In the furnishings market, the terms underlay and underlayment refer to a layer of cushioning under a carpet, in order to provide insulation and deaden sound. However, in the construction industry those terms refer to a layer of waterproofing material under the roof covering.

This underlayment seals the roof a second time, and controls any leakage past shingles, tiles and other roofing materials. There are various types of roof underlays on the market. Building code requirements, the choice of roof overlay, and the local climate are important decision criteria.

Two Types of Roofing Underlay for Shingles

The International Residential Code sets guideline standards and model building codes, and supports roof underlays under shingles. Although these have no legal status in Canada, our national state and building standards generally follows those norms.

Roof underlayment is generally regarded as good practice, although is not a specific legal requirement in Alberta. Where it is installed, which very often the case, there are two types in general use namely traditional felt, and synthetic, polymer-based roof underlayment.

Lower Cost, Popular Felt Roof Underlayment

Felt is a generic term for matting produced by condensing and pressing fibres together. These fibres may be organic wool or fur, or synthetics such as petroleum derivatives, or wood-pulp-based rayon.

Manufacturers turn this felt into ‘tarpaper’ by soaking it in asphalt or bitumen. This increases its resistance to moisture passing through. Felt underlay is popular because of relatively low price and traditional custom.

However, it’s important to note that tarpaper comprises organic materials including recycled paper or sawdust in a cellulose case. This progressively degrades, and admits some moisture over time as it gradually fails. It also weakens if exposed to sunlight before the roof covering lays over it.

Slightly More Expensive Synthetic Roof Underlay

Artificial roof underlayment comprises woven synthetic polypropylene fabric. This is considerably lighter and stronger that traditional felt tarpaper, and withstands degradation by ultra-violet light and adverse weather for months. This is a distinct advantage if shingle or roof tile deliveries are delayed.

Synthetic roof underlayment resists damage when the roof is laid over. It does not decay and should last the lifetime of the shingles.

Where to Go for Further Advice

Valiant Exteriors Ltd in Calgary, Canada are expert building water proofers in the roofing, eavestrough, soffit, downpipe and siding niches. We are proud of the big reputation our small, privately-owned company has earned, and we cherish it in every job we do.

Please call us on (403)829-1661 if you are planning an underlay and shingle roof repair, or replacement. We will be delighted to provide our free estimate. We are waiting for you to reach out here.

More Information

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Tarpaper Going Over Heat Shield: Unknown Author: CC 3.0