Roof Underlays/underlayments For Continuous Supports

This is a much different case compared to non-continuous supports which we examined in previous articles.

 

Roofing underlays should exhibit two principal characteristics:

 Thus the following types of underlay are automatically excluded:

 

 * all underlays based on microperforation techniques

 

* most of the breathable underlays and especially those with a two-layers configuration and/or weight less than 150gr/m2.

 

* all thin bituminous underlays covered with talcum or light sand.

 

 Roofing underlays: possible options for continuous supports

 



 1) SBS, self adhesive, elastomeric membranes.

 

This is by far the best option when asphalt shingles are used as roof covering.

 

These membranes are also ideal for low slope roofs and when ice dam formation is possible.

 

 2) All types of thick asphaltic membranes, conventional roofing felts etc.

 



 3) Many breathable 3 or 4 layer underlays with a high grammage.

 



 4) All types of non-breathable synthetic foils destined for roof waterproofing.

 

 When tiles are used as roof covering, they should be fixed on tiling battens. Counter battens are indispensable for ventilation and drainage reasons.

 

Counter battens will be fixed through the support into the rafters (not just to the support!!).

 

The roofing underlay will be fixed in parallel with the eaves with side overlaps from 10-20cm depending on the slope. A slope of about 30% is usually the tidemark for the overlap change.

 

The fibrous part of the overlay will be in contact with the support. This will provide the antiskid properties.

 

Underlays should be fixed with big head diameter, galvanized nails just like shingles nails. Nail heads will be covered, when feasible with underlay overlaps.

 

Ventilation under the continuous supports is obligatory both for warm and cold roofs!!!

 

 

 Chris Strogilis

 

Civ. Eng. - MBA - DipM

 

http://monosimacon.blogspot.com/