Underlayment protects the sheathing from moisture until the roofing material is installed and also afterwards as additional sheathing protection from wind driven rain.There are 3 primary types of roofing underlayments; asphalt-saturated felt, rubberized asphalt, non-bitumen or ‘Synthetic’.
Asphalt-Saturated Felt/Paper
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 For residential steep-slope applications, the most common underlayment used is asphalt-saturated felt or paper. This type of underlayment is made from either organic (cellulose base) or fiberglass substrate, with organic being most common. This type of felt is water-resistant but not waterproof. 15lb felt is commonly sufficient for most fiberglass/asphalt shingle roofs, with 30lb felts being reserved for those desiring a heavier duty system or for other roofing material types (wood shakes, metal roofs, etc). Roofing felt is held down with roofing nails or staples with just enough fastening to stay in place until shingle installation. Eave flashing is ommonly installed along the eave for ice dam resistance which typically consists of two layers of 15lb roofing felt. For higher sloped roofs 4:12 pitch and up roofing felt is commonly side-lapped 4″ and top lapped 2″. For lower sloped roofs 2:12 to 4:12, top overlap should be 19″, which ultimately provides a double layer across the entire roof. 15lb roofing felt most commonly comes in 36″ x 144′ long rolls and the amount needed will be calculated by the actual exposure after lap.  5-8% is a common overage to estimate when figuring amount of rolls needed when typical lap and waste are considered. (example. (2″ lap / 36″ wide) multiplied by total roll square footage). Like eave flashing, felt at hips and ridges are also doubled and should extend out on either side at least 6″. For additional ice-dam resistance, on some occasions (particularly low sloped roofs; 3:12 to 4:12) an extra course of felt is asphalt-cemented in place from the rake to at least 36″ past the inside wall. Underlayment is typically fastened with staples but plastic windstrips or plastic nail caps may be used in higher wind areas.  
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Many roofers feel that the proper installation at the roof edges is to overlap felt with drip edge metal at the rakes and lap the felt over the drip edge at the eaves.  There are many low end roofing felt manufacturers which do not provide ASTM certified felt which has a relatively low asphalt saturation content. Excessive UV exposure and time will break down the volatile compounds in the asphalt so leaving them exposed for long lengths of time is undesirable. Asphalt Saturated felt is becoming installed less and less common as synthetic underlayments have begun to compete in terms of performance and price.
Rubberized Asphalt Underlayment
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This type of underlayment is also an option and typically has an adhesive side with a peel-off membrane for sticking directly to a roof deck. This rubber quality of the underlayment makes it seal better around fasteners. There are several different versions/types of rubberized asphalt underlayment:
-Polyester or Polyethylene surface bond layer. Has skid-resistant qualities.
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-Polymer film surface bond layer – Provides improved moisture resistance.
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-Fiberglass Reinforced.
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-Mineral Coated.
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-Polymer modified- Adding plastic modifiers to the asphalt which saturates the underlayment, creating a stronger, ‘rubber-like’ characteristic to the product.
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Rubberized Asphalt Underlayments are often formulated for use in high temperature areas and are designed to resist heat to maintain the adhesive qualities, especially in situations with harsh and hot metal roof environments. Many Rubberized underlayments come with a feature called a ‘selvedge edge’ which lines one side of the roll and creates a tougher watertight seal on the overlap edges.
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This edge should be on the top of each roll course.
Synthetic Underlayment
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These underlayments are commonly made from polypropylene or polyethylene and are in general lighter-weight, higher strength, UV-resistant, and low skid. They are also resistant to fungus, generally wrinkle free, and most are moisture resistant near or to the point of being considered a moisture barrier.  Lastly, they are also popular because they can be left exposed to the weather for long periods of time, in some cases as long as 6 months or more depending on the manufacturer’s recommendations. On the other hand, ASTM certified synthetic underlayment is in it’s infancy and the product also has a higher tendency to wick and create moisture problems. When installing underlayment lapping requirements will vary from manufacturer. With synthetics it is very common to wrap the material around the eaves and rakes and place the drip edge metal on top of the paper. Fastening is commonly done with plastic caps or roof nails with staples being avoided because synthetics aren’t self-sealing. Synthetic underlayment can often be ordered in rolls of around 400SF.