Felt vs Synthetic Underlay: The Pros and Cons of Each

When deciding on roof materials, most homeowners only think of the outer layer. However, your roofing will also require an underlay which is placed between the outer material and the decking. Underlay is important to form a water barrier, prevent damage to the roofing deck and to offer additional sound and heat insulating properties.

In general, you will be choosing between either synthetic or felt underlay, so you'll have to consider the pros and cons of each option to come to the right decision.

The Pros and Cons of Synthetic Underlay

Synthetic underlay can be made from many materials, including fibreglass, polyester, polyethylene and polypropylene, and it comes with a number of strong advantages over felt.

Perhaps most importantly, synthetic underlay is much lighter than felt underlay. What this means for the homeowner is that enough can be added to form an effective water barrier without adding a significant amount of weight to the roof. Its lightness also makes it very easy and safe to install, and it degrades at a much slower rate than felt.

On the other hand, synthetic underlay is far more expensive than felt underlay. There are also a few things that can go wrong during installation that can weaken the water barrier. Finally, one reason why many homeowners need to give up on synthetic underlay is because their local building codes do not allow it, so make sure you check these before you begin work.

The Pros and Cons of Felt Underlay

Felt underlay is manufactured using asphalt-saturated paper. It is a far better option for any homeowners that don't have a huge budget to play around with since it is dramatically cheaper than synthetic underlay but still performs well. If you're planning on installing it yourself, there is less that can go wrong compared to synthetic, giving you further opportunity to cut down on costs. Additionally, felt underlay is very seldom prohibited by local building codes.

That makes felt a good choice if you want to keep your costs down or are planning to move home in the next few years anyway. However, it does add more weight than synthetic underlay and will take longer to install, even if less go can wrong. It also degrades a lot faster than synthetic, so you'll be looking at replacing your underlay sooner. Since the exterior roofing material will need to be removed in order for work on the underlay to be carried out, this can be a very costly and inconvenient procedure.

Talk with roofing companies if you have specific concerns about the underlay your roof will need.