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How to apply roofing felt adhesive

How to apply roofing felt adhesive

Posted on 10 March, 2022

Applying roofing felt adhesive is no easy task, it can’t just be done with no instruction or preparation, as with anything it’s best that interested roofers follow a guide; at BC Profiles we’re more than happy to be that guiding figure. 

This guide will also have a rough outline of how to install the first and second layer of felt onto a flat or shed roof but to find out in more detail our blog post “How to felt a flat or shed roof”.

Preparation and tools

In order to apply roofing felt adhesive effectively, you’ll need to carry out the relevant surface preparation and have the correct tools. 

To prepare the surface where the adhesive will be applied, you’ll need to check the following:

  • Measure the surface area of the roof so you know how much felt you’ll need to cut and how much adhesive will need to be applied. 
  • Ensure that the felt adhesive is stored in a warm room before otherwise, it will thicken in colder weather
  • That the surface is clean and dry and is free from debris, rust, dirt, and grease.
  • Ensure there’s no existing felt or adhesive on the surface which needs to be felted. 
  • Any traces of algae or fungus growing on the surface should be removed immediately using a stiff-bristled brush.
  • To add to this, any areas where algae or fungus have been present should be treated with a fungicide to kill any remaining spores, which will discourage the return of the growths. 
  • If the surface is a porous surface, use a bitumen primer and allow it to dry before you proceed. 
  • Also, ensure that the felt which will be laid is cut to an appropriate size and positioned flat for 24 hours to assist application

The following tools will need to be used to apply the roofing felt adhesive as well as the roofing felt:

  • A stiff brush (50mm), mop, or squeegee to apply the roofing felt adhesive
  • Appropriate roofing felt adhesive such as Black Jack roofing felt adhesive
  • A hammer
  • Flat pry bar
  • Galvanized clout nails
  • A Tape measure
  • Suitable lengths of roof felt
  • Gloves
  • Mineral cap sheet
  • Underlay felt
  • A 15KG roller

Now you’ve carried out the preparation for the job, it’s time to apply the adhesive and felt.

Applying the adhesive and felt:

Whether you’re laying felt on an apex shed roof or a flat roof, applying the adhesive is mostly the same in principle although laying the felt is a different process.

Laying the first piece of felt on a shed roof

Lay the first piece of felt across the bottom of the slope for a shed roof, nail in the top edge with the clout nails, you’ll need to leave 50mm gaps between the nails. Once done you’ll need to fold the edges of the felt and nail them into place, again leaving 50mm gaps.

Laying the first piece of felt on a flat roof

Lay the first piece of felt on the lowest edge of the roofing deck and roll it out at right angles to the direction of the fall of the roof and check it’s aligned, once it’s aligned with the fall, nail with the clout nails from the centre and work outwards leaving 50mm gaps 

Applying adhesive and laying the second piece of felt on a shed roof

Take the second piece of felt that’s been cut out and lay it across the roof, ensuring there’s an overlap on the first roll, once this piece has been laid it can then be nailed. Once it’s nailed, use the brush to apply the felt adhesive to the joint overlapping the first and second layer, this adhesive can be left in the open for 15 to 30 minutes. You can then overlap the first and second layer of felt and nail over the joint for extra security. Roll over the joint with the 15KG roller to ensure there are no air bubbles and to ensure maximum contact between the layers and adhesive. 

Applying adhesive and the second layer to a flat roof

Apply felt adhesive to the first layer, making sure it’s an even coat otherwise it will cause issues with future layers. Again, leave exposed for 15 to 30 minutes before the second layer of felt is placed, overlapping the first and nailing it down using 50mm gaps. Once nailed down go over it with the roller to ensure there are no air bubbles. 

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