Roofing felt is a very useful material that can underlay shingles and act as a moisture barrier between the wood of the wood and the shingles. In this way they act similarly to underlayment with asphalt flooring. This adds the extra benefit of a water resistant roof. Roofing felt is made of fibreglass or polyester fleece combined with tar or another bituminous material. It can enhance the ability of your roof to resist all types of weather conditions.

Correct installation of roofing felt is just as important as installing the shingles themselves. Because of the weight of the felt and the special handling methods, installing felt underlay is not an easy task. The optimal time to install roofing felt is in warm weather as this creates a better seam bond, adding the problem of hot weather to the work.

It is best to remove the old roof before installing roofing felt on an existing roof, and it is a requirement for a new roof.  If you are replacing shingles, make sure all the old material is gone and that there are no remaining nails sticking up which could damage the felt. You can buy felt in rolls of around 36 inches (91cm) and in lengths of 30 feet to 100 feet. Some felt also has a light grit on one side, which stops the felt sticking together when in storage.

How to Install Roofing Felt

First off, lay the first roll at the edge of the roof. Make sure you leave a drip edge of approximately one-inch (2.5cm) over the gutter. If possible this should all be in one piece, but you have to make a seam, the overlap should be 4 to 6 inches. On both sides, leave an overhang of around 2 inches to act as a drip edge or to extend underneath metal flashing.

Then second roll of felt should overlap the first piece by at least 6 inches. If you have to use another seam, try not to put it in the same place as the first seam. The seams should be at least two feet (60cm) apart.  If you have felt with grit on one side, you should lay the grit side facing up. Cold tar sealer should be used in the overlap between the two rolls.

If you have to make cuts to the rolls of felt, make sure you don’t do this on top of the other layers of felt. Use a cutting board underneath to protect the already laid felt from damage. This will ensure a nice neat cut, and will stop the knife accidently cutting into the other felt. Keep on adding rolls to the rest of the roof, and once you reach the peak let the felt lie over the top to act as the first layer on the peak.