a building with dark patches on its brick exterior under every window

I don't recall seeing the walls like this before, so I don't think it's just accumulated soilage. If it's wetness, what design flaw causes the windows to do that? Today was very hot and muggy so it's likely the AC was running all day.

  • $\begingroup$ What is the weather when this is first observed? Hot sunny days? Cloudy, rainy days? Before it even rains? $\endgroup$
    – Abel
    Jul 3, 2022 at 12:37
  • $\begingroup$ @Abel The day this was taken was more or less like you see in the image all day: cloudy, diffuse, humid, hot. It did rain for about ten minutes that morning, very light, and this was taken in the evening. $\endgroup$ Jul 4, 2022 at 16:01

4 Answers 4


The building is in a place (Halton Hills, Ontario, Canada) where the relative humidity is about 100% in the summertime (see part Climate in this https://www.britannica.com/place/Toronto)

The building must be heavily air conditioned due the numerous medical businesses which do not stand uncomfortable indoor conditions. As already said by others, it's well possible that those single layer glasses are wet all the time in the summertime. The continuous water flow condenses dirt just below the windows which have nothing to keep the wall dry.

I guess the the tiling below windows has holes which leak out a substantial amount of indoor air. In the summertime the walls get wet also because the wall near the holes are cooled by the indoor air. In a cold weather the walls get wet because there comes out warm indoor air which carries out water. See this Google Maps image:

enter image description here

The image is shot in march (=early spring). The windows looked the same in all sides of the building when I drove around with Google Maps.

When looking older versions of Google images one can notice that the stain seems to be much darker in the autumn - like some midew has developed in the moisture on the stones during the summertime. I guess it tears off during the winter.

  • $\begingroup$ I guarantee the humidity is not 100%. 100% is fog. $\endgroup$
    – Tiger Guy
    Jul 2, 2022 at 22:11
  • 2
    $\begingroup$ I used poetic term "about" to make a difference $\endgroup$
    – user33233
    Jul 2, 2022 at 22:24
  • 2
    $\begingroup$ @TigerGuy: en.m.wikipedia.org/wiki/Fog: “[fog] can form at lower humidities, and can sometimes fail to form with relative humidity at 100%”. viz super saturated, dew, nucleation, mist… $\endgroup$
    – Krazy Glew
    Jul 3, 2022 at 19:13
  • $\begingroup$ @TigerGuy I fixed the nomenclature $\endgroup$ Jul 5, 2022 at 12:32

OP states that the staining is not always present. here is a possible reason for it:

If the building is air-conditioned and has single-glazed windows, then the outside of the window glass might be below the dew point temperature of the exterior air (which varies from day to day) while the AC is running. This would cause occasional condensation on the outside of the window glass, which would then run down and wet the building walls under the windows.

  • $\begingroup$ Maybe, but if so, why are they so uniform, and why isn't it like this every day? $\endgroup$ Jul 2, 2022 at 3:10
  • $\begingroup$ @LukeSawczak What do you mean by uniform? They don't look uniform to me. $\endgroup$
    – DKNguyen
    Jul 2, 2022 at 3:40
  • $\begingroup$ @LukeSawczak: it requires a certain level of atmospheric humidity for condensate form on the outside of windows for a given temperature inside the building. $\endgroup$
    – Fred
    Jul 2, 2022 at 8:40
  • $\begingroup$ @DKNguyen Not carbon copies, but every window, about the same amount, to about the same height. Feels like it probably wouldn't be the result of animal behaviour (but not impossible, of course). $\endgroup$ Jul 2, 2022 at 12:30
  • 1
    $\begingroup$ Critical flaws with the bird suggestion: the parapet wall (the top part) would also attract birds yet it's completely fine; secondly the ledges are way too small here for birds to perch. It's actually this lack of ledges why this happens; glass gets dirty and rain water washes it off, unlike on the brick itself because brick is more absorbant. Protruding ledges force the water to fall straight down and on to the ground with specially designed channels underneath them. $\endgroup$ Jul 2, 2022 at 16:14

To add to the other answer:

When ledges protrude out from the wall then the water, and dirt, falls away from the wall surface. Also, the underside of the ledge usually has a drip channel or groove to stop the water running back onto the wall.

So flush window ledges have this problem. Windows with protruding ledges can have the opposite problem where the wall under the ledge actually stays cleaner than the rest of the wall.


Cold air is spilling down the wall from the window. This permits condensation on the block. I bet it was a really calm day too.


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