Roof Underlay

About Roof Underlay

Roof framing and lining play a vital role in maintaining the integrity of the whole structure for roofs in New Zealand. Each part of the roofing system supports each other with specific properties. 

The roofing underlayment is a protective layer that is found on the roof deck surface. It’s composed of a water-resistant material to protect the deck from water intrusion. While barely visible, roof underlayment also serves as a vital barrier from excessive heat, rain, and snow.

Advantages of Installing a Roof Underlay 

• It provides protection against weather conditions which can be harsh in New Zealand that shingles can’t handle on their own. 
• It prevents water build-up by allowing water droplets to slide off the surface. That way, it prevents the shingles from growing brittle and cracking over time due to extended exposure.
• While roof tiles and shingles make the first line of defence in preventing potential water leakages, waterproof underlayment supports the function as the outermost layer of the system.

Types of Roof Underlay

There are various types of roofing underlayment in New Zealand, each with its own set of advantages that vary in performance towards elements. 

Non-Breathable Underlays

Traditional bituminous products and watertight polypropylene roofing membranes belong to the category of non-breathable underlays. They are a cost-effective solution for establishing an effective secondary barrier against wind pressure and weather infiltration. Used in combination with eaves (low level) and ridges (high level) ventilation systems, they are similarly effective in allowing air circulation as their breathable counterparts. 

Non-breathable underlays are particularly convenient for use in New Zealand where roof systems require counter-battens to ventilate the batten void. Both lightweight or reinforced bitumen membranes work well in this case. 

Breathable Underlays

Breathable membranes are a modern, low-resistance variety of underlays. Along with a higher initial price tag in New Zealand, they bear some additional benefits. Breathable roofing papers are lightweight, durable and easy to install.

There are two different types of breathable membranes in New Zealand: vapour permeable (VP) and vapour-and-air permeable.

• Vapour permeable (VP) is the prevalent type, sometimes wrongly perceived as a sole means of roof ventilation. However, it’s best to use them in systems that will ensure sufficient cross-flow ventilation to eliminate all water vapour from the building.
• Air and vapour permeable breather membranes allow the condensation to move out by passing through. This method helps to reduce the amount of moisture build-up within the roof or wall and insulation, preventing issues such as mould and rotting, which can lead to severe structural concerns in the long run.

Breathable membranes can either be placed on the outer edge of the insulation board or roofing board or fastened to wall structures. 

Asphalt-Saturated Roofers Felt

Asphalt-saturated felt is also known as tar paper or felt paper in New Zealand. It’s a water-resistant type of underlay, traditionally in use before synthetic products appeared. While it’s typically made as a mixture of asphalt, polyester, and bitumen, it’s also available as an organic substances compound with a cellulose base.

The asphalt-saturated felt offers durable protection from debris, hailstones, and heavy solid objects.

Felt underlayment type is often found on steep-sloping roofs where water can run off quickly. The reason is that felt is prone to wrinkling. Flatter roofs sometimes retain water for extended periods, during which the felt will absorb a certain amount. It can also deteriorate after prolonged exposure to direct sunlight and heat. 

Rubberised Asphalt

Rubberised asphalt is a waterproof material, composed of high concentrations of rubber polymers mixed with asphalt. It is also the only type of membrane with a 100% waterproof seal, making it an ideal solution for cold or snowy regions. Rubberised asphalt is typically available with its own self-adhesive in New Zealand, protected by a peel-off membrane. Some products also include polyester film or fibreglass to prevent moisture build-ups.

Non-Bitumen Synthetic Underlayment

Modern non-bitumen synthetic underlayment type is saturated with asphalt, enhanced with fibre-reinforced plastic (fibreglass). It’s water-resistant, resilient to fungal growth, and exceptionally durable and flexible. Another notable characteristic of the synthetic underlayment is the high UV resistance, suited to prevent damage to the roof’s surface exposed to excessive heat, extremely important in New Zealand.

Synthetic Roofing Underlayment VS Felt

Asphalt-saturated felt is often a solution of choice for steep-sloped roofs. It’s easy to install and maintain. Non-bitumen synthetic underlayment is favoured by professionals due to its strength, followed by a series of advanced functions. While less affordable than other types, it becomes cost-effective over time, offering more robust and reliable roofing reinforcement.

Things to Consider Before Buying Roofing Underlays in New Zealand

When installing vapour permeable membranes, it is important to use strong tapes. Fastening each layer of membrane, particularly on the joints, will secure it to maximise leakage protection. 

The fire-resistance rating is an essential factor that needs to be considered when looking to buy an underlay. While the majority of underlayment types offer some level of protection against smoke, their flame protection properties can fluctuate a lot. A rating of A, B, and C will make the picture more clear. A marks the best performance, while C is the worst.

The price of the underlayment should fit with the available budget. However, it’s good to keep in mind that a more water-resistant, more durable membrane with higher overall quality can save the costs of repairing the whole roof sooner than later. 

If you need assistance to find the right roofing underlays for your home or office, ArchiPro provides a connection to various professionals in New Zealand, ready to assist with all requirements and questions. Simply click "Contact supplier" on the product's page, or visit the supplier's website directly from the product's page.