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Fibreglass Roof Problems

Fibreglass roofing is sometimes referred to as GRP roofing, and GRP is the acronym for ‘Glass Reinforced Plastic’. Property owners typically have questions about fiberglass roofing. In fact, a few of the most common statements/questions that property owners ask include the following: 

• I have problems with my fibreglass flat roof. 

• I've just gotten my fibreglass flat roof installed. However, there seems to be a problem with it although the contractor says this is normal. 

The Most Common Problems of Fibreglass Flat Roofs 

This article discusses the most common problems of fibreglass flat roofs, and it provides the answers to these problems. The most common problems are: 

  • The surface of the fiberglass flat roof has pinholes.
  • The GRP flat roof has cracks on its surface. 
  • The roof shrivels up due to the alligatoring of the GRP top coat. 
  • Pools of water sit on the GRP roofing surface, which is known as ponding.
  • Leaking comes through the fibreglass roof.
  • The GRP roof has noise problems. 
  • The fiberglass roof has flashing problems. 
  • The swollen timber board causes cracks in the GRP roof.
  • It doesn't use the OSB Board.
  • The collar details are not finished correctly.
  • There is a cracking expansion joint.
  • The render of the wall that the roof is attached to has cracks.
  • The roof and capstones have lichen and moss growth. 
  • Faulty capstone joints are where they meet on the wall.
  • The guttering is failing.

The Solutions to These Problems

For Ponding  

If you notice patches of a lighter or darker color than the rest of your roof or huge pools of water, contacting a reputable roofing professional immediately is the answer. 

For Alligatoring That Causes the Shriveling of the roof 

In most cases, roof replacement is needed to resolve this situation.  

The Solution to the Crackling of the GRP Flat Roof  

Improper installation of the GRP flat roof is the most common reason for the crackling of the GRP flat roof. Another reason for this crackling could be thermal movement. Other possible reasons for crackling include using incorrect timber boarding, applying GRP resin on top of wet laminate, and applying GRP resin too quickly.

When it comes to using incorrect timber boarding, you should use OSB boarding. OSB boarding greatly handles the rigidity of these flat roofing systems. Otherwise, if there is a problem with the boards, re-installing the boards may be the solution.  

If the use of GRP resin is the problem, then you should use a PU liquid roof coating instead of repairing it with GRP resin. To do this, you must first clean off the damaged area. Next, use the PU liquid coating, and then use PU sealant to fill in any cracks. Afterward, prime the surface. 

For Leaks Coming Through the Fibreglass Roof

For roof leaks coming through the fiberglass system, you must first contain the leak. Then, find out what is causing it. And lastly, get it repaired 

The Noise Problems 

The temperature and the rigidity of a GRP roofing system cause noise problems with GRP flat roofs. To minimise noise caused by a fibreglass roof, it must be designed for maximum movement. Installing certain expansion joints can achieve this. Alternatively, using a more flexible fibreglass roofing system, such as UltraFlex, will accomplish this. 

The Flashing Problems 

Either repairing or replacing the fibreglass flat roof is the answer to resolving flashing problems. It just depends on the cause of this problem. For instance, if water ingress occurs due to a cracked flashing, then you most likely need to remove the cracked flashing and replace it. If the flashing, for instance, has detached from the wall, it must be reconnected to it.  

For Swollen Timber Board Causing Cracks

The timber board must be replaced. If not, water will build up over time, if it hasn't already. This water buildup causes an unsightly crack or a hole. 

The Solution to the Incorrect Collar Completion

You must apply a new mastic dressing that is at least 150mm. You must also include separation in and around any lead seams. 

For the Cracks That Came From the Wall Render Attached to the Roof 

You must render the wall in the problematic area again to resolve this. 

The Roof and Capstones Having Lichen and Moss Growth 

It's critical to remove lichen and moss growth from the roof to prevent any damage to the coating of the fibreglass flat roof. Also, obtaining a yearly roof maintenance survey is an amazing preventive measure. 

To Solve Faulty Capstone Joints 

The capstone joints must be replaced. 

The Failed Guttering

Obtaining routine maintenance to get your gutter or roof cleared away is critical. If you don't obtain routine maintenance, replacing the gutter is in order.  

The Cracking Expansion Joint Solution

In this situation, you must make sure there is room between the timber board and the perimeter edges. And for large roofs, you must ensure your expansion joint is installed every 60 SQM of the GRP roof.

To Ensure the OSB Board is Used

If the OSB board isn't used, the resin of the GRP roof may detach, which may lead to a failing roof system. And if this happens, you must cut out the damaged area and replace it with an OSB board. Then, you can redo the GRP system.

If there is no damage, however, just redo the GRP roof. Then, you can attach an OSB board and redo the roof system.


There are some disadvantages to fibreglass roofing. The majority of these can be avoided by choosing a reputable roofing contractor and properly maintaining your roof. However, choosing a more elastic roofing system which is less dependant on the skill level of the contractor will be a better option for most of the people. Especially, if it is not possible to find a sufficiently reputable roofing contractor.