Clay Tile Roof

What do you put under roof tiles?

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 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.

What type of roofing underlay should I use?

Roofing underlays are one of the most important elements of a roofing specification, and choosing the right product is essential to ensuring the finished structure is weather-resistant and provides years of trouble-free service. But there are many different types of roofing underlays on the market – so how do you know which product is the right choice for your project?

The first step to selecting a roofing underlay is to understand the available types, and what’s right for your project. Then you can narrow your selection by considering installation requirements and building regulations, as well as the unique features and benefits that certain products offer.

The different types of roof underlays

According to BS 5534, the British Standard for slating and tiling, and BS 5250, the code of practice for the control of condensation, there are two main types of roofing underlays: high resistance (non-breathable underlays) and low resistance (breathable or vapour-permeable membranes). Let’s look at these in more detail:

Non Breathable Underlays

These include both traditional bituminous products, as well as impermeable roofing membranes made from modern materials like polypropylene. They are often perceived as the affordable and functional option and are designed to provide an effective secondary barrier against wind pressure and water penetration.

Breathable Underlays

This is a newer type of product, and low-resistance underlays are generally more expensive than their non-breathable counterparts. However, they offer several benefits in addition to their breathability. For example, they’re lightweight, durable and easy to install, and they also provide some advantages when it comes to roof ventilation.

The basics of fitting

The felt is held to the rafters by first nailing direct and then by the wooden battens which are used for fixing the slates or for locating the nibs on the back of tiles.

The fascia board of most roofs stands above the level of the rafters so before laying the underfelt, you may want to cut and fix filler wedges to each rafter, or alternatively nail narrow strips of sheet material (such as exterior grade plywood) across the gaps between the rafters and the top of the fascia (see diagram below) – either method supports the underfelt otherwise there is a tendency for the felt to sag behind the fascia allowing water to collect which will eventually rot the felt and cause dampness to the soffit or wall below.

Fitting the felt

Starting at the bottom, run the felt along the roof and align the felt so that the lower edge extents over the fascia board by enough to reach the middle of the gutter. Starting at one end, nail the felt to the rafters using galvanised 25mm (1 inch) clout nails, put a nail in the middle of the width, and about 250mm (10 inches) in from the fascia on every other rafter (leave the top of the felt unsecured at this point). Work along the roof taking out any excess slack in the felt, but do not pull it tight; a slight sag between rafters is ideal as it will allow any water to drain down the felt.

When the first length is complete, lay the next layer of felt on top of the first so that it overlaps by at least 100mm (4 inches) – horizontal overlaps (where one roll ends, and the next one is started) should be at least 150mm (6 inches). Nail the second strip of underfelt to the rafters in the same manner as the first, with the lower nails positioned about 50mm (2 inches) from the edge so that it secures both this strip and the previously laid one.

Repeat this sequence with further strips of felt up to the top of the roof.

  • At the ridge, take the underlay over the top of the ridge by at least 150mm (6 inches), then, when installing the top run of felt on the second side of a pitch roof, take the felt over the felt from the first side and nail it through to the top of the rafters on the first side.
  • At a verge, lay the underlay about half way over the outer wall skin (or the outer rafter on an overhanging verge).
  • Where the roof abuts to a wall, either at the side or top, trim the underfelt to allow about 50mm (2 inches) onto the wall.
  • At a hip, take the underlay from the first side around the corner, and overlap from the second side by at least 150mm (6 inches). Make sure that all folds are done so that no pockets are left where would could collect.

Fixing the battens

Start at the lower edge of the roof and position the first batten so that the slates/tiles give the required overhang over the fascia – set the distance at both ends of the roof and run a string line between, check along the line to make sure that the distance is fairly consistent and satisfactory (fascia boards may sometimes wonder a little bit, but this can be checked by eye looking along the front face).

Cut the first batten to length, if one length will not cover the full roof length, cut is so that it is half way across the rafter where the next batten needs to meet it.

Nail the lower batten in place using the string line as a guide, use galvanised wire nails (60 mm ( 2½ inch) are normally adequate) to secure to each rafter – ideally the nails should penetrate the rafter timber by at least 40mm.

Once the lower batten is fixed, use the two ‘gauge’ spacers to position the next batten, again measure the length and cut before nailing.

  • At verges, take the battens to within about 25mm (1 inch) of the end of the underfelt.
  • At hips, allow enough space between the battens from the two sides so that a batten can be fixed along the top of the hip board.

Repeat this sequence for each batten up to the top of the roof.

Should any of the horizontal felt overlaps not coincide with a batten, fix an intermediate batten just to hold down the overlap, but ensure that if using tiles, there is enough room next to the primary batten for the tile nib.

You may need to slightly deviate from using the gauge for the top batten (any minor cutting errors in your gauges may add up over the height of the roof), adjust it as necessary.