Buying a Metal Roof Part 9: Underlayment

What’s underneath the new roof of your North Carolina home matters. Here’s why:

If your metal roof is to be installed over solid decking or over existing shingles (an environmental benefit), a layer of quality underlayment between the two is essential. Underlayment is also a good idea when the roof is installed over battens though a natural tendency for the underlayment to drop between the battens can be a problem.

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For many years, the standard underlayment beneath a metal roof was 30-pound felt. In some cases, local building codes may require a more fire resistant underlayment or may even require the use of self-adhering “ice and watershield” type underlayments over the perimeter and valleys of the roof. Always make sure that you’re adhering to the more stringent of either building code requirements or manufacturer specifications. If a self-adhering underlayment is used, it must be a smooth surface product rather than a granulated surface product.

One issue with 30-pound felt was that, because it is an oil-based product, it would sometimes “stick” to the underside of the roof panels during hot temperatures. Expansion and contraction of the roof panels could then damage the underlayment. For this reason, red rosin paper was often placed over the roofing felt to act as a “slip sheet”and prevent oils out of the underlayment from sticking to the roofing.

Recent years have seen the development of many new underlayments generically called “synthetics”. These polymer-based products are longer lasting and easier to install than 30-pound felt. They also serve as both a slip sheet and an underlayment.

So, why use underlayment on your North Carolina Home? Here are the primary reasons:
  • It is required by the International Building Code.
  • To protect your home during the installation process. Even if there is an existing roof in place, it is getting a lot of foot traffic while the new metal roof is installed as well as having nails pounded through it. Underlayment provides extra protection. And, of course, if your old shingles are removed, it is very apparent why underlayment would be necessary to keep water out of your home while the roof is being installed.
  • Metal roofing depends upon precise flashing and metal fitting for watertightness. While sealants and adhesives may be used, they are as secondary back-ups for protection against water infiltration. Fact is, there is always a chance that during a particular wind-driven rain storm that may happen only once every five years, you may bleed a little water through a flashing or some other area of a metal roof. Underlayment prevents that from causing a problem.
  • Certain weather conditions may generate condensation on the back of the metal roofing panels. Ventilation allows that condensation to drain and/or evaporate without reaching your home’s structure.
  • If your new metal roof is installed over composition shingles, the rough surface of the composition shingles can scratch the back of the metal panels as they expand and contract with temperature changes. These scratches can damage the metal roofing panels and lead to premature failure of the roofing system.

For answers to your questions about the need for underlayment on your new roof, contact McCarthy Metal Roofing Systems, your local metal roofing experts.