Every shingle roof needs an underlayment. Because shingles are not adhered at all joints, wind driven rain can get beneath them. Underlayment acts as a water shedding membrane in such case.
For more intense issues, like shingle blow off, underlayment is necessary temporary protection for the time before repairs can be conducted.
Underlayment is not designed to withstand the brunt of the weather by itself, but it is designed for those seldom (but inevitable) times where shingles do not shed all water.
Asphaltic roofing felt (also known as tar paper) has been a mainstay for many decades in the roofing industry. It’s relatively inexpensive and functions very well for its purposes. But synthetic underlayments have been taking over the market. Given that synthetics are more expensive than traditional felt paper, there must be some benefit to using synthetics. This article will cover the differences.
Unless you go for the most expensive synthetics (most don’t), felt paper has improved breathability over standard synthetics. There is much debate in the roofing industry about how important breathability is since asphalt shingles don’t breathe, but there is some evidence to suggest that sheathing can more effectively dry upwards with a more breathable underlayment (like felt).
But few contractors go for these very expensive synthetic underlayments. Most opt for the standard synthetic underlayments instead, which are still a bit more costly than felt paper. If installing roofing designed as a vented rain screen — like tile or wood shakes — then high permeability is very important and felt paper may be the best option.
The question is why are so many contractors using synthetic underlayment? It’s more expensive than felt, and felt performs its job very well.
We believe there are four main reasons contractors gravitate towards synthetics:
Synthetic underlayment is safer
This is a big deal, because roofing can be dangerous. Synthetic underlayment grips shoes better and doesn’t tear like felt does. Everybody benefits from a safer installation, and synthetics being safer may be the chief driver of their growth. Some roofing contractors are known to have switched to synthetic underlayment for the safety issue alone.
Synthetic underlayment is also safer because it is lighter and easier to install than felt. Roofing can be very tough work and managing energy is important. Roofers that install synthetic underlayment instead of felt paper may be safer simply by not getting as tired out by the underlayment application.
Felt paper can telegraph unevenness
Plywood decks have bumps, grooves, and other asymmetries that cause roofing shingles laid directly over them to telegraph, making the roof look uneven and poorly finished. Felt paper serves a purpose of providing a smoother, more uniform surface to lay the shingles on to avoid telegraphing. However, even felt paper experiences telegraphing issues due to tendency to wrinkle when wet or crack when cold.
Exposure during installation is a real risk. Felt can deform after a day of exposure, and a day of exposure is not uncommon during installations. Not every installation will have a day or more of exposed underlayment, but many do. Since roofers know to look out for this, sticking with synthetics is a good way solve a potential problem before it begins. Synthetic underlayment can be exposed for weeks without deforming.
Synthetic underlayment makes the roofing job more reliably high quality because it significantly reduces the risk of the underlayment deforming.
Felt is not compatible with top warranties
Manufacturers tend to require synthetic underlayment when providing upgraded warranties. Manufacturers tend to allow only their best contractors to provide upgraded warranties, which means that if you want a quality contractor, you’re probably looking for a contractor that uses synthetic underlayment. Even if traditional felt was “better” than synthetic underlayment, if all the contractors that do the quality work use synthetic, it would mean opting for an installation with felt would more likely result in a lower quality job.
Synthetic is more robust to extreme events
The same high winds that blow shingles off can blow underlayment off. Felt paper tears easily enough that wind can catch it and carry it off the roof. Synthetic underlayment, however, comes in much larger rolls and has excellent tear strength. Where a storm could rip felt paper off a roof, it is very unlikely to do the same to synthetic underlayment.
Performance in extreme events is one of the primary functions of underlayment. Shingles handle almost all the weathering, but when outlier events cause water to bypass the shingles, underlayment is designed to shed it. If felt also performs poorly in some extreme events, then it doesn’t function as intended.
Typically called leak barrier or ice and water shield, waterproof underlayment is a rubberized membrane that self-seals around fasteners. It’s significantly more durable than the felt paper and water resistance synthetic underlayment discussed so far. It should be used only in critical leak areas like valleys and transitions, or along eaves in regions where ice damming occurs. Where water resistance membranes are designed to shed water that bypasses shingles, waterproof membranes are designed to handle water that doesn’t easily shed.
Felt paper is a generally reliable as a shingle underlayment, but there are a handful of improvements made by synthetic underlayments that are making many roofing contractors switch to it in spite of felt being less expensive.