What Roofing Underlayment Should I Use

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What Roofing Underlayment Should I Use? Where do the irregularities in the selection of roofing materials and foundations for them come from? Many, even experienced, designers and contractors do not fully understand what is happening inside the roof - how its individual layers work together. In the article, we describe what are the types of underlays for roofing and what functions it fulfills.


Types of Undercoats for Roofing

The Undercoat is Available in Many Variants:

payment and high-vapor permeability foil;payment and low vapor permeability of the initial covering; self-payment without a film; formwork (stiff cover) without tar paper or foil; formwork and tar or foil with low vapor permeability; formwork and film with high vapor permeability; formwork and drainage mat.


Some versions are very similar, because they differ, for example, only with the type of roofing membrane used. However, such a seemingly small difference can be very important.


In order not to get lost in it and choose the right solution in a given situation, one must understand what functions the foundation plays :

Stiffens the roof structure; it is the support for the cover;it is a barrier to rainwater that will get under cover;it can be a barrier to water vapor escaping from the attic or allow it to freely penetrate. Functions of undercoats for roofing.



The backing is a support for the cover and transfers its weight to the roof truss. At the same time, however, it stiffens the roof structure itself, connecting all the girders together. It is best to do this task with large wood-based panels, forming a stiff shield. The most susceptible to deformation is the grate with battens and counter battens. Therefore, changes in the type of plating, as well as spacing and cross-section of patches should be agreed with the designer. The differences can be very large, for example, the modification of the method of laying traditional carp tile tiles - instead of "husk" for "lace" - means a two-fold increase in the spacing of patches (from about 15 to 30 cm).



The backing is a support for the cover and it depends on its stiffness what solution will be appropriate. Roof tiles, roof tiles and profiled sheets (above all trapezoidal) are so rigid that they do not need to be supported all over their surface - a grate with patches is enough. For flat plates, to avoid buckling, solid support is needed in the form of formwork. However, spacing of several centimeters between the boards can be left.

However, Jasper Contractors suggest that for roofing felt and shingle (bituminous) shingles it is necessary to provide the most even foundation - preferably in the form of large wood-based panels, or boards combined with a tongue and groove. Bitumen covering materials are simply very flexible and adapt to any unevenness in the substrate. If it is uneven, the roof will look unsightly.


In practice, no roofing is completely waterproof to rainwater. Small amounts of it can always get under cover, eg as a result of strong wind during rainfall. Tightness depends both on the characteristics of the coating itself (a large sheet of roofing sheet will always be more tight than small ceramic tiles) and the angle of the roof (the steep roof runs faster and the risk of leakage is lower). A tight undercoat is like a second covering, securing a truss, insulation and attic against rainwater. The Germans sometimes refer to them as the "roof of the suburbs". Due to the tightness, the sleepers should be arranged in the following way :

Roofing felt or roofing foil on a stiff cover (boards or boards);roof foil glued on plants; roof foil arranged with at least 10 cm of the plant.


Roofing Underlayment

What Roofing Underlayment Should I Use? There are different types but depending on weather, your preference, or what is just gernerally the better option is going to be up to you. The source of moisture may be rainwater and water vapor, penetrating from the attic. Only on non-utility attics there is no problem of water vapor and ventilation of the roof structure, because it is uncovered from the attic side. On the attics in the roofs, we have thermal insulation , which is protected from the utility rooms by vapor barrier foil. However, this is not a perfect barrier and a certain amount of water vapor penetrates the roof. Layer system, including the construction of the undercoat, must allow its removal outside.

Ventilation slots must be unobstructed over their entire length - from the eaves to the ridge. However, in roofs with a complex shape, it is very difficult to achieve this, it can help, for example, the use of special roof tiles.


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Chrissy Boltan

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