Underlayment: a crucial part of every roof system

Do you agree that the roof is one of the most important parts of your house, especially as it protects you and your family from bad weather conditions?

Take a look at the structure of a correctly installed shingle roof offering protection against all types of weather.

Types of underlayment

There are two basic types of underlayment used beneath roofing materials:

  • Bituminous underlayment
  • Non-bitumen synthetic underlayment

On the European market most synthetic underlays and breathable membranes are used as diffusion foils beneath wooden decks or wooden battens. They protect the insulation in case water would infiltrate through concrete or ceramic tiles. Diffusion foils cannot be penetrated with nails because the penetration between the membrane and the nail is not 100% watertight.


The big advantage of a bituminous underlayment is that every penetration by a nail is waterproofed. A bituminous underlayment is mostly used over wooden decks, especially under shingles or sometimes even under metal roofing as a safety hydro-insulation. We know many types of these underlays:

  • reinforced with fibers or polyester
  • finished with the sand or polypropylene
  • self-adhesive or non-self-adhesive

Installation of underlayment and slope limitations

An underlayment should always be installed as flat as possible without any wrinkles, buckles or waves which can later be visible on the shingle surface. It is recommended to start applying the underlayment from the eaves and continue to upper parts of the roof. Drip edge flashing must be installed first and then the underlayment comes over the flashing. The underlayment must be glued on the drip edge flashing. Rake flashings on the gable part must always be installed on top of the underlayment. It is recommended to fix the underlayment with the standard IKO roofing nails.

Underlayment failure

Some underlayments may fail for a number of reasons:


Poor quality

Poor quality underlayments absorb more water. Water absorption can cause wrinkling as the product expands. These wrinkles may telegraph through to the roof covering products, such as thinner bitumen shingles. Underlayment as felt paper is not recommended to apply over roof decking. Water and moisture from a wet underlayment may be absorbed by the roof deck, which can cause problems with expansion and contraction of the deck. The typical result of this failure is wrinkled shingles.


The underlayment is a key component of the roofing system. The price difference between underlayment options usually adds up to a few hundred euros in your overall roof replacement project. If you are investing in high quality IKO shingles that will improve your home’s value for years to come, choosing a better quality, more durable, water-resistant underlayment is well worth the extra cost. That is why IKO the professional range of Armourbase underlayments is highly recommended.


Want to see the high quality of our Armourbase underlayments before making your decision? Or are you having difficulties deciding between two types of Armourbase?