How to fix a leak in a bay window

Written by steven white
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How to fix a leak in a bay window
Bay widow (bay windows image by Aaron Kohr from

Leaky bay windows can be a big problem for homeowners. Not only can water leak into the house, but it also can seep between the frame and the wall, eventually leading to dry rot and potential mould. Thankfully, fixing a leak in a bay window is a straightforward task that can make a great weekend do-it-yourself project.

Skill level:

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Things you need

  • Screwdriver
  • Paint scraper
  • Goggles
  • Gloves
  • Mask
  • Acetone
  • Rag
  • Pencil
  • White butyl rubber seal
  • Putty knife

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  1. 1

    Unscrew the window and frame's fastening screws and slide the window out of the window hull.

  2. 2

    Scrape off as much of the mastic seal as possible using a paint scraper.

  3. 3

    Remove the rest of the mastic seal. Put on proper safety gear, including gloves, goggles and a mask, and rub the remaining mastic seal off with an acetone soaked rag.

  4. 4

    Set the window back into the hull and mark any areas where the window is sagging or simply not fitting snugly.

  5. 5

    Replace the seal with a fresh white butyl rubber seal. Connect the ends of the seal at the bottom of the window, making the seal a little long and pressing the two ends together. This will create some additional pressure at the bonding point to ensure proper waterproof protection.

  6. 6

    Re-install the window and screw it back into position.

  7. 7

    Let the new liner set for 24 hours.

  8. 8

    Cut off the excess seal, with a putty knife, around the outside of the aluminium frame.

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