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Flat Roof Sealant: Which Sealant Is Best for Flat Roofs?

It is often asked which is the best sealant for a flat roof. The answer will always depend upon the projects budget and conditions in which the flat roof will operate in. A domestic roof will have to endure minimal foot traffic in the most extreme case and zero-foot traffic in the best case. Whereas a service deck will need to withstand both heavy foot and vehicular traffic. This is before considering the weather conditions which the roof will have to provide waterproofing in.

In A Rush? Here Is Our Top Flat Roof Sealant

MPM Starflex Mono Tix 100

What Would Make The Perfect Sealant For A Flat Roof?

The most common reasons a flat roof will fail are; poor installation, a lack of maintenance, age, thermal movement and unsuitable usage (an example of this would be a service deck which has not had a top coat added to provide extra hardness). With this in mind, the perfect roof sealant would be easy to install, easy to maintain, come with a long guarantee, resistant to thermal movement, durable and seamless.

Let’s look at each of these characteristics in detail:

Easy To Install

Most new roofing systems fail which fail within the first 2 years of their service life do so because of a poor installation. The easier a system is to install the higher the probability it will be done so properly.

Systems like mastic asphalt are extremely good systems which tick every box for a good flat roof sealant aside from the ease of the installation. Not only is it hard to install, the supply of skilled asphalter is extremely low. However, systems which require skilled labour are not where you are most likely to find problems with a bad installation.

The type of sealants for flat roofs which are most problematic are relatively simple ones which only require a course lasting between 2-5 days to be accredited in it. The type of systems which cause these issues are normally liquid based or GRP based. This does not mean these systems are bad but you should pay attention to 2 areas. What are the credentials of the installation team, and does the sealant manufacturer provide the guarantee.

Liquid based sealants are the easiest types of systems to install. But this means they still need to be installed by an approved roofing contractor accredited by the sealant manufacturer. The reason liquid sealants are the easiest type of waterproofing for a flat roof are it is cold applied, as an overlay or on a new installation. It provides a seamless finish. And can be easily installed using a brush, roller or airless spray. The speed at which a liquid installation can be applied is also beneficial from a cost perspective especially on commercial buildings as large areas can be covered in a single day.

Easy To Maintain

A lack of maintenance is a reason many roofing systems fail before their expected life expectancy. A key factor in this is cost. Many building owners put of regular maintenance for two reasons, to save the cost of a survey and through fear of the costs of repairs. The cost of repairs can largely be mitigated by choosing a flat roof sealant which is easy to repair. Systems which are easy to repair are easy to maintain which in turn means the cost of repairs is low. For ease of maintenance mastic asphalt and liquid sealants are best. If any problems arise with either a local roof repair can be carried out quickly and cost effectively. In most cases these repairs are patch repairs, where the defective areas is removed and a new coat of sealant is applied. Liquid is better than asphalt in terms of cost and the ease of finding the appropriate labour to carry out a roof repair.

Long Lasting With A Long Guarantee

Most flat roof sealants are long lasting. The reason a system fails is often not because f the sealants itself, but the quality of the roofing contractors. Any sealant installed by a good contractor with a guarantee should be long lasting. Where you can wrong into problems is if you choose a poor contractor. There are a few reasons for this; the potential for bad workmanship, a system being installed which is not suitable for its working requirements and void guarantees. Bad workmanship is self-explanatory. As mentioned most new roof systems which fail in the first two years of installation are caused by bad workmanship. What most building owners get caught out on here are a lot of manufacturer guarantees require the contractor to correct any problems which occur in this period. This can mean an disreputable contractor could cease being a company, instead of putting any problems right, leaving the building owner to pick up the cost of repairs.

The use of a inappropriate system can result in a roof failing early due to being designed for less rigorous demands. All the problems mentioned here can be overcome by choosing a reputable contractor with a guarantee backed by a reputable sealant manufacturer. In both cases a reputable contractor and sealant manufacturer will have been in business for a reasonable period of time with a healthy balance sheet. This will largely mitigate any problems with guarantees and help make sure when you purchase a roof it is a long lasting waterproofing selection. The key point with choosing a long lasting sealant is a guarantee, installation by a reputable contractor and choosing a sealant made by a reputable manufacturer.

Resistant To Thermal Movement

Thermal movement is a problem which all flat roof sealants must endure. The severity of the thermal movement a roof must endure depends on the weather conditions they are subjected to. In the UK, thermal movement places great strain on roofing systems as in a single day the deltas between daily high and daily low temperatures can be vast. Rigid systems such as GRP/fibreglass are especially prone to the effects of thermal movement. As they do not have the elasticity to cope with thermal movement as well problems such as cracks are likely to develop.

Mastic asphalt without solar reflective paint is also prone to problems with thermal movement as the heat of the sun can cause the asphalt to bake and expand then as the temperature cools the asphalt contracts. This can lead to cracks, water ingress and blows. With an asphalt roof the way to minimise these problems are to use solar reflective paint on the finished asphalt membrane. And to also use it on any parts of the roof connected to the asphalt such as lead flashings. This will minimise the delta between the high and low temperature for the asphalt, making problems a lot less likely.

There are some liquid systems such as MPM Starflex Mono Tix 100 which are outstanding at negating the effects of thermal movement. Such systems are a single component permanent elastic liquid-applied polyurethane coating. These systems can provide solutions where other applications have failed, this includes roofs with challenges such as stagnating water, frost, humidity and extended exposure to the sun.


Any roofing system must be robust enough to withstand the task it is used for. If a system is not sufficiently durable it will fail before the it's expected service life.


Any flat roof which has seams has a higher probability of failure. A seam is a weak point of any roof. This area of a flat roof is prone to fail due to slack workmanship. By choosing a flat roof sealant which is seamless, one of the parts of a roofing which is most susceptible to failure is eliminated.

What Is The Best Flat Roof Sealant?

1. Best Overall: Starflex Mono Tix 100 Liquid

2. Best Value: Starflex Mono 100 Liquid

3. Best For Leaks: Acrypol Acrylic Coat

4. Best For RV Roofs: LRS Rapid PMMA Liquids

5. Best For Flat Roofs: Starflex Mono Tix 100 Liquid

6. Best For Metal Roofs: LRS Rapid Roof

7. Best For Flashing: Soudall Fix All

Need Help Choosing The Best Flat Roof Sealant For Your Project

If you would like any further information about the best flat sealant for your own personal situation, whether for a new or existing roof please contact us or call us now on 01277 375 511. One of our friendly team of liquid waterproofing specialists will be delighted to help.