Waterproof or Protective Roof Coating?

What is the difference between a roof waterproofing system and a protective coating?

When specifying roof coating systems, it’s important to know the difference between a waterproofing system and a protective coating.

Whilst it is true that water cannot permeate through a protective coating, this in itself does not constiute “waterproofing.”

Instead, pitched roof coatings, including our own metal roof coating system, Metalseal, are typically classed as protective coatings.

Flat Roof Waterproofing

Image showing a flat roof waterproofing system. Liquasil Ltd

Protective Coating

Image showing a flat roof waterproofing system. Liquasil Ltd
image showing a liquid applied flat roof waterproofing system in progress

This image shows a flat roof waterproofing system being installed. The lighter colour is the embedment coat containing an embedded chopped strand mat. The darker area has received a second top-coat.

Roof Waterproofing

Flat roofs will always have a waterproofing system applied to them, whether it be a felt overlay, a single ply membrane, or a liquid waterproofing system.

In simple terms, a liquid waterproofing system is commonly built up in stages;

  1. Embedment coat
  2. Fleece or glass fibre reinforcement mat
  3. Top coat

The fleece or mat is used to add tensile strength to the liquid coating.

The top coat provides additional strength as well as UV protection.

Protective Coating

Protective coatings are typically used on pitched industrial roofs.

A full waterproofing system is not required, because the substrate already provides the waterproofing protection.

In the case of a metal roof, a coating is used to protect the substrate from the elements and corrosion.

A new metal roof will have a factory applied finish in the form of Plastisol or other coating.

It is when this original finish deteriorates, that a maintenance recoat will be required, to ensure the substrate remains protected.

If a metal roof leaks, it is usually because other defects are present, such as loose or missing fixings, lack of ridge profile fillers, defective roof lights, missing seals between overlapping roof sheets, etc.

As each of the ususal defects can be solved independently, a protective coating only needs to consider corrosion, environmental considitons and aesthetics.

image showing Liquasil's Metalseal protective metal roof coating

This image shows Liquasil’s Metalseal 20, protective roof coating on a pitched metal roof.

Can You Waterproof A Metal Roof?

Whilst it is possible to use a waterproofing system on a metal roof, it is likely to be very expensive, simply because of the amount of work involved in applying a fleece or glass fibre mat reinforecement layer and an additional top coat.

Even if this approach was taken, there would still be vulnerable areas like ridges, which should not be fully sealed, and roof lights.

Also, sealing around fixings, mid-lap overlaps and other details can prove difficult and extremely labour intensive.

GRP reinforced waterproofing systems are unlikely to cope with movement caused by foot traffic at the mid-laps, due to lack of flexibility.

The Liquasil Metalseal system contains optional components such as Lapsil Sealant or HP Butyl Tape to seal overlapping roof sheet joints and either of these can be used in our stand-alone cut edge corrosion treatment too.

Out of the two options, HP Butyl Tape is far more robust, since it is itself a 20 year waterproofing tape, but we recommend leaving mid-lap joints unsealed in most cases.

See this article on cut edge corrosion for more detail.

Fixings and details like flues and other protrusions can be sealed with Liquasil Non-Sag Sealer, the same unique silicone compound we use to seal joints in our liquid gutter liner.

Where possible, roof lights should be replaced, as coating them is often a false economy, because often, a degraded roof light will be brittle, cracked or holed.

On ridges, we usually recommend that profile fillers are replaced with new, vented versions, which adequately seal the ridge line from water penetration, whilst providing ventilation to reduce condensation build-up in the roof itself.

For more information regarding pitched roof coating, please call one of our friendly & knowledegable building surveyors on 0121 709 5352

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