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While you can technically install new roof sheathing over an old layer, it is generally not recommended for several reasons. Installing new sheathing over old can trap moisture, which may lead to future problems, especially in humid climates. It also adds significant weight to the roof structure and prevents you from inspecting the original sheathing for rot, mold, or other damage.

However, in some cases, if the old sheathing has been thoroughly inspected and is in excellent condition with no signs of rotting, warping, or damage, a roofer may install new sheathing over it to add strength and stability. Despite this, the more common and recommended practice is to remove the old roof sheathing entirely before installing a new layer, as removing it is not a difficult task.

There are several signs that indicate you’ll need new underlayment. Clear signs include missing or damaged roofing and exposing the underlayment to the elements. Additionally, if you notice water damage in your home by the roof after a storm, it could indicate that your underlayment may need to be replaced. If your gutters aren’t draining properly, it could also lead to rotting or water damage to the underlayment.

Your deck may pop, and your neighbor’s deck may not for a number of reasons. Whether or not a deck pops will depend on how it was made, the conditions it’s exposed to, and the materials. Improper spacing is a common reason for increased pressure between deck boards. Moisture penetration is another.

Fall is widely considered the best time of year to replace a roof. The moderate temperatures are ideal for roofing crews to work and for new shingles to seal properly. Additionally, autumn weather is often drier, reducing the risk of weather-related delays compared to the rainier spring season.

While spring is also a viable option, the higher chance of rainfall can slow down the project. The main drawback to a fall installation is that it's the peak season for roofers, which can lead to higher prices and scheduling challenges.

To potentially save money, you could consider scheduling your roof replacement during the summer or winter off-seasons. However, both seasons have significant disadvantages. Extreme summer heat can make shingles too soft to handle, while winter's cold, snow, and ice can create dangerous working conditions and prevent shingles from sealing correctly, as they typically require temperatures above 40-45°F.