How to Install GRP Fibreglass Roofing: A Step-by-Step Guide

Table of Contents

• What You’ll Need Before Installing GRP Fibreglass Roofing
• Preparing Your Roof Deck for GRP Installation
• Why Flat Roof Drainage Is Important
• Step-by-Step GRP Fibreglass Installation Process
  • Laying the First Resin Layer
  • Applying Fibreglass Matting
  • Adding Subsequent Layers
• Building Regulations Part L: Flat Roof Compliance
• GRP Roofing Cost Per Square Metre in the UK
• Common Mistakes to Avoid During Installation
• Frequently Asked Questions

Last Updated: September 18, 2026

GRP fibreglass roofing is one of the most durable and cost-effective solutions for flat roofs across the UK. Whether you’re tackling a repair or planning a complete roof replacement, understanding the installation process helps you work with contractors more effectively and make informed decisions about your property. At Evershield Roofing, we understand the questions that matter most when it comes to flat roof projects. Below, we’ll walk you through exactly what you need to know about how to install GRP fibreglass roofing, from preparation through completion.

What You’ll Need Before Installing GRP Fibreglass Roofing

Before any work begins, gather the right materials and tools. This step determines whether your installation holds up for decades or fails within years.

You’ll need polyester resin, fibreglass matting (typically 300-450gsm weight), a roller for pressing layers down, a brush or spray gun for resin application, and safety equipment including gloves, goggles, and a respirator. The roof deck itself must be solid, either concrete, timber boarding, or existing asphalt. A primer suitable for your deck type helps the resin bond properly.

Most installations also require a topcoat or sealant to protect the fibreglass from UV damage. This extends the life of your GRP fibreglass roofing installation significantly.

Preparing Your Roof Deck for GRP Installation

A weak foundation ruins even the best materials. Your roof deck preparation determines how well the GRP fibreglass roofing installation performs over time.

Start by clearing the roof completely. Remove debris, moss, and loose material. If you have standing water, that’s a drainage problem you must fix before installing new roofing, we’ll cover this next.

Inspect the deck for damage. Soft spots in timber, cracks in concrete, or blistering in existing asphalt all need attention. Sand down rough areas and fill cracks with appropriate filler. The surface should be clean, dry, and smooth.

Apply primer according to the product instructions. This step is often skipped, and that’s where problems start. Primer ensures the resin bonds to the deck rather than sitting on top of it.

Why Flat Roof Drainage Is Important

Standing water is the enemy of any flat roof. This is why flat roof drainage matters so much, it’s the difference between a roof that lasts 20 years and one that fails in five.

When water pools on a flat roof, it adds weight and creates pressure on the structure. The water penetrates small cracks and seeps into the insulation below. Freeze-thaw cycles in winter make this worse. Over time, the roof fails from the inside out.

Good drainage means water moves off the roof quickly. Traditional flat roofs use a slight slope (typically 1:80 gradient) to direct water toward gutters or drains. Without this slope, you need a high-performance system that handles pooling.

GRP fibreglass roofing installation includes checking your drainage setup. If your roof doesn’t slope naturally, install additional drains or adjust the slope using tapered insulation boards. This investment prevents thousands in water damage later.

Step-by-Step GRP Fibreglass Installation Process

The actual installation follows a proven sequence. Each layer builds on the previous one, creating a waterproof membrane that bonds to your deck.

Laying the First Resin Layer

Mix your polyester resin according to manufacturer instructions. Temperature affects cure time, warmer conditions speed it up. Pour the resin onto the roof deck and spread it evenly with a brush or spray gun. Aim for a consistent thickness of about 1-2mm.

Work in sections if your roof is large. Don’t try to cover the entire roof at once, the resin will start curing before you finish. A typical section is 3-4 square metres.

Let the resin reach a tacky (not fully cured) state before applying fibreglass. This usually takes 30-60 minutes depending on temperature. You want the surface to feel sticky but not runny.

Applying Fibreglass Matting

Unroll the fibreglass matting over the tacky resin. Press it down firmly with a roller, working from the centre outward to remove air pockets. Air bubbles create weak spots that fail under stress.

The matting should lie flat with no wrinkles or folds. If you see bubbles, roll over them again. Ensure the matting overlaps at seams by at least 100mm, this overlap prevents water from seeping through joints.

Once the matting is pressed down, apply a second coat of resin over the top. This saturates the fibreglass and locks it in place.

Adding Subsequent Layers

Most GRP fibreglass roofing installations use 2-3 layers total. After your first layer cures (typically 24 hours), repeat the process: resin, matting, resin.

Each additional layer adds strength and waterproofing. A three-layer system is often used for residential roofs. Commercial properties may use additional layers for extra durability.

Allow full cure time between layers. Rushing this step compromises the bond between layers and weakens the entire roof.

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Building Regulations Part L: Flat Roof Compliance

Any roofing work in the UK must comply with Building Regulations, specifically Part L (Conservation of Fuel and Power). This applies to new installations and major renovations.

Part L requires that flat roofs meet minimum thermal performance standards. This means your roof must include adequate insulation beneath the GRP membrane. The U-value (heat loss rate) must not exceed specified limits depending on your property type.

For residential properties, typical requirements mean 100-150mm of insulation beneath the roofing system. Check with your local building control office before starting work, they’ll confirm exact requirements for your property.

GRP fibreglass roofing installation itself doesn’t provide insulation. The insulation sits below the membrane. This is why preparation matters, you’re not just installing roofing, you’re building a complete thermal system.

Failure to comply with Part L can result in enforcement action or difficulty selling your property later. It’s not worth the risk. Get confirmation from building control before work starts.

GRP Roofing Cost Per Square Metre in the UK

Material costs for GRP fibreglass roofing vary based on system specification and supplier. Labour costs depend on roof complexity, access, and local market rates.

Material costs for GRP fibreglass roofing vary based on system specification and supplier. Labour costs depend on roof complexity, access, and local market rates.

The most important factor isn’t the cheapest price, it’s getting a system that actually lasts. A poorly installed budget roof costs far more in repairs than a properly installed quality system.

At Evershield Roofing, we provide free surveys and quotes so you know exactly what you’re paying for. We break down material costs, labour, and any additional work your roof needs. No surprises, no hidden charges.

Common Mistakes to Avoid During Installation

Most GRP fibreglass roofing installation failures come from skipping steps, not from material defects. Here’s what to watch out for.

Skipping surface preparation. A dirty or damp deck undermines everything that follows. Take time to clean, repair, and prime properly.

Rushing between layers. Applying the next layer before the previous one fully cures creates weak bonds. Each layer needs 24 hours minimum in normal conditions.

Ignoring drainage problems. Installing new roofing over a roof that pools water guarantees failure. Fix drainage first.

Using the wrong resin type. Polyester and epoxy resin cure differently and have different properties. Match the resin to your system specification.

Inadequate overlap at seams. Fibreglass matting must overlap by at least 100mm. Skimping here creates leak paths.

Skipping the topcoat. UV exposure degrades unprotected fibreglass. A quality topcoat extends roof life by years.

Not checking Building Regulations. Part L compliance isn’t optional. Work with building control from the start.

Installing GRP fibreglass roofing requires careful planning, quality materials, and attention to detail at every step. The process isn’t complicated, but it demands respect for each stage.