What is a roofing cap sheet?

RBM felt abbreviation of Reinforced Bitumen Membrane is a felt used for roofing, it comes in many types such as torch-on and self-adhesive for any range of budget. Felt waterproofing is a number one option for various buildings such as residential, commercial, retail, health care and education.

Felt roofing normally consists of two or more layers formed on site, with the top layer being cap sheet. One of the most important components of a building to prevent water ingress, cap sheet is the finally layer in the felt system as it provides a watertight surface and protection layer. Cap sheet has been designed to have many characteristics, it must be strong and durable to resist degradation from extreme weathers and must have an aspect of flexibility to withstand building movements. If the wrong capsheet has been chosen or incorrectly installed it will then become extremely week and eventually split allowing water ingress.

What are they made of?

The process of manufacturing roofing felt is continuous and consists of three core components:

  • Reinforcement base – Most High performance felts are made on a polyester fibre base making it strong enough for roofing purposes whilst allowing the cap sheet to have a high-elongation at break which exceeds all demands likely to be met.
  • Polymer Modified bitumen – the addition of a polymer in the bitumen improves many properties of the roofing material making it more flexible and durable, allowing the cap sheet to cope with the thermal and structural movements of the building.
  • Protective finish – To protect the cap sheet membrane from UV rays and the installation process, they will have a protective surface finish, this could be a mineral finish, Stone chippings finish, reflective coating or even an aluminium or copper finish.

Methods of applications

There are a number of ways to install cap sheets such as:

  • Torch applied
  • Pour and roll – using heated and melted bitumen as an adhesive to adhere the roofing felt
  • Cold applied systems refer to either using a cold PU adhesive to adhere the membrane or using a self-adhesive cap sheet containing a specially formulated adhesive into its underside


Cap Sheet Selection – What to think of:

  • Check the product credentials such as weight, fire performance and 3rd party certifications
  • Its method of application
  • Look for meaningful guarantees. Make sure to read the small print
  • Ensure the supplier/installer is part of recognised trade association(s)
  • Work with a reputable manufacturer/supplier that has a network of approved installer, such as ourselves.
  • What technical/support do you get from the supplier? This is invaluable!

Undeniably the roof and its waterproofing protect our most valuable assets. Therefore, it is important to select and buy the best waterproofing product you can afford in order to avoid costly repair or damages to the building further down the line.