When the rafters and the frame of the roof have been fitted or, if you are simply replacing the tiles or slates, when they and the batten underneath have been removed, the next stage is to lay the roofing felt or roofing underlay. This is to ensure that the roof is ready for the battens to be fitted, which will secure the tiles or slate in place. At JJ Roofing, we stock and supply a complete range of sarking felt products, check out our breathable membrane products. If you have been asking: "Is roofing felt waterproof?" then please keep reading below to get your questions answered.

Sarking Felt

What is a Roof Underlay?

The practice of lining roofs with a permanent sheet roofing underlay has become a universally accepted practice in the UK over the last fifty years and people are now asking: "Is roofing felt waterproof?" The sheeting or sarking felt, is laid over the supporting rafters or counter battens, and beneath the tile or slate battens. Previously it had been standard practice to underline slate or tile roofs with a mortar filling consisting of sand-lime mortar and reinforced with animal hair.

The main purpose of roofing underlay is to reduce the effect of wind loading on the slate or tile roof covering, according to BS5534 – the Code of Practice for Slating and Tiling.

The secondary purpose of sarking felt is to provide a waterproof barrier and allow for the safe disposal of water that has collected on the upper surface of the underlay. This prevents damage to the internal spaces of the roof and building and creates an effective barrier against rain ingress as a result of damaged and cracked tiles/slates. So is roofing felt waterproof? Yes.

The third benefit of roofing underlay is that it provides an extra layer of insulation to prevent heat from escaping the building.

Types of Roofing Felt

Traditional bitumen sarking felt is the cheapest option for roofing underlay, and is readily available. It is slightly different from flat roofing felt as it includes a strong woven base, but it can become brittle over time and can rot into the guttering where it has been exposed to sunlight. There are other, more modern alternatives available, which tend to be lighter and more durable if a little more expensive.

Pitched roof underlays are classified by the British Standard BS 5250 as:

  • HR (High Water Vapour Resistance) Underlays – These membranes have a high water resistance, but will always require some form of ventilation. In the case of cold roofs, ventilation of the loft space will be required; and in the case of warm roofs, between the insulation and the underlay.
  • LR (Low Water Vapour Resistance) Underlays – These membranes are also known as ‘Vapour Permeable’ (VPU), or ‘Vapour Open’ underlays, and provide an extra method of vapour transfer which can be used in conjunction with ventilation for cold or warm roofs.

LR underlays are divided into two general sections:

  • AirTight – These consist of a three or four layer vapour permeable membrane. The middle layer of the sarking felt is a vapour permeable film which provides vapour transfer and high water hold-out properties.
  • Air Permeable –These are generally made up of three layers, to create a vapour and air permeable membrane. They tend to have high vapour permeability but lower water hold-out properties.

Laying and Fitting Roofing Underlay

The felt roof underlay is secured by nailing direct to the rafters and then held by the battens which will be fitted to secure the tiles or slates too. As the sarking felt can be easily damaged by strong winds, it is advisable to wait for a period of time where the felt and battens can be fitted in quick succession, during fair conditions.

The fascia board of most roofs tends to stand above the level of the rafters, and because of this, it is advisable to cut and fix filler wedges to each rafter. An alternative is to nail narrow strips of plywood across the gaps between the rafters and the top of the fascia board. Either of these methods will help to support the roofing underlay and prevent sagging behind the fascia, which can eventually lead to rotting of the sarking felt and dampness of the soffit or wall below.

  • Starting at the bottom, run the sarking felt along the roof, and align it so that the lower edge extends beyond the fascia, roughly to the middle of the gutter.
  • Beginning at one end, nail the felt to the rafters at the mid-point of the width of the rafter and approximately 250mm in from the fascia on every other rafter. Leave the top of the sarking felt unsecured at this stage.
  • Work along the roof removing any excess slack in the felt, but don’t pull it too tight – a slight sag between rafters is ideal to allow water to run off.
  • When the first length is secured, lay the next layer of sarking felt on top so that it overlaps by approximately 100mm. Nail the second strip to the rafters in the same manner, with the lower nails positioned 50mm from the edge so as to secure both strips of sarking felt.
  • This sequence should be repeated until you reach the ridge of the roof.
  • When you reach the ridge take the felt approximately 150mm over the top of the ridge. Bring the felt from the other side of the roof over the top of the felt from the first side, and nail it through the top of the rafters on the first side.
  • At a verge, lay the roofing underlay approximately halfway over the outer wall skin, or the outer rafter on an overhanging verge.
  • If the roof abuts to a wall, trim the roofing underlay to allow roughly 50mm to go onto the wall.
  • At a hip, take the underlay from the first side around the corner, then overlap from the second side by approximately 150mm. Take care to fold the underlay in such a way that no pockets are left which could collect water.

At this stage, the roofing underlay is vulnerable to prevailing conditions, as the battens which secure it are yet to be fitted. Felt tacks or batten nails turned over, can be used as temporary fixings until the battens are installed, but it is essential to fit the battens as soon as possible to prevent damage. In Scotland, there is a requirement for counter battens to be fitted prior to the underlay being installed to prevent damage by strong winds and bad weather conditions.

The next stage after laying the roofing felt is to fit the battens to secure it, ready for installation if the slates or tiles.