The concrete footings for the front steps stopped at the top of the foundation wall. This allowed me to cover the Zip in the rimboard area with a layer of butyl tape and 2” of foam board on top. I notched a 1”x1” portion of the foam board where it meets the sill to allow me to recess the cumaru sill extension. I used cumaru for the sill because of its durability and I used it in the portico ceiling above the door so it pairs well (as you can see by the picture I need to oil it). Supposedly it is FSC certified. Underneath the cumaru is a 5/4 piece of pressure treated decking that is covered by ¾” of Azek. We then poured the landing and finished it with paver stones. This way none of the masonry touches the sheathing.

    I also have a French patio door on the front exposure (opens inward) that I had the same problem. We used aluminum flashing to make the sill extension and it works fine – but only because it will not see any foot traffic. Typical aluminum flashing will get beat up pretty quickly if used as a sill. I think if you go the metal route you will need to get something more durable like what Michael posted.

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