What is the problem with butting brick against a window?

Question: What is one of the most common mistakes that building contractors make when installing windows and doors?
Answer: Not allowing enough clearance between the brick and the installed window.

If you read the installation instructions, on just about every window manufacturer’s labels, put on every window, you will find that it is recommended for a gap of 1/4″ to 3/8″ be allowed between the brick and the window.

So what’s the problem with butting the brick to the window?
Most of the time it is not a problem.  However,  if your home is built in the winter time, and perhaps the lumber package sat in the rain for three or four days, then the lumber is going wick up the moisture and move as it dries out. Since the winter is typically less humid, the result is dimensionally unstable lumber (i.e., the lumber will shrink). If the lumber moves; and the window is nailed to the lumber; and the brick is stationary because it does not have a stability issue with moisture; and the brick is butted next to the window–what do you think will happen?  Well…the answer is, the window will to move with the lumber to which it is fastened.  If there is a brick wall in it’s way I can tell you from 25 years of experience–that brick ain’t goin’ nowhere, and the windows are going to be crushed by the stationary brick wall.  In my experience the most common movement is for the window to try to move down against the brick sill plate, which will crush the window sill. I’ve seen, and our salesmen and service people have seen,  wood windows, aluminum windows  and vinyl windows, crushed by the immense forces created by this issue.

It’s a numbers game (you can skip this section if you don’t like math)
I watched PBS’s episode on Einstein’s Theory of Relativity last night, so I’m in the mood for a thought experiment.  (Oh Geshhh! you had to go there, Abe.) Knowing that you should leave a 1/4 to 3/8″ gap, I can tell you from all my 25 years in the window business, that probably 99+% of all windows installed have the brick butted right next to the window.  On the sill it is the norm to see the brick angled and butted right under the sill of the window.  In 99+% of the installations, even though it goes against recommendation–it does not create a problem.  Contractors can probably tell you something like: “I’ve been building homes for 25 years and I have NEVER seen a problem with butting brick up to a window.”  If you figure the average contractor builds 20 homes a year and the average home has 15 windows, that’s 300 windows that the contractor has exposure to on a yearly basis.  Over 25 years, he or she has seen 7500 window installations (25 years times 300).  That’s a lot of windows until you start comparing that to the number of windows MGM makes in one year and then multipling that by 25 years.  If you assume that MGM Industries manufactures about 300,000 windows in one year and multiply that by 25 years, you will calculate that MGM would have produced over 7.5 million windows during that time frame.  Assume, for a moment,  that butting the brick to the windows creates problems in 0.01% (i.e., 1/10th of 1%).  The math says that there will be 7,500 window installations with problems (7.5 million times 1/10th of 1%).   One tenth of one percent is statistically insignificant: unless you are caught up in the 1/10th of 1% that has the problem. When your caught up in the problem, all the statistics go out the metaphorical “window”.

If you use the same percentage to the mythical contractor that has been responsible for 7,500 windows, he/she will see 7.5 windows with issues over a 25 year period–not much: until he/she finally gets bit by the bug created by butting brick to windows.  The volume of windows built  is why manufactures know about this issue and many, professional contractors, know nothing about the problem.  Take the mythical contractor that I created.  If he does not have a lumber package exposed to the rain in the winter, then the likelihood of he/she experiencing the problem is even more remote.

If you extend the math of 7,500 problems created per year times 25 years, our company would have seen 187,500 windows with issues caused by brick being butted next to the window.

I created a hypothetical math scenerio in order to make a point, so don’t take the numbers verbatim.  To be accurate, we have not averaged 300,000/year over 25 years.  This year, in 2011, we will come close to manufacturing that many windows, so we see a lot of window installations.   In actuality we probably see a handful of jobs in which the windows are crushed as a result of the lumber package shrinking and moving.  So probably the real percentage of problems is far less then 1/10th of 1%.  My question is: “Do you want to be the one struck by lightening?” Why not leave the 1/4″ gap that most seasoned window manufactures recommend?  It makes such a better installation.  It’s such a cheap insurance policy against problems.  Just leave the 1/4-3/8th’s of an inch and caulk the gap.  Even better: put in the backer rod and then caulk it.

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