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Roofing Underlayment - What is it?

Shingles, or whatever roofing materials you choose to use to cover your roof, are your first defense against the elements but there is an unsung hero working in the shadows. Roofing materials consist of far more than just shingles, tile, or metal panels. One of the most

Shingles being installed by Flat Rate Roofers over hybrid underlayment
Roofing in progress in Medford, OR

important things other than your shingles is the underlayment. Underlayment serves multiple purposes and I am going to do my best to explain some of them to you in this article.

It can take a while to install a roof - especially with a small crew and/or a large job. Also, in wet states like Oregon, a job that would otherwise be quick can be interrupted or slowed down by rain. Building materials are usually made to withstand some rain for a short period of time. This is to make building in states like Oregon possible. Imagine having your house framed and then having an unexpected rain storm and needing to completely re-frame the building. That would be expensive and time consuming. Covering an entire building in tarps for unexpected rainstorms before it is ready would be impractical at best. Once the house is built, however, there are many things that can get damaged by water easily. Things like drywall and your belongings. When a house is complete, there is also less airflow because it's no longer wide open. This lack of airflow could also cause mold and dry rot if something gets wet.

When you are getting your roof replaced, as long as there is sufficient attic ventilation, a little sprinkle of rain shouldn't hurt your roof deck. Even so, a good roofer will do his or her best to get the underlayment installed before even a sprinkle occurs. Underlayment serves multiple purposes. There are also many different types of underlayment for different budgets and different purposes. I'm going to give you some information on underlayment in general. I'll probably go into more detail in another post.

The first purpose of underlayment is to protect the roof from rain while it is being installed. underlayment generally goes down very quickly compared to the shingles. This makes it so a roof can be torn off and the underlayment can be rapidly installed before a rainstorm. In fact, it is common to install shingles while it is raining on underlayment that was installed just before the bad weather. This is especially common in the Portland area where it rains even more than here in the Medford area. Both the underlayment and the shingles water resistant and can handle moisture. Installing while it's raining isn't ideal because it can take a while for it to dry out between the underlayment and the shingles but it is sometimes necessary in rainy climates. Properly installed underlayment can protect the roof for several months if absolutely necessary. Even so, it's best to get the roofing installed over the underlayment within a few weeks because the underlayment will get sun damaged in the summer and some types can get damaged after a few good rains. Waiting a few months is generally a bad idea and is probably never necessary.

Another purpose is traction on the roof during installation. Different types provide better traction in various conditions. Roofers may decide to change types depending on the weather or the pitch of the roof.

Properly installed underlayment also has another very important purpose as a second layer of defense for your roof. Wind can drive rain under your shingles. If there wasn't underlayment it would soak into your roof deck and you could get mold and/or rot. As your roof gets old, some of the shingles may loose their seal and get wind damaged. this second layer of protection will help minimize leaks while you wait for the repair or replacement. Ice, leaves, and moss can also re-direct water underneath your shingles.

This is why we don't take underlayment lightly and why you should make sure the contractor you choose won't cut corners by eliminating important materials like roofing underlayment.