If you have ever watched a roofing crew install a roof, then you may have noticed that they nail down a layer of some rolled product on top of the roof’s wood decking before the shingles are installed. This rolled product is called “underlayment”.

Many homeowners believe that underlayment is some kind of secondary protection. That when water gets past the shingles, the underlayment is the roof’s last defense against water incursion. Though this may be true to a degree, it is not the main reason it is installed. There are, in fact, two reasons:

1. A roof installation can take a day or several before complete. And sometimes rain storms seem to come out of nowhere. (Or, as I have experienced, form directly over the roofing job). But the underlayment, which can be installed rather quickly, actually waterproofs the roof. If a storm should pop up out of nowhere, the underlayment will actually prevent water from getting inside the house.

2. A second benefit of roof underlayment is to protect the interior of the home if, for some reason, shingles are blown off or torn off. It could take several weeks or more before the repair can be implemented, so the underlayment ensures that, in the meantime, no water gets inside.

Just a few years ago, almost all roofing contractors installed what is commonly called “felt”, which is a black paper infused with asphalt. But felt is relatively heavy and dries out when exposed to the sun within a couple of weeks. The newest technology, and that which most contractors now use, is called “synthetic” underlayment. It is a bit more expensive, but much easier and faster to install. Moreover, if an area of shingles are blown off, it will shield the house up to six months, rather than the two weeks that old fashioned felt offers you.

The advantages of newer technologies can often be debated, but in this case, it is a clear cut improvement.