What Is Underlayment In Construction?

When you look at even the most technically advanced roof, you probably won’t be able to assess the type of underlayment that was used to install it. You will be in a very similar situation when you look at your siding or flooring. Regardless of the application, this group of building materials is extremely important in any home. So let’s find out what the role of underlayment is and what details to pay attention to when choosing it.

What Is Underlayment? Definition

What is underlayment in construction

As Builder-question mentions, underlayment is any material used as a base for rooftop material, siding, floor or any other flat surface. In many cases, underlayment allows an even and sufficiently strong bond between the finishing material and the building’s supporting structure. In the case of exterior finishing, a desirable feature of the underlayment is its partial water resistance – the membrane should not let water in, but should also wick away water vapor and moisture. Typically, the underlayment also stabilizes the surface finish, protecting it from cracking and crumbling. Underlayment can take a variety of forms, but typically, it is sold in the form of rolls, or possibly sheets. Popular manufacturers include Owens Corning, PAC-CLAD, Roofnado and GAF.

Roofing Underlayment

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In roofing, underlayment takes the form of a rolled membrane or film, which is most often made of a material resembling fairly soft rubber or foam, covered with a special coating. Sold in rolls, the material often has adhesive properties – so that shingles, metal sheets or other roofing product stabilizes. Typically, roofing membranes are installed in such a way that they maintain a complete seal. Otherwise, the roof sheathing could get wet, creating the perfect conditions for mold growth in the attic.

Depending on the roof design, your Chicago roofing contractor should use breathable or non-breathable underlay. There are no better or worse solutions – the membranes used for in cedar roofing must have different specifications from those that will be covered with asphalt shingles or aluminum.

Part II of the first chapter of the 2023 International Residential Code (IRC) defines underlayment as ‘One or more layers of felt, sheathing paper, nonbiutminous saturated felt, or other approved material over which a roof covering, with a slope of 2 to 12 (17-percent slope) or greater, is applied.’ In doing so, note that according to this source, underlay is not used in the construction of flat roofs. Technically, this is not true – flat roofs, made, for example, with TPO technology, popular in commercial roofing, also include an additional sealing layer.

Siding Underlayment

james hardie siding contractor lake bluff

Basically, siding underlayment resembles solutions used in roofing in many aspects. In this case, we are also dealing with adhesive (or self-adhesive) membranes, which usually allow water vapor to pass through. Exactly as in the case of roofing underlay, the layer installed on the walls should not leak, so that water does not cause mold growth.

It is worth mentioning the insulating properties that wall underlayment has. Admittedly, heat transfer is blocked primarily by the layer of mineral wool surrounding the structural elements of the house, but the airtight layer composed of the membrane reduces the cost of heating and cooling the building.

Wall underlay also stabilizes finishing elements such as vinyl siding or ACM panels. Thanks to its adhesive properties, the panels or boards are additionally held in place, which of course does not preclude the need for screws.

What Material Is The Underlay Made Of?

roofing underlayment materials

We mentioned that roofing and siding underlay can be made of many different materials, such as rubber, saturated felt, sheathing paper as well as many other polymers. Modern materials used by Owens Corning, for example, can somewhat resemble film tape or a thin, soft layer of acrylic glass. The manufacturing process is, of course, very complicated, and no brand reports what raw materials it uses, but it is usually made of polyethylene or polypropylene.

Some companies also use saturated felt, both asphaltic and non-bituminous. This is an older and still very good solution that provides an excellent seal. Asphalt has a relatively low melting point, so by heating it, you can create an excellent, completely water-tight barrier. Of course, this makes the transport of moisture more difficult, the weight of the material is also quite high. Durability of saturated felts is also not the best – as with many asphalt shingles, after 15-20 years you need to do a comprehensive reroofing.

roofing underlayment materials

Building paper / sheathing paper is another interesting technology. This solution has been known in the construction industry for many years. Perceptive people, however, may be confused – paper is not waterproof, after all. While it’s no good when it comes to insulating a house against leaks, paper works well for removing moisture through capillary forces – by soaking in, the underlayment transfers water vapor to the outside.

Is It Always Necessary To Use Roof Underlayment?

rotten plywood because of leaking underlayment

Roof underlayment is basically essential, especially for homes covered with either cedar shakes or slate tiles. Regardless of the material used, there is always the risk of a leak, which can cause very serious damage within a few months. Rotting plywood or OSB will instantly become covered with mold and begin to fall apart. The roof structure itself can also suffer – after a long time, rot will cause the rafters and other wooden beams to completely disintegrate.

Of course, it is possible to install a roof without underlayment – it is technically possible, but completely unjustified and harmful. The short-term and apparent savings are unjustified, because after just a few years, the entire roof on your house will be eligible for replacement. No sensible person should opt for this, because the underlay is an absolutely crucial shield to protect your home from water.

What Kind Of Underlayment Should I Choose?

Whether for the roof or the walls of the house, it is not worth saving on underlayment. The differences between the various products manifest themselves:

  • Weight – there are rolls of lighter or heavier weights, 15-lb and 30-lb as standard;
  • Material – synthetic underlayment made of polymers is more durable, but also more expensive;
  • Price – bituminous underlay is usually cheaper, but also less durable;
  • Longevity – asphalt ages much faster than polypropylene;
  • Duration of warranty – more expensive and reputable brands such as Owens Corning, Henry and GAF provide longer and more comprehensive warranty protection for their products.

In our opinion, in 2024 it makes sense to opt for a polypropylene, breathable membrane with a higher thickness. Replacing the underlayment always involves the removal of the rooftop material, which often means having to buy new shingles, but this occurrence does not happen in reverse. This makes it definitely not worth saving on the underlay and sometimes it is better to pay for a premium product.

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