Monarflex Roofing underlay

Monarflex Roofing underlay


What is a roofing underlay and where is it installed?

A roofing underlay is placed under the exterior roof and its primary function is to work as a shield that protects the thermal insulation material and the building structure from the effects of the weather. The roofing underlay thus works as a secondary roof underneath the primary roof.

Secondly, the roofing underlay must function as the primary weather shield during the construction period. The construction period should naturally be kept as short as possible as the roofing underlay are constructed to work as the secondary roof, not the primary.

How does roofing underlay function?

The roofing underlay acts as a condensation barrier, which leads condensed water away from the underlay, and thereby also the insulation material.

Roofing underlay’s can be divided into two groups depending on the construction of the roof where it is used:

  • Ventilated roofs – roof construction which needs ventilation gap above and below the roofing underlay. The insulation material must never touch the roofing underlay.
  • Diffusion-open roofs – roof constructions where only ventilation above the membrane is needed. The insulation material can be applied just under the membrane.

Ventilated roofs

Ventilated roofs 

Diffusion-open roofs

Diffusion open roofs


Before starting to install a roofing underlay it is extremely important to consider the slopping angle of the roof, as there are some minimum demands. These demands are often dictated by national traditions or requirements. However a general rule is that the roof slop must never come beneath 12,5° slopping angle when using a plastic based roofing underlay. Please consult local rules for minimum overlap and slopping angle for either vertical or horizontal installation.

Importance of accessories

In order to get the optimal protection out of the roofing underlay it is extremely important to recognize that the accessories used for the installation is just as important as the membrane itself. Choosing the right accessories will ease installation as well as help ensure the best possible protection for the whole roof construction. Monarflex carries a wide range of accessories that has proven it worth on thousands of roof. Further information can be found under accessories.

Choose the right product

When looking at the tremendous selections available on the market for roofing underlay it is quite overwhelming to get an overview. However choosing the right product for the construction is of upmost importance if the product is to last the lifetime of the exterior roof, which for the most common roofs is approximately 25 years.

The following conditions most be evaluated before choosing the right product:

  • What kind of exterior roof is used and can UV (sun light) shine in on the roofing underlay or not?
  • The construction type must be considered, is it a ventilated or non-ventilated construction?
  • Is it a boarded or non boarded (only rafters) roof construction?
    Angle of roof slope?
  • Duration of the construction period where the roofing underlay will be exposed to conditions of the weather e.g. sun light, rain, snow, wind etc. - long term or short term?
  • Climate conditions should be considered – condensation build-up?
  • Position of the building, e.g. sea site or in land site?
  • When looking at which product to use for different projects, it is difficult to give an answer, as it is very important to get the right product for the right construction. Please consult our data sheets or consult the sales office in your country for further information.


At Monarflex "we are setting the standard"


Contact Us


Monarflex contact details

Monarflex s.r.o.
Továrenská 1
943 03 Štúrovo

Slovak Republic

Phone + 421 36 7563865

UK & Ireland

Please visit the dedicated website at

Tel: 0161 8654444 (UK office)

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