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Waterproofing Of Pitched Roofs:Types Of Underlays

 

 

There are two broad categories of roofing underlays for the waterproofing of pitched roofs:

 

A) Asphalt underlays

B) Synthetic underlays/underlayments.

 

 

A.  ASPHALT BASED WATERPROOFING ROOFING MEMBRANES

 

Here we can find generally three subcategories:

 

1) Traditional roofing felts which most usually are asphalt saturated felts. In Europe these felts are produced at a weight of 2+ to 3+ Kg/m2 and are mostly reinforced with a glass fiber mat of 50-60gr/m2 (they tear easily).

These roofing felts are outmoded and gradually replaced by more technically advanced and sophisticated products.

They are used exclusively for continuous supports.

 

2) Thin asphalt membranes with weights ranging from 270gr/m2 (the lowest I ‘ve met, a nanotechnology based, avant-garde product) to about 1Kg/m2.

There are again many versions of these asphalt membranes with:

 

- different weights

- different reinforcements (mostly polyester of polypropylene fabrics)

- different finishings: plastic films, light sand, talcum, spun-bonded polypropylene fabrics etc.

 

Usually they are very strong products due to their reinforcement.

Most of them are used in non continuous supports e.g. on rafters.

Many of these products are prone to the tent effect: Please check the relevant entry.

These thin asphalt underlays are heavier, more robust and less liable to flapping noise compared to the synthetics.

A serious disadvantage is that they don't breathe and thus they need strict ventilation measures.

In Europe they are in constant retreat with breathable membranes steadily gaining ground.

 

3) SBS modified, self adhesive asphalt membranes.

These membranes are by far the preferred solutions in the following occasions:

 

3A) Asphalt shingles as roofing material: Nailing asphalt shingles on continuous support causes myriads of perforations through the underlays. In this case you would like to have an elastomeric, self healing membrane under your shingles.

 

3B) Generally when the slope is low. The lower they slope the more imperative the need for self adhesive underlays.

 

3C) In every climate that the formation of dams is possible. These special waterproofing underlays are the best defense for the dams consequences.

Next entry will be devoted to synthetic underlays.

 

 

B. SYNTHETIC UNDERLAYS

 

Synthetic underlays are lighter and wider than their asphalt based cousins. Usual weights range between 100-250gr/m2. There is a big variety of them.

A somehow arbitrary classification might be as follows:

 

1) Vapor barriers:

These underlays don't carry any incorporated technology to make them breathable or semi-breathable.

They are fabricated from polyolefins - mainly polyethylene - and are usually reinforced with a polypropylene grid.

Sometimes they carry a laminated non woven layer for antiskid properties.

These products must be used with very strict ventilation guidelines.

 

2) Microperforated underlays:

The usually consist of a polypropylene reinforcing grid sandwiched between two polyethylene films. The microperforation is achieved with special needles.

They are low cost products, used as economical alternatives in low importance constructions.

Due to the tent effect their use on continuous supports is prohibited.

 

3) Breathable-underlays: There is a wide variety of these products. The most typical underlay configuration is one of three layers where a special breathing, microporous polyethylene or polypropylene film is sandwiched between two spun-bonded polypropylenic fabrics (might also be polyester of polyethylene based).

There are products of up to 5 layers and there are also monolithic products.

Regarding breathability standards vary a lot.

BS for example, consider as breathables all roofing underlays with a vapor resistance inferior to 0,25 MNs/g.

Breathable underlays can be used in practically all roof configurations and are a must for warm roofs and also well recommended for cold unventilated roofs provided that necessary sealing measures will be taken.

When using breathable underlays check carefully for the following parameters:

 

3.1) possibility of flapping noise

3.2) temperature resistance. High temperature is a critical aging factor.

3.3) UV resistance: there is an enormous span.

3.4) Lamination method with ultrasonic method being the king.

3.5) The "tent effect": not all products are suitable for continuous supports.

3.6) Water penetration class.

3.7) Coefficient Sd.

 

Breathable underlays are getting better and better every year.

Light, strong, water vapor permeable, wider, non shrinking, antiskid and with a gradually more competitive cost, these products are destined to conquer the underlayment market.

 

 Chris Strogilis

 

http://maconwaterproofing.blogspot.com

 

 

About the Author

Civil engineer with postgraduate studies in MBA and Marketing