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Re-Roofing: A Step by Step guide – Part 2

Part 2 - Roofing Felt

If the rafters are the heart of the roof, them the felt or underlay that lies on them is the last line of defence against water getting into the property.  It doesn't matter what type of roofing material you elect to put on your roof, a layer of felt is always needed.

Felt actually does two jobs; it works to add an extra waterproof barrier to the roof and provides an extra layer of insulation to prevent heat from escaping.  There are various types of roofing felt available with bitumen felt being the most widely used and least expensive.  It differs from roofing felt because it has a strong woven base that roofing felt doesn't have, though it can become brittle over time with sunlight exposure as roofing felt does.

breathable membrane from Cromar

The use of felt under the roofing material became common practise around fifty years ago.  Before that, it was normal to use a mortar filling composed of sand-lime mortar with animal hair added to underline slate or tile roofs.

Types of felt

Pitched roof underlays come under the standard of BS 5250 and come in two main types: HR underlays and LR Underlays.

HR Underlays are high water vapour resistance underlays.  These will need the ventilation of the loft space when used on a cold roof system or between the underlay and the insulation in a warm roof system.

LR Underlays are low water vapour resistance underlays.  This means they are vapour permeable or vapour open and uses an extra mechanism of vapour transfer.  This means they can be used with designed ventilation for both cold and warm roof designs as well as in non-ventilated cold and warm roof styles, as long as there is air permeability of the roof covering.

LR Underlays can be further divided into two categories: airtight and air permeable.  The former is normally a three or four layer vapour permeable membrane with the middle later being a vapour permeable film providing vapour transfer and high water hold out properties.  Air permeable underlays have three layers of vapour and air permeable membrane and have a high vapour permeable.

Underlay standard

To put it in another way, for an underlay to be defined as fit for purpose it must do the following jobs:

  • Provide a barrier that minimalizes the wind uplift on the roofing material
  • Work as a secondary barrier to stop water getting in
  • Work with the roof drainage system to funnel water into the guttering and not into the roof
  • For the LR underlays, it must provide adequate transmission of water vapour away from the area where roof battens are found
  • Give temporary weather protection (normally 3 months) before the installation of the roof material

Laying the underlay

The first thing to learn about when it comes to laying the underlay is lapping.  This is where each strip of the material overlaps with the ones on either side and makes sure that water cannot get past the layer.  Horizontal laps, also called headlaps, are the easiest to install because they can be measured down from the top edge of the underlay.  This can then be marked with a wet chalk line or even a temporary batten until the next layer of underfelt is rolled into place.  The lower edge of this second sheet is placed against the line drawn and then the two layers are nailed into position.

iko breathable membrane installed over the rafters and under the batten

The normal amount of a lap is 100mm based on a pitch roof of 35 degrees or steeper.  If the pitch is less than 35 degrees, the lap needs to be increase so for between 15-34 degrees, it should be 150mm while between 12.5 and 14 degrees, it should be 225mm.

Vertical, or sidelaps, should be 100mm regardless of the pitch of the roof.  The 100mm lap should start from the end of the new piece of underlay to the furthest edge of the last rafter, as opposed to the end of the lower piece of underlay.

Where the underlay is laid across rafters and there is no support, there should be a sag in the underlay.  This then forms a trough into which water can run and be kept away from the battens.  The sag will vary but around 25mm is recommended to do the job but not get sucked up by the wind.  A small sag is better than none so should an underlay material shrink and there be no sag, pulling it apart to create the sag is necessary.

Fitting the underlay

Unless stated otherwise, the underlay is nailed directly to the wooden battens that make up the structure of the roof.  Fascia boards on most roofs are found above the level of the rafters so filler wedges may be needed before laying the underfelt in these areas.

When fitting the underlay, start at the bottom, run the felt along the roof and line the felt up so that the lower edge extends the fascia board sufficiently to reach the middle of the gutter.  Usually, 25mm clout nails are used, putting the nail in the middle of the width and around 250mm from the fascia.  Pull the felt to remove slack but don't pull it too tight.

When it comes to the ridge, take the underlay over the top of the ridge by a minimum of 150mm.  When fitting the second side of the roof, take the felt over the felt from the first side and then nail it into the rafter on the first side.

At a verge, lay the underlay so that is it around half way over the outer wall skin.  Where a roof abuts a wall, trim the underlay to give around 50mm onto the wall.  At a hip, lay the underlay from the first side around a corner then overlap it with the underlay from the second side by around 150mm, similarly to a ridge.  Make sure there are no pockets of underlay where water could collect when doing any of these finishings.

Klober roofing felt installed under the batten and around the position of a new roof window for a loft conversion

Conclusion

Laying roofing underlay is an important step in the process of building your new roof and not one to be undertaken if you are not confident climbing up onto a roof with no roofing materials on.  Done properly, a good underlay will form a line of defence should a tile come loose or other weather incident happen and will also add to the insulation of the house.

 

Reference URLs

http://www.diydata.com/general_building/roofing/battens_felt.php

http://www.nfrc.co.uk/docs/tbs/tb06-pitched-roof-underlays-feb12CD69B50ED149.pdf%3Fsfvrsn%3D4

http://www.roofconsult.co.uk/articles/tiling/tips35.htm

 

Published at: 11-11-2014

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