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What do I need to do in order to caulk this huge gap above an arched window?
Should I put some sort of barrier on top of the wood window frame and pipe cement to fill the gap. It is too large for even the biggest backer rod I can find. I am using Quad caulk and various sizes and lengths of backer rod on the other windows as their gaps are much smaller.enter image description here

  • Wheres the flashing or at least flashing tape? All I see is a cut up nail strip...
    – Jack
    Nov 1, 2017 at 3:41
  • I just purchased this house, and apparently, it was constructed without the use of flashing or flashing tape.
    – J.M.
    Nov 2, 2017 at 16:03
  • How old is the house and how many round top windows are there? I know the question is off topic but this would be pretty important to me to be remedied. If the roof overhangs the area really helps solve the issue.
    – Jack
    Nov 3, 2017 at 3:18
  • The house is about 20 to 25 years old. There are 2 round top windows. The roof overhangs both rounded windows.
    – J.M.
    Nov 3, 2017 at 19:03

2 Answers 2


I'd be tempted to try and get a piece of wood in there! Cut a piece over length and leave it out in the rain leaning against a wall etc. Wet wood will eventually bend (or you can encourage it over a couple of blocks by adding a bag of sand etc.) and get something the approximate curvature you're looking for and then cut it to length.

Not the quickest solution, but cement will crack up eventually as the window moves about through the seasons.

If it's too thick to bend naturally, (it looks to be on the upper limit at around 3/4 inch or so) you could laminate a piece up. Run some timber through a table saw to get 2 or 3mm thick strips and then glue them up over a former the same radius as the window. Again, not quick but oh, so pretty!

  • 2
    Given exterior use, I'd go for a piece (or pieces built up) of Azek (or generic PVC) trim.
    – DaveM
    Oct 31, 2017 at 20:08
  • Good idea Dave. I'd almost forgotten about PVC trims (it's wood all the way here in Norway!). Plastic is also much easier to bend into shape than timber.
    – handyman
    Nov 1, 2017 at 20:57

You really need to leave some room in there for seasonal movement. My choice would be backer rod and caulk that has been tooled in after masking off the area. Larger backer rod is out there.

  • Sure 2mm or 3mm, but over 20mm is overkill surely?
    – handyman
    Nov 1, 2017 at 20:58
  • @handyman, Yes 20mm is way too big for a gap, there absolutely needs some room for season movement. 5-6mm is best depending which floor the window is on, second floor windows need more room if the house is platform framed as it is called here in the States. The idea of caulking it only and tooling it in place is the simpler method, IMHO, than adding bentwood and then caulking the remaining gap. A carefully placed tooled caulk joint, with the right size backer rod behind it will appear as a larger flat piece of the actual window trim. This will require taping the brick and a flat putty knife.
    – Jack
    Nov 2, 2017 at 2:38

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