When a house is brand new, all of the siding and walls are straight as an arrow. Over time, however, the siding, or the walls, may have developed bulges or concave areas, making it look wavy. There are some definitive reasons for this, and knowing and understanding the reasons is the first step in correcting the problems.
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Most materials expand when heated and contract when cooled. In a state such as Michigan or Wisconsin, where the temperature fluctuations between winter and summer are great, the constant expanding and contracting will eventually warp the siding, causing it to look wavy. There is really no way to prevent this; it is normal for very old siding to look wavy.
Understanding that materials expand and contract due to temperature fluctuations, the siding itself has to be installed correctly to compensate for the expansion/contraction cycle. If the aluminum or vinyl siding was pulled too tightly against each other when installed, the contractions pull it even tighter, causing it to buckle. If panels were used, such as T-111, the panels can not be butted too closely or tightly together, but rather must be installed with a little "breathing room" of no more than 1/8 inch between each overlap joint to allow for the expansions and contractions.
Over the years, house foundations start to sink into the ground and walls develop sags. These sags then cause the siding to buckle, and waves start to form. This is a normal part of the aging process; many very old houses even had their foundations removed and replaced. If a relatively new house starts to sag, structural problems may be present according to Bob Vila, requiring qualified attention to the matter.