May 8

DIY Air-tight Sump Hole Cover


The sump hole is original to the house. The drainage tiles originally diverted downspout run-off into the sump hole, where it was pumped out. When we moved into the house, a 1/3 horsepower pedestal style pump failed twice, once during a power outage, and once when the float bent and caught, keeping the unit from kicking on. After dealing with inches of water in the basement a second time, I purchased a Basement Watchdog unit from Lowe’s. I discovered, when I installed the new unit, that the old sump pump had kept the water level high enough that the tiles were filled with water.  This meant that the tiles were always full of water: hundreds of gallons of water was always hanging out, and the difference between a flooded basement and an unflooded basement was only a few gallons. The new pump kept the water down to just inches. from the bottom of the hole. So this was the first advantage to the new pump.

Sump Hole Without Cover
Sump hole without cover.

Now the downside, literally. With the water level so low, air was flowing through the tiles from outside the house (remember, the downspouts used to run into the tiles; although they don’t any more, they are only covered with grates to keep things, but not air, out). Over the last couple years after heavy rains the basement begins to smell. You can tell me: have your septic inspected; something is leaking. And you would be right. But here’s the problem: the town is installing a sewer system and every septic tank must be destroyed before the end of the summer. It would be a waste of money to dig up and fix a septic tank whose time has come. After reviewing a number of sites that discuss the advantages of their air-tight sump hole covers, I decided to make my own.

I chose to make a drain with a trap to allow the water to go down while preventing the air from coming up. Some covers I looked at on the internet used a ball and funnel type design, where water would lift the ball  as it drained, then the ball would create the seal. That seemed ingenious, except for one thing: all plumbing in houses uses traps to stop odors, so why not use the same concept here?

Here are some additional advantages to an air-tight sump hole cover: It’s supposed to stop Radon infiltration, it keeps moisture contained within the sump hole instead of spread throughout the basement, and it keeps the air that was blasting through the tiles from the outside from penetrating into the house.

Material List

  • 10 foot long 1″ x 4″ PVC trim board (cost around $17)
  • 1 PVC Trap (1 ½”)
  • 1 PVC Reducer (from 3″ to 1 ½”)
  • PVC Glue
  • Silicon Caulk
  • Assorted scrap 1 ½” PVC

Total Project Cost: Less than $50


First, let’s take a look at the original cover. It was constructed from assorted pieces of 2x treated lumber. It was not sealed and was constantly wet since water drained onto it. BUT, it kept my children from falling into it.

Original Sump Cover
Original sump cover made from treated wood.
  1. I decided that like the original wood cover, the PVC cover should run from the north to the south (left to right in the picture). Any cut-outs needed to be set into a side of the trim board rather than cutting the board in two. Using the existing cover as a template I cut the PVC trim board to width and used a jigsaw to make rough openings for the sump pump.
  2. I traced the PVC reducer and used a jigsaw to cut an opening for it. I actually meant to have it on the other side, but due to a mix-up in what is up and what is down, cut it on the opposite side I meant. Since the location of the drain was nothing more than a preference, I didn’t worry about it. If I had cut the hole for the sump pump itself, however, that would have been another story…Side view of the trap and underside of cover
  3. Edge glue the trim boards and put the trap together with glue. I glued two pieces with the idea that I would seal the remaining edges with caulk so I could remove it as needed.

    Underside of Sump Cover
    Underside of sump cover, including PVD drain with trap.
  4. Used caulk to line the perimeter of the sump hole as well as the edges where the two pieces would meet. Set both pieces in the sump hole.
  5. Ran another bead of caulk around the perimeter, smoothing the edges to seal.
  6. Cut additional pieces to stair-step and glue to form tight fit around the PVC from the sump pump.
  7. Caulked the remaining gaps.
  8. I then bought assorted PVC to re-route any drains (in this case, the furnace and the reverse-osmosis units) to run directly into the drain. The remaining PVC line that doesn’t make it to the drain in the photos is from the softener, which isn’t running at the moment. When I have it running it will also extend to the drain.
Finished Sump Cover
Finished Sump Cover
Finished sump cover detail
Finished sump cover detail

Final Thoughts

Since the cover was installed there has been no smell in the basement. The cover may also help the humidity, and if so, that will mean running the dehumidifier less, which will be a cost savings also.

Disclaimer: This page is not intended to replace the advice of a professional. It simply relates what I did. And what I did may be wrong. That’s one of the ways I learn, by trying things and discovering that I did something wrong. I’m a lot smarter now than I was five years ago, and I hope I’ll be even smarter five years from now.

Update July, 2015:
I wrote a post detailing modifications/improvements to the sump cover described in this original article. View the new post by visiting:
DIY Air-tight Sump Hole Cover Redux.


  1. By Mark Sherrill on

    Hey, thanks for taking the time to post this. I thought the trap idea was great. I ‘ve had one of those ball-float drains for several years and recently realized it doesn’t work. I think the suction from my radon mitigation system is enough to prevent the ball from floating up. In your design, you have a constant supply of water to keep the trap full. I don’t have that. I’m thinking I will extend your idea for to fit my situation… I’m thinking of putting a J-shaped pvc pipe that is long enough to reach to the bottom of the pit. That way, when the water level rises a few inches (due to rain), the J-shaped trap will fill up with water before the sump pump kicks in. That should keep the trap from drying out. After the sump pump runs, there should still be water in the J-shaped pipe.

  2. By Hillie on

    I really enjoyed your article. I want to do something similar. I was thinking I might use a waterless drain so that I don’t have the problem Mark mentions. Also nice website. I echo your thoughts on Lord of the Rings.

    1. By Andy (Post author) on

      Thank you for the comment and the compliments. I’d love to hear what you come up with for a sump cover and how it works for you.


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