Before going into the types of roofing underlay I feel the need to discus the reasons why it is used.
Asphalt shingles did not used to require underlay. This was because there was an asphalt saturated paper already in the shingle.
Today most of the shingles are a fiberglass core & with some the fiberglass core is somewhat porous. Maybe that has changed but I am seeing problems with 8-10 year old shingles in that they are losing granules prematurely, the asphalt is breaking down & water is weeping through them. These are not good shingles(I will not name the manufacturer). In a case like this a good underlay is helping hold out the water for a little longer.
Some shingles do not meet their fire rating without the underlay as well
All that aside, it is a good idea to have a second layer of protection. I have seen storms tear off a roof & only the underlay has kept the water out. Animals causing damage & homeowners letting their roofs go far too long, all protected by that thin membrane.
Cedar Shakes did not used to have underlay ((felt: asphalt saturated paper) either, but they also used to triple layer all the shakes as well. a 24″ shake only had a 7 1/2 inch exposure. If you wanted to install them like that today an underlay would not be required. Today they are all double layered with a double layer of felt, interlaid between each row of shakes. As long as it is protected from the weather it will stay intact for the life of the roof. Cedar shingles are triple layered & do not require felt.
Metal Roofs need a good underlay because they tend to sweat with moisture & temperature changes
Synthetic shingles are impervious to water but also require an underlay. I would not think one would be required but most of these are very smooth products & i would suppose that a strong wind could blow some degree of water up through the spaces in harsh conditions. Most of these products do not go on below a 5/12 pitch without lowering the exposure probably for the same reason
Concrete Tiles is something I don’t do anything but repairs on & I personally do not agree with the installation method of underlay on the west coast of British Columbia. We live in a rain forest & this product was made for a desert. If these roofs leak it is anybodies guess if the underlay will hold out the water.
Types of Underlayment:
Felt or Asphalt Saturated Paper: This is just what it sounds like. A heavy paper with an asphalt product on both sides & saturating the paper. Comes in 15 lb & 30 lb ( that would be 30 lbs per square -1 square = 100 sq ft) 30 lb is usually required on the eaves & valleys then 15 lb throughout the remainder of the roof. This product has been used for 100 years and to this day is still a mainstay in the market. Personally I like a 30 lb felt on the entire roof. You can drive nails into it & it remains watertight. Consider that your roof is going to have 5000 nails driven through it, this is not a bad feature. You cannot leave it out to the weather as it wrinkles when it gets wet but as long as it is covered with shingles it should last the life of any roof . 30 lb would be hard to blow off in a storm as well. (fairly inexpensive)
Synthetic Felts are either a paper/fiberglass mix or saturated fiberglass. These tend to be lighter weight & wrinkle free. Definitely easier for the roofer. If you leave them exposed just be careful as exposed nails & tacks tend to weep water. We tried them many years ago & found that they were not superior to regular felt. (Slightly more cost than a regular felt)
Synthetics. Most are a variety of plastics. They put material inside them to prevent the ultraviolet light from breaking down. These do work for covering a project you cannot roof immediately but keep in mind that nails & tacks will leak. Stinger makes a plastic plug that goes on with a special tacker that will not only hold the underlay down against winds but also stops the leak issue. These products do not wrinkle, come in wide rolls & are light weight. You are still going to drive 5000 nails through this roof so it works but is only a plastic & does not seal up behind the nails. (2-3 times the cost of a felt)
Breathable Synthetics are in fact breathable & very tough, cannot be ripped & I have seen them exposed for 6 months & still be intact. Same nailing issue though, they do not seal up behind nails & although you cannot tear them, they can be stretched. You need to be careful about installing it if you are not going to roof right away. They do work well on new construction when the house needs to dry out. (3 times as much in cost as regular felt)
Rubber based Synthetics are a mixture of SBS rubber, asphalt, fiberglass with a mineral (sand covering) These are amongst my favorites. Slightly more fragile than synthetics, but wrinkle free, able to withstand long periods in the weather & can just be applied with nails & tack & will remain watertight. Those 5000 nails through your roof are also watertight. It is a sanded finish so keep in mind that it may be a little slippery on steeper pitches. Also needs to be nailed well in warmer weather. (About 3 times the cost of regular felt)
Ice & Water Shield is the same as the rubber based underlay except it has a peel off back that can not only be stuck to the roof deck & walls but also seals to itself. You could probably make a swimming pool out of it. It is great for low slopes, detail work, ice damming & anywhere that you want to just make sure it will never leak. My favorite is one that has a plastic top layer attached with a scrim in it that helps to walk on it. This one is great because it is very tough & cannot be stretched. This is the best & used almost always in colder climates. (Cost is about 6-8 times the cost of regular felt depending on the product)
Whatever you want to use or are recommended by a roofer just make sure it is right for you & your building. Not just easier for the installer.
This is an opinion that I have as per my experience working with these products. I have no doubts that there are people that disagree me as well as manufacturers. Keep in mind that in different locations in North America some of these products may be more suitable than others. There is no one size fits all.
DC Roofing Inc.