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Building Science

Don’t Let This Happen to You

Omitting the sill pan under an exterior door is risky

Doors leak.
Image Credit: Mike Maines

Door design details

The photo at right is from an entry that’s just 15 years old. Fortunately, it was able to be repaired. I haven’t always been so lucky. Let’s just say that replacing subfloor and framing is no fun. A safe assumption is that, for one reason or another, doors always leak. They shouldn’t, but they do. Seals wear out. Wind blows. Jambs rot. Sills crack. Weepholes clog. Following are some ways to mitigate the chance of damage.

  • Always use a sill pan. I used to make my own out of lead, a practice (and a material) I would not recommend. Then I used custom-soldered copper pans, but only on the most exposed locations, due to cost. Less-exposed locations received pans made from bituminous membrane. It is imperative that the corners are sealed, which is hard to do. Simply adhering the membrane to the subfloor without creating sealed, upturned corners only relocates the water. Now it’s easy: I specify one of the ready-made sill pans. It is important, however, to seal the sill pan to the subfloor and the door sill to the pan to prevent air leakage and flexing of the door sill.
  • Choose the right sill type. When it’s important to be historically correct, I call for real wood sills, protected with epoxy paint or a penetrating sealer such as Penofin or Sikkens. Otherwise I prefer adjustable aluminum or fiberglass sills. They are usually available in brown or tan, if mill finished aluminum is not desired. Turning a few screws is enough to snug the sill against the door to stop water and air from getting through. Where the side jamb meets the sill there is usually a big dollop of butyl caulking. Don’t cut it away the way I did as a novice carpenter—it is necessary for sealing the transition between components. I like to order doors with “long horns” on the sills, meaning that the sill extends to the outside edge of the casing instead of the inside edge, where a vulnerable corner is susceptible to moisture. It takes more care to work with and is usually a special order, but I’ve repaired enough doors to consider it a worthwhile detail.
  • Use rot-proof jambs. An added benefit to an adjustable sill’s air-sealing capability is the fact that it’s rot-proof. Rot usually starts at the bottom of the jambs. The end grain wicks water and rots the wood from the inside out. It shows up as paint failure after it’s too late to fix. This can be minimized by priming all sides of the jamb and leaving the caulking at the sill joint. Some manufacturers offer an even better alternative: rot-proof lower jambs. While as a rule I don’t consider pvc or composites to be green, this is one case where it solves a real problem.
  • Storm doors are often discouraged because excessive heat buildup can damage the doors they are meant to protect, and if you live in a warm climate, you wouldn’t want them anyway. Anywhere that’s cold part of the year, though, I would still like to use them. They are a first line of defense, the same way siding takes the brunt of the weather but has housewrap behind it. Plus, in warm weather the storm panel can be swapped out for a screen panel, allowing for natural ventilation.
  • Overhangs should protect every door. I know it’s not always possible, but a porch roof or some other form of protection is a good goal. Not only does an overhang keep rain from hitting the door on its way down, it also minimizes damaging splashback. An overhang also keeps the summer sun from baking south-facing doors. My favorite reason for overhangs: They offer a protected location to ease the transition between indoors and out.


  1. jimblodgett | | #1

    Flex Wrap
    At $2.00 per linear foot, DuPont's "Flex Wrap" isn't cheap. But it's available at several local lumber yards here in the Great Northwest. We keep a box of it handy and use it for door pans and in the lower corners of window openings.

    I'd like to try plastic door pans, but until the local yards stock them, we'll be using Flex Wrap. Good stuff.

  2. Expert Member
    Michael Maines | | #2

    Flex Wrap
    Jim, how do you treat the inside edge of the flex wrap? The first time I'd used flex wrap as a sill pan was under the windows on job pictured above. I was impressed how easy it was to form the outside corners. As with Vycor or Ice & Water Shield though, I find it difficult to get the guys to create a sealed dam on the inside. Without a dam, leaks are just redirected to the edges of the pan.

    Good point about availability. Our regular yard stocks one brand of sill pan, but I have found it best to order pans when ordering the windows and doors. They are kind of a pain to deal with, but so is repairing rotten floors!

  3. jimblodgett | | #3

    Back Dams
    I've read a lot of accounts of people using various materials for back dams - Mike Guretin was the first I read who recommended a piece of beveled siding to creat a slope towards the exterior on windows.

    Then I think it was in one of the Building Science books I read to put a strip of wood, maybe 1/2" tall, paralell to the window then Vycor over that. I guess that would work okay on a window, but in the case of a door, how do you support the rest of the sill?

    And if you say same thickness boards, perpendicular to the back dam (thereby supporting the sill and providing an escape route for water) but on top of the Flex Wrap, I gotta ask how you keep the critters,cold north wind and our old friend wind driven rain out from under that area under the sill.

    And besides, I always run a few healthy beads of silicone under the door sill. If I back dam the water from flowing into the house, how's it going to get past half a tube of silicone?

    I can't see a good method to back dam under doors, Mike. Windows, maybe, but doors? I just don't see it.

    What do you recommend? I'd love to find a better way.

    Wait. I think I get it. Do those plastic door pans allow for drainage and still provide adequate support?

    I think I've just been led to the slaughter like a curly haired little sheep.

  4. Expert Member
    Michael Maines | | #4

    Back dams
    Mike G. explains his method here:

    But you're right--what about doors?

    In my experience they are much more prone to leaking than windows, and you can't use beveled siding or a back dam. What I do when using a flexible membrane is adhere it to the subfloor below the door sill, but leave another couple inches of membrane beyond the sill with the paper on. After the door is installed, peel the paper off and stick the membrane to itself, then fold THAT piece up and staple it to the door sill. At the same time, make hospital corners at the sides of the membrane pan. The doubled layer of membrane becomes its own back dam.

    It's hard to explain, and just as hard to do, which is why I like the manufactured pans!

    You caulk the pan to the subfloor, but then you have a decision to make--a perfect seal between door sill and sill pan, or water drainage? I split the difference--caulk the high points, but leave a few gaps for drainage.

  5. jimblodgett | | #5

    Door pans
    Yeah, most exterior doors are in a 38" opening anyway. I could probably find a source online and buy a dozen to keep on hand until my local yard starts carrying them. What brand do you use? What would you use if given a choice?

  6. Expert Member
    Michael Maines | | #6

    Good idea
    Jim, our primary yard stocks these: I have only used the brown ones with the backdam.

    I've also used They are ok but their drainage system isn't great. looks pretty similar to Jamsill, but I haven't used them.

    A couple other options are and

    Now that I've ranted on about backdams, it looks like several manufacturers are offering sloped pans with drainage slots and no backdam. It appears they would solve your concern about sealing (check out Marvin's instructions), and they probably work well. I still like the idea of a dam though.

  7. jimblodgett | | #7

    thanks Mike
    Thanks for the links, Mike. Sure seems like there has to be a better way, though, doesn't it?

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