Roofing - underfelt and battens.
Safety: Working on a roof involves working at height (even on a bungalow), so always use scaffolding or other stable platform, never use just ladders.
When the new roof timbers have been completed or the old roofing has been cleared away and any necessary repairs made to the roof timbers, the next job is to fix the roofing underfelt and then the wooden battens for fixing the roof tiles/slates in place. (Note: in Scotland there is a requirements for the roof to be boarded and counter battens fixed before the underfelt is fitted.)
As the felt may be damaged by strong winds, it is best to wait until it can be quickly followed by fixing the battens and roof covering in reasonable weather.
What is felt underlay?
The underfelt provides a layer of insulation and an extra waterproof barrier for any moisture which gets under the roof covering. Traditional bitumen felt is readily available and is the cheapest option, this differs from flat roofing felt in that it incorporates a strong woven base. The traditional bitumen felt does tend to become brittle over time and rot into the guttering where it is exposed to sunlight. Modern alternatives are available which are generally lighter and more durable.
To obtain a combination of cost and performance, one option is to fit one of strip of the more expensive alternatives along the bottom of the roof, while using traditional bitumen felt for the rest of the roof.
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.
The manufacturer/ supplier of the slates or tiles will recommend the size of battens to be used but typically they are 40 x 20mm (1.5 x ¾ inch) sawn timber, where the distance between rafters is large and/or the roof covering is heavy, larger battens may be needed. Always use tanalised battens as they may become damp - most timber merchants carry stocks of tanalised roofing battens as a standard product.
Spacing the battens
The manufacturer/ supplier of the slates or tiles will also recommend the spacing up the roof between the battens (known as the 'gauge'), this usually varies according to the pitch of the roof and the degree of exposure. However, if using reclaimed slates/tiles, you will have to work this out yourself or refer to the literature for similar products. The spacing is in fact the same as the 'margin', which is the length of the slate or tile exposed. In some cases the manufacturer or supplier will specify the required overlap, typically it is around 75mm (3 inches), so the 'margin' and 'gauge' are just the overall length minus the overlap - be careful when working with slate as the 'head overlap' is sometimes quoted, this is from the fixing holes (halfway down the slate) to the lower edge.
The 'gauge' quoted by the manufacturer is the minimum required, you need to establish the 'gauge' you require for your roof. Things can get a bit difficult where there are different slopes on adjoining roofs or where there is a window in the roof. The manufacturer will specify a different gauge for different slopes of roof, where there are different slopes on one roof, choose the gauge for the lowest angle (which will be the smallest gauge) for all the roof slopes so that the rows of tile/slate will align.
- First establish the position of the batten for the lowest row of slates/tiles from the fascia - remember to allow for the overhang of the stales/tiles in front of the fascia.
- Now establish the position of the uppermost batten for the top row of slates/tiles - typically the top of the uppermost row needs to be at least 75mm (3 inches) under the edge of the ridge tile.
- Take the measurement between the top of the upper batten and the top of the lower batten and divide this by the manufacture's 'gauge' to give the number of slates/tiles required.
- The chances are that it won't be a complete number, so round it up to the next whole number.
- Now divide the distance between battens by the rounded number and you have the 'gauge' YOU require.
- Measurement between battens = 2.8m,
- Manufacturers 'gauge' = 250mm, so total 'gauges' required = 11.2.
- Round up to 12 then divide into the original measurement = 233mm.
This is only a rough guide - but it should illustrate that if you use the manufacturer's 'gauge' you could end up with an awkward part tile at the top of the roof.
If you have a window in a roof slope, work out different gauges for:
- From the lowest batten to the batten under the window.
- From the bottom of the window to the top of the window.
- From the top of the window to the upper batten by the ridge.
If you end up with 3 very similar gauges, you may choose to adjust the tile line above and/or below the window by using extended flashings so that you can use just one gauge. If you use different gauges, use them also on adjoining roof slopes to keep the tile line the same.
Once you know YOUR 'gauge' or 'gauges', cut a couple of pieces of timber for each 'gauge' length minus the width of the battens, you can then use these as spacers to set the distance between battens as you fix them to the roof.
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.