It is not surprising that many people take short-cuts. And this fact also applies to contractors and sub-contractors. However, I’m still amazed when I see shoddy work when it would only take a small amount of additional time to do the job right.
When outside doors and windows are installed there is more to it than just securing them into their space. Steps like filling cracks with foam insulation, and putting flashing around the opening help keep the area air tight, and also prevent water from seeping into the surrounding structure. That water not only causes rot, but also mold.
I just replaced a few patio doors for a client and I was surprised to see how poorly that the original ones were installed. In the photos below you can see that the upper portion of the exterior sheathing was both swollen and black. Clear signs of water damage and a poor installation. It is always best to use a reputable contractor who specifically knows how to install doors and windows. You don’t want to go cheap when it comes to this home repair.