Flat roof problems that won't go away — and why

A flat roof that leaks once is a problem. A flat roof that leaks, gets patched, leaks again, gets patched again, leaks a third time — that’s a different problem. The patches aren’t doing what they were meant to do, and the underlying covering has usually failed.

Why patches on flat roofs often fail

Flat roof materials — felt, single-ply, EPDM, GRP — don’t take kindly to being patched over and over. Each patch:

• Leaves a seam between old and new material. Seams are the weak point of any flat roof system.
• Traps water in different places. A new patch can redirect water to a different weak point.
• Usually patches only what’s visible. The water that’s been getting in has often been doing damage to the deck underneath.

The second patch usually lasts a bit longer than the first because you’re now catching water over a wider area. The third patch usually fails quickly because the deck underneath has gone soft.

The signs the underlying covering has failed

• Multiple leaks in different places — water is finding multiple paths through the covering
• Visible blistering or bubbling — usually means water has got underneath the covering and is lifting it from the deck
• Ponding that takes days to drain — water sitting on the roof for longer than 24 hours after rain means the falls are no longer working, usually because the deck has sagged
• Soft spots underfoot — the deck is rotting. Particularly common on older timber-decked flat roofs.
• The covering is cracked or torn in multiple places — usually age-related failure of felt, or delamination of GRP

If you’ve got two or more of these, the covering has had it. Patching is not going to extend its life by a meaningful amount.

What the options actually are

When a flat roof covering has failed, you have three real choices:

1. Patch again

Short-term fix. Buys you maybe one more season. Cheapest option, but you’re almost certainly going to be back for the full replacement within 12 months. Sometimes the right call if a full replacement is impossible right now and you need the roof to last one more winter.

2. Overlay a new covering on top of the old

Cheaper than a full strip because you skip the labour of taking the old covering off. Risks: the old covering underneath stays wet and continues to rot, and the overlay adds weight to a deck that’s often already compromised. Sometimes the right call when the existing covering is sound but old — felt over felt can work. Sometimes a disaster.

3. Full strip and replace

The right call in most cases where the covering has failed. Strip the old covering, inspect and replace the deck if needed, install a new covering with proper falls. Costs more upfront but resets the clock properly.

What we usually recommend for Bath flat roofs

For most flat roof situations on Bath properties — extensions, dormers, garages, bay windows — we recommend single-ply membrane systems: Fatra, Armourplan, or Danosa. Plus EPDM for smaller or simpler installations.

These are:

• Welded at the seams, not glued or taped — the seam is the strongest part of the system, not the weakest
• Compatible with most deck types — timber, concrete, OSB
• UV-stable — won’t degrade in direct sunlight
• Fully bonded — no ponding water getting underneath

We don’t install GRP fibreglass. It’s a perfectly serviceable system when installed properly, but the failure mode is bad — once it starts to delaminate, it goes quickly, and there’s no way to repair it cleanly. We prefer single-ply for that reason.

What a proper flat roof replacement looks like

1. Strip the old covering — including any overlay layers
2. Inspect and replace the deck if needed — usually 50-70% of the cost of a flat roof replacement is the covering itself, the rest is the deck
3. Check and correct the falls — flat roofs shouldn’t actually be flat; they need a minimum 1:40 fall to a gutter or outlet
4. Install the new covering — with proper edge trims, upstands against walls, and outlet detailing
5. Test for watertightness — either flood test or visual inspection of all seams

A typical flat roof replacement on a Bath extension takes 2-4 days, depending on size and access. Scaffolding is usually minimal.

If you’re at the patching-again stage, book a survey. We’ll tell you honestly whether the covering has life left or whether it’s time for a proper replacement.