Common building problems caused by human errorMost of what I do is remodeling/repair/restoration, as opposed to new construction. Thus, I'm in a good position to observe the long-term viability of various techniques and materials. This area of the site is my chance to gripe about all the dumb stuff people do that I have to fix, and what seems to be the reasoning (or lack thereof) behind such errors. Most of these problems are easily avoidable, if common sense and some basic knowledge is applied.
If you're a homeowner, this will hopefully arm you with knowledge of what to look for in a contractor's workmanship; if you're a contractor who does these things, you're probably not reading this, because you have no real interest in improving your work.
Failure to acknowledge the force of gravityTo paraphrase the venerable plumber's proverb, water flows downhill. It also flows into tight spaces via capillary action, and generally will go anywhere it can. One would hope that most contractors are aware of this, but apparently many are not.
The most common offense is to rely solely on a sealant to shed water at a critical joint between two components, rather than using gravity to one's advantage. Roofers are frequent offenders; many of them would rather use BlackJack (tar) than flashing (sheet metal) to prevent leaks around chimneys, vents, etc. As millions of homeowners probably already know, tar dries up and cracks open within a few years; it will never last as long as the roofing material. In fact, no sealant can be relied upon to keep water out of your home. Sealants lose flexibility with age, fail to adhere, and generally cannot serve as a primary barrier to prevent leakage in any exterior building system, period.
"So what do we need to do to prevent water damage?"
Let's review the plumber's proverb: stuff flows downhill. Take a look at the shingles and siding on most houses. Notice how it overlaps? That's all you need to know. Stuff gets nailed on from the ground up, and if it overlaps, it sheds water. If it doesn't, water gets in. Now, go and explain that to your roofer.
The most common building component in which this principle is ignored is chimneys. A chimney and a roof are two totally separate systems, with unrelated means of support; thus, any relative movement between the two systems is independent as well. As a roof deck moves with settling or temperature changes, its position relative to the chimney changes. This should make it clear why we cannot rely on a sealant to prevent leakage at this critical joint. Use gravity instead; if the chimney is properly flashed, the sealant can fail (and it will!), and there will still be no significant leakage.
"What???! Thou speakest blasphemy!!!"