Underlay for profiled metal roofing

An isolating layer is usually installed over the supporting framing before roofing is installed. There are several types used in domestic construction:

• Fully supported (on wire mesh) breather-type (R1 or R2) bitumen-impregnated roofing underlay or kraft building paper. Building paper is most commonly used in domestic buildings, garages and some light commercial buildings. 
• Fully supported breather-type fire-retardant kraft building paper. Fire-retardant building paper is used in some domestic buildings but is more common in commercial and industrial buildings.
• Self-supporting roofing underlays designed to span between purlins where the purlin spacing is not more than 1200 mm. They are a breather-type material consisting of bitumen-saturated heavyweight kraft paper or fire-retardant solvent-free laminated (two layers) reinforced heavyweight kraft paper, coated with a water-based water repellent. They are designed to function in the same way as building paper. Support of self-supporting roof underlay may be required where the roofing material is dark coloured or when exposed from underneath. Self-supporting underlays are used in both domestic and commercial buildings. 
• Fully supported or self-supporting proprietary breathable absorbent synthetic roof underlays. Typically, these products consist of micro-porous water-resistant film laminated between layers of spun-bonded polyolefin. Currently, synthetic roof underlays are outside the scope of E2/AS1, but some products are covered by a BRANZ Appraisal.
• Fully supported or self-supporting proprietary breathable absorbent synthetic fire-retardant roof underlays. Currently, fire-retardant roof underlays are outside the scope of E2/AS1, but some products are covered by a BRANZ Appraisal.

While material can be installed vertically (down the slope) or horizontally (across the slope), some manufacturers and BRANZ recommend horizontal installation. Horizontal installation is mandatory when the building is being designed in accordance with E2/AS1 and the roof pitch is 10º or less. The location of roof underlay must always be shown on construction drawings and the material type identified in the specification or on the drawings. 

Running the sheets vertically allows the paper to be quickly covered by the roofing, but it is possible to progressively lay the paper horizontally by using a number of rolls on the roof and progressively unrolling by one roofing sheet width. 

Considerations for which way to run the underlay 

For horizontally laid material: 

• Condensation or water that is stopped by the underlay can be shed to the gutter without the risk of leaks. (When underlay is installed vertically, water can migrate through the side lap, particularly where it may have sagged or shrunk.) 
• The material width may not suit purlin spacing, particularly with self-supporting material, and can make fixing difficult. 
• The logistics of running multiple rolls can be difficult on large roofs. 
• Rolls can ‘creep’ down the slope as laid. 

For vertically laid material: 

• Each length can be readily covered by the roofing to reduce the risk of wind action on the material without the need for multiple rolls being kept on the roof — for any roof, handling long lengths of light flexible material can be difficult. 
• It can be fixed to each purlin. 
• The risk of sideways leaking of any condensation is greater. 
• There is a lack of fixing along the lapped area (except for foils, which can be taped). 

Minimum laps of 150 mm are required for both horizontal and vertical applications under NZS 2295:2006 Pliable, permeable building underlays.

Installing underlay

Good practice for the installation of supported roof underlays: 

Good practice for the installation of self-supporting roof underlays: 

• Don't expose them to the rain or dew as it will cause them to sag.
• Don't rely on them to arrest falls. 
• Ensure lap distances are maintained.