Flat roof overlaying: a guide for roofing contractors

Overlaying an existing flat roof can be an efficient and cost-effective method to extend its life, but only if done correctly. As detailed in the National Federation of Roofing Contractors (NFRC) GN71 guidance and other industry sources, skipping vital assessments or cutting corners when flat roof overlaying can lead to structural issues, non-compliance, or costly repairs down the line.

If you’re a roofing contractor considering an overlay project, here are some essential things to consider before you get started.

Spotlight on GN71 – NFRC’s Guide to Flat Roof Overlays

GN71, published by the NFRC, is a comprehensive technical guidance note specifically focused on overlaying existing flat roofs. It provides contractors with a solid framework to determine if an overlay is a viable, compliant, and long-term solution.

Rather than opting for a complete roof replacement, GN71 supports the idea that, under the right conditions, a new waterproofing layer can be installed directly over the existing one, saving time, cost, and environmental impact.

Key points from GN71:

• Overlaying should only be carried out if the existing membrane is dry, clean and free of structural damage.
• Overlaying makes the roof structure heavier. GN71 advises conducting a structural survey to ensure the existing deck and supports can handle the additional load.
• The new membrane must adhere properly to the existing one, which may need priming, particularly on older bitumen-based systems.
• GN71 reinforces the importance of using overlays as a chance to upgrade insulation to meet UK Part L regulations, while also preventing condensation.
• Proper drainage must be maintained, and tapered insulation is often required to achieve the standard 1:80 fall for water runoff.
• GN71 encourages contractors to consider how the roof will be used. The roof surface might need extra layers if used as a walkway, for plant and equipment or solar panels.

GN71 is an essential resource for roofing professionals. It encourages careful planning, helping contractors avoid premature failure and liability while delivering high-performance overlays.

Now that you have an understanding of the guidelines, we share a few practical tips to consider when overlaying.

Initial assessment and viability

Inspect the existing roof:

Ensure the current roof is dry, structurally sound, and free of damage. Spongy areas or visibly failing sections must be stripped and replaced, not covered over.

Look beneath the surface:

Moisture, rot, and thermal bridging can go undetected. Overlaying may mask these issues, leading to bigger failures later. Always assess for hidden damage.

Use diagnostic testing:

Use core samples, moisture mapping, or thermographic imaging to assess moisture and damage. For large areas, consider adhesion tests every 50m². Document findings with photos and compile a condition report.

Check substrate suitability:

Overlaying is only viable on stable, clean substrates like concrete, asphalt, or bituminous felt. Materials like strawboard or degraded insulation boards are not suitable. Correct surface irregularities over 5mm.

Structural load considerations:

Overlaying adds weight. A structural survey may be required to confirm that the existing deck and support structure can handle the additional load of the new waterproofing system.

Surface preparation

Adhesion is key:

The new system must bond effectively to the old one. Some older bitumen roofs may need priming before adhesion.

Clean thoroughly and repair defects:

Remove dirt, moss, grit, and grease using a brush and pressure washer. Any blisters in the old membrane can be cut and flattened. A smooth, even base is essential.

Prepare edges and flashings:

Check that the areas where the membrane meets walls, trims, or penetrations are properly prepared, detailed, and sealed to prevent water ingress.

Prime appropriately:

If a primer is required, apply during stable temperatures to avoid outgassing, especially on concrete decks, which could lead to pinholes or defects.

Thermal performance and Building Reg compliance

Consider upgrading insulation:

Overlaying presents a perfect opportunity to improve thermal efficiency to meet Part L of UK Building Regulations and consider long-term energy savings.

Consider the elements

Maintain drainage falls:

Overlays must maintain (or improve) drainage falls. Tapered insulation can help achieve a minimum 1:80 fall to avoid ponding.

Fire and wind uplift compliance:

Your overlay system must meet external fire standards and be structurally sound against wind uplift, especially at edges and corners. All systems should comply with BS/EN norms.

Consider how the roof will be used:

If the roof is home to plant, equipment, machinery or solar panels, or has heavy foot traffic, it is important to ensure the surface has adequate protection.

Select the right products and partners

Choose trusted suppliers and manufacturers:

Look for systems that are BBA certified and/or produced under ISO 9001 quality management systems. If unsure, speak to your roofing materials supplier. Lancastria supplies BBA Certified waterproofing systems manufactured to our exacting standards, providing our customers with assurance of their quality and longevity.

Final word: plan, test, and follow guidance

Done correctly, overlaying delivers excellent performance, less disruption, and long-term value for customers and contractors alike. However, to be successful, every overlay project needs careful planning and adherence to guidance.

For more information, or to find the right solution for your next overlay project, speak to our experienced team. xxxxxxxxxx