A general concern of the UK public will always be “Is my roof waterproof?” due to the bad weather conditions the UK is famous for. This is mainly due to the fact that although the practice of lining a roof with permanent sheet roofing underlay is a universally accepted one in the UK and has been for the past fifty years or so, not many people know what roofing felt actually is, does, or how it benefits their homes. 

This guide will walk you through any concerns you may have about your roofing underlay, especially if you’ve asked yourself “Is roofing felt waterproof?”

What is roofing underlay?

As stated above, nowadays it’s common practice to line a roof with permanent sheet roofing underlay (also known as sarking felt), such as (Easy Trim’s Master Breather Membrane), the sheets are laid over the supporting rafters or counter battens in the roof, but underneath the tile/slate battens. Before this, it was standard practice to use a sand-lime mortar that was reinforced with animal hair to underline a slate or tile roof. 

The code of practice for slating and tiling tells us that the main purpose for roofing underlay is to “reduce the effect of wind loading on the slate or tile roof covering”- BS5534

There’s also a secondary purpose of sheeting or sarking felt is to also provide your property with a waterproof barrier, as well as to allow for the safe disposal of ant water that has collected on the upper surface of the sheeting. Due to this, it prevents any damage to the internal roof space and building, and it also creates another barrier but this time against rain ingress in case any tiles or slates become cracked or damaged due to bad weather.

So in short, is roofing felt waterproof? The answer is yes, it is. 

Another, third benefit of roofing felt underlay or sarking felt is that it provides an extra layer of insulation for the property as it will prevent heat from escaping. 

The different types of roofing felt

There are different types of roofing felt with different classifications and sections, which all have slightly different qualities and are all different costs.

Traditional bitumen sarking felt is the cheapest option for roofing underlay and is the most available, it differs slightly from flat felt roofing because it includes a strong base that is woven, although it can become brittle over time and rot into the guttering where it’s exposed to sunlight. More modern alternatives are available to purchase, although they’re lighter and more durable than the traditional sheeting, it tends to be a little more expensive. 

There are two classifications of pitched roof underlays that fall under the British Standard BS 5250. The two classifications are:

• HR (High Water Vapour Resistance) Underlays- These types of membranes have a high water resistance (hence the name) but they require a form of ventilation. For cold roofs, the loft space will require ventilation and for warm roofs, the space between the insulation and the underlay will require ventilation. 

• LR (Low Water Vapour Resistance) Underlays- These membranes are commonly known as “Vapour Permeable” which can be shorted down to VPU or “Vapour Open” underlays. These types provide an extra method of vapour transfer, this can be used along with ventilation for either cold or warm roofs. 

• AirTight- These membranes consist of three to four vapour permeable layers. The middle layer of the sheeting or sarking felt is a vapour-permeable film that provides vapour transfer and also contains high water hold-out properties. 

• Air Permeable- Unlike AirTight membranes, they only contain three layers which create a vapour and air-permeable membrane. These types of membranes tend to have a higher vapour permeability but lower water hold-out properties because they do not contain the vapour-permeable film layer. 

What happens when roofing felt gets wet?

When any of the types of sarking felt gets wet, it acts more like a barrier as opposed to a sponge due to its water repellant properties. The felt absorbs and repels the water, but if the surface beneath the felt gets wet, it can lead to the dissolution of the subsurface as well as the roofing felt.