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Roof System Components

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Understanding your roof surface and the terminology of the components that go on your roof will help you make a better buying decision when it comes time to have your new roof installed. Not all product are the same and don't withstand the weather elements as long as others. 

1. Roof Covering: Shingles or metal are the most common types.  You could also have slate or tile.

2. Sheathing:  These are the boards or sheeting material that is fastened to the rafters to cover your house.

3. Underlayment: Roofing underlayment is what lies between the shingles and the roof sheathing, or roof deck, which is typically either plywood or OSB. It’s installed directly on the roof deck and provides a secondary layer of protection from the elements, including rain, snow, and wind. 

Types of Roofing Underlayment

There are two main types of roofing underlayment:

  • Felt

  • Synthetic


Each product has its pros and cons, and the type you choose may depend on your geographical area, roofing materials used, roof design, budget and what your roofing contractor may suggest.

Felt Roofing Underlayment is one of the oldest types of roofing underlayment. It’s created by saturating paper or fiberglass mat with asphalt.

Felt roofing underlayment is typically available in two types: No.15 felt and No. 30 felt. Compared to No. 15 felt, No. 30 felt is typically thicker, stronger, and may be less prone to tearing or ripping off during installation or weather events. 


The main advantage of using felt roofing underlayment is cost. Felt underlayment tends to cost less compared to synthetic underlayment, which is why it’s often the go-to for budget-conscious homeowners.


There are several disadvantages to using felt underlayment on a roof. One disadvantage of traditional felt roofing underlayment is it generally can’t be left exposed for more than a few hours. The material may dry out or leach oils in the heat.  This would impact the felt’s ability to protect against moisture. 

Other drawbacks of felt underlayment include:

  • Prone to tearing in high winds and during the strain of installation. 

  • If exposed to moisture, the mat can absorb water and wrinkle the felt, making it harder for the shingles to lay flat.  Therefore, shingles should be installed immediately after felt roofing underlayment is installed if possible to ensure optimal protection.

  • Felt underlayment also weighs more, which can make it harder for roofing contractors to drag rolls of it up a ladder and onto a roof.

  • It also has a slippery surface, which can sometimes make it more difficult to install. 

  • The weight also leads to less material per roll.  This means more potential seams instead of a single course with no laps.

Felt Roofing Underlayment and Warranties

If felt underlayment is installed it may also prevent you from being protected under the manufacturer’s warranty, which may require synthetic underlayment.

Synthetic Roofing Underlayment

For enhanced water-resistance and protection from the elements, many roofers are choosing to go the route of synthetic roofing underlayment. These products are usually made from long-lasting polymers, which provide added strength and longevity. This type of underlayment is typically moisture-resistant, and when it’s installed correctly, it offers better protection from the weather compared to felt.

Synthetic roofing underlayment materials are not standardized, so different manufacturers may make their products differently, and therefore may have different levels of performance. 


There are four main advantages to installing synthetic roof underlayment rather than felt. Compared to felt, synthetic roofing underlayment is:

  • Tough

  • Fast to install

  • Safe

  • Repels water

Synthetic underlayment has a tough and durable construction with an extremely high tear strength compared to felt.

Synthetic roof underlayment is extremely durable. It typically doesn’t tear and is suitable for extended UV and moisture exposure in some cases, which is especially helpful if there’s a bit of lead time before your roof covering is installed.

Synthetic roofing underlayment also tends to be:

  • Lighter* – Up to four times lighter in some cases

  • Fast to install – Because there is more material per roll compared to felt (synthetic roofing underlayment comes in wider and longer rolls), it results in fewer trips up the ladder for your roofers, saving them time and perhaps helping the job move along faster. For instance, a typical 2700 square-foot home might require three rolls of synthetic underlayment compared to 14 rolls of No.30 felt to cover the same area.

  • Safe – Synthetic underlayment is also advantageous for worker safety — the surface of many synthetic roofing underlayments, including those offered by ResPros Roofing Siding and Gutters, features a variety of slip-resistant surfaces for enhanced walkability. It’s also usually well-marked with overlap guides and indicators of where fasteners should be placed, helping to improve consistency and accuracy during installation.

  • Moisture-resistant – Where felt products tend to absorb water, synthetic roofing underlayments are built to repel water. This is important for homeowners concerned about moisture infiltration, especially if they plan to leave the underlayment exposed for a prolonged period of time.

Because it’s made of plastic, synthetic underlayment is typically resistant to mold growth, a definite advantage over felt.


Many synthetics are competitively priced, but when compared to felt, the main drawback of synthetic roofing underlayment is the cost. The upfront investment in higher quality roofing materials, however, could save you money down the road. You can’t put a price on the peace of mind knowing that your roof is sufficiently protected from moisture.

The Right Underlayment for Your Roof

Whether you’re embarking on a reroofing project or new home construction, there are many factors to consider about the type of underlayment to use. Synthetic roofing underlayment has many advantages over felt and may be a worthwhile investment to protect your roof and home from the risks of water and moisture infiltration.

4. Roof Structure:  Rafters and trusses make the support for your roof.

Rafters are typically 16" apart 

Trusses are 24" apart

Trusses can cause the roof sheathing to look wavy because of the span and distance in between the roof members.

5. Flashing: This is the material that's installed at your walls or fireplace. Flashing is also the pipe flashing that protects your ventilation pipes from leaking.  

6. Roof ventilation and ridge caps 

7. Leak Barriers: Ice and water protector, sometimes also referred to as “peel and stick”, is a waterproof roof underlayment membrane developed to protect vulnerable areas on a roof from ice and water damage. Ice and water protectors (sometimes called ice and snow shields in cold climates) are made with polymer-modified bitumen.  

Ice and water protector products have a modified bitumen adhesive back surface that is covered by a release film. The film is removed during application so that the membrane will adhere to the roof deck and also form watertight end laps and side laps when properly applied. The membranes are designed to form a watertight seal around the nail penetrations when shingles are applied.

Potentially, covering an entire roof with ice and water protector can prevent leaks even after high wind conditions blow shingles off the roof—preventing further damage and making repair easier.  However, because the modified bitumen back surface is continuous, these membranes can result in a vapor barrier. 

8. Roof Ventilation: Roof ventilation is based on the simple fact that warm air rises. In summer, the sun heats air in the attic. In winter, heat from your home warms attic air. In either season, good venting occurs when cool air can enter the attic near the eaves and exit near the peak. Ideally, half of the vent area should be low and half high. The ultimate goal is that the temperature and humidity levels in the attic space match the conditions outdoors.

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