5 Key Considerations When Replacing Your Flat Roof

When a flat roof has reached the end of its life, the decisions made at replacement stage decide how it performs for years afterwards. A well-specified roof can improve insulation and reduce maintenance. A poorly specified one leads to repeat leaks and condensation problems. These are the five things worth getting right.

1. Choosing the right flat roof system

There is no single best system. The right choice depends on the size and shape of the roof, how it will be used and the budget. See flat roofing for more detail on each.

• Liquid-applied systems form a seamless membrane and suit roofs with complex shapes, lots of penetrations, balconies and terraces.
• GRP (fibreglass) gives a rigid, neat finish and suits smaller roofs such as dormers, bay roofs and porches.
• Single ply membranes (EPDM, PVC, TPO) are lightweight, quick to install and suit larger open roofs and modern extensions.
• Modern bituminous felt uses reinforced multi-layer membranes and is a cost-effective choice for garages, outbuildings and extensions.

When comparing systems, think about expected lifespan, foot traffic, exposure and maintenance. Typical lifespans vary by system; the table on our flat roofing page gives typical ranges.

2. How the roof will be used

A roof that only sees the occasional maintenance visit has different needs from a balcony, terrace or a roof carrying solar panels or plant. If you plan to add solar or create access in future, build that into the specification now. Walkways and protected access routes reduce accidental damage on roofs that are walked on for maintenance.

3. The condition of the deck and structure

Many failures come from what is underneath, not the new covering. Before replacement, check:

• The deck for rot, soft spots and sagging. A new covering on a failed deck will not last.
• The insulation to see whether it can be kept or is wet and needs removed.
• The joists and supports, especially if the new build-up adds load, such as thicker insulation or a terrace finish.

A survey with photos before work starts shows what is needed and why.

4. Warm roof or cold roof?

The main difference is where the insulation goes, and that affects condensation risk and thermal performance. Most replacement flat roofs are now built as warm roofs.

In a warm roof, insulation sits on the deck under the waterproofing, with a sealed vapour control layer below it, so the deck stays warm. In a cold roof, insulation sits between or below the joists and the void above must be ventilated to remove moisture.

5. Drainage, detailing and maintenance

• Falls. The roof should shed water to outlets or gutters. Tapered insulation can be used to create falls and reduce ponding.
• Upstands and thresholds. Upstands at walls, rooflights and door thresholds must be high enough above the finished surface. Poor upstands are a common cause of leaks.
• Outlets and gutters. Check outlet size and position, and consider an overflow so a blocked outlet does not flood the roof.
• Access for maintenance. Plan safe routes and protection where the roof will be walked on. See roof maintenance.

If the existing roof only has a local defect, a repair may be enough. See do cheap flat roof repairs save you money?

Building warrants and insulation

Depending on the scope of the work, a building warrant may be needed, particularly where the build-up or insulation is being changed. We will advise at survey stage on what is likely to apply, and the specification should be checked against current Scottish building standards.