One of the most important things that you need to know when buying a home is the condition of the roof. Specifically the roof covering. Here in sunny Phoenix, Arizona the two most common roof coverings are composition shingle or concrete tile. I'm going to be talking about something common to both: the underlayment. The underlayment is a water-resistant layer of "roofing felt" or "tar paper" that lays between the roof deck and the shingles or tiles. It's primary purpose is to keep the wood roof deck dry while shedding any water that may get under the shingles or tiles down the slope of the roof. It's a very important job, and sadly, one that is often done incorrectly by both DIYers and roofers.
The roof inspection, as part of a home inspection, includes assessing the condition of the underlayment. Put a ladder up against the lower horizontal edge of the roof, aka the eave, and with a bright flashlight, peek under the first row of shingles or tiles at the drip edge. Can you see the end of the felt underlayment? If so, great, the felt was properly installed on top of the metal drip-edge flashing. If the felt is under the drip-edge flashing at the eave, any water running down (on top of) the felt will flow under the metal drip-edge and get trapped against the top of the wood fascia. Even here in dry Arizona, this will eventually lead to a rotted fascia. This is a very common DIY mistake, thinking the drip edge is supposed to hold the felt in place at the edge. The photo below is from a recent inspection in which the underlayment was under the drip-edge at the eave.
Now head over to the sloped edge of the roof, aka the rake. Looking under the shingles or tiles in this area you should see the opposite of what you saw at the eave. The felt underlayment should be under the drip edge. But why, you ask? Isn't the water going to get under the drip edge and rot the rake board? Well, this is a case where the goal is to keep the damage to a minimum, given the weather. The theory is that any water that gets under the shingles/tiles will be traveling down the roof slope and so will not get under the edge of the drip-edge flashing. Much. The bigger priority is to keep wind-driven or slanted rain from getting under the edge of the underlayment.
OK, time to get up on the roof. As a home inspector, the most dangerous part of my job is that moment when I step onto or off of a roof. Practice ladder safety an be careful up there. You're looking for missing or damaged shingles/tiles that expose the underlayment below. This is a very bad thing. The purpose of the shingles and tiles, other than impact protection, is to keep the blazing hot Arizona sun from deteriorating the felt underlayment. An underlayment should last up to 50 years, but if exposed to the sun, it won't last through two summers. If the felt is exposed and you can see the wood roof deck below, it's time to call in a licensed roofer. You may also need a drywall repair contractor for the ceiling below. Here's a picture from a recent inspection showing the overhead view of a cracked concrete tile that has slid down and exposed the underlayment. The underlayment is deteriorated and the wood roof deck is showing. The ceiling below had extensive drywall damage.
So there you have it, the wild life of inspecting the roof underlayment. It's usually unseen, but is the most important part of your roof covering system.