How can I know the size of the rough opening of the original window?


My house is from 1951. Before us, the house was rehabbed by a DIYer who did a lot of things with a lot of errors. I've had professionals in several trades independently comment that things looked really "frankensteined together." Other houses in the neighborhood which look similar do not have windows that look like this.

The problem

A few of the window areas have started leaking when it rains and is windy. However the windows themselves don't seem to be leaking, just water getting in behind the flashing, messing up drywall around the window, and dripping into the house.

What I have tried

I have tried having window replacement companies come out, but they always measure the current window in order to generate quotes.

My questions

My hunch is that the person who rehabbed the house bought a bunch of standard sized windows and caulked flashing above them, hoping it was enough.

  • How can I know for sure?
  • Is it worth it to find out, or might a reputable window company replacing the windows be sufficient to stop the leaking?
  • What type of professional would I contract to find the size of the original rough opening?

top floor window 1 top floor window 2 window first floor


2 Answers 2


Your hunch is probably correct. This is often the case with vinyl replacement windows. Various trim and flashing methods are used to close the gaps.

To know your rough opening size, simply remove some interior trim. You'll either encounter original framing or masonry, depending on construction type and age.

In the case of the first example, I suspect that the opening will have been reduced with drywall. You'll have to decide whether you want to return the opening to the original size, in which case you can do a stepped exploration to find the original opening.

You could do the same from the outside, but you then have to deal with even worse weather intrusion conditions.

Any professional carpenter (contractor, handyperson) or other person with basic framing and trim skills can do this for you if you're not up to it yourself.


The only way to know for sure what's behind the wall is to open it up.

Measuring the existing window is exactly what's expected when ordering a replacement window. Replacements do not have a nailing flange around the outside because you'd have to remove the exterior siding to nail that in. Instead, they measure the opening, build the window to fit, then slide the new window in and attach it to the framing that held the old window in place.

If the old framing isn't suitable, and decent company* will come with some 2x4 lumber somewhere in one of their trucks and they'll adapt your existing opening to fit the new windows. They should also insulate around the windows (usually with spray foam which is 100% acceptable).

*When I had windows replace a couple of years ago in my 1890s house, none of the existing windows quite fit the rough openings. I went with a national company (though not one of the big brand-name window manufacturers) and they seemed pretty decent. Until there were some very significant delays getting 2 broken window sashes replaced. I ended up talking with one of their VPs trying to get things sorted out. I wouldn't recommend the company, to the point of not even remembering their name. Despite this, they still adapted the house framing by attaching necessary 2x4 lumber to make the new windows fit the existing openings properly.

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