Fix Flat Roof Ponding Issue

Before looking at costs, the absolute first thing you should check is how old this roof actually is. If it was installed recently, it should still be covered under a guarantee (usually 10 to 20 years for a professional felt roof). If that's the case, get straight on the phone to the original contractor—they should come out and fix this structural defect free of charge. However, if the roof is out of guarantee, trying to fix a center-ponding issue on a 5.5m x 3m roof without replacing it is highly risky, and any money spent on a quick fix will likely be wasted. When water pools right in the center, it means the timber joists or the decking boards underneath have already sagged or bowed. Felt has no structural strength of its own—it simply follows the shape of the wood beneath it. If you try to 'fill' the dip with a leveling compound or pour a liquid overlay on top, you are just adding significant extra weight to a timber structure that is already struggling. This will cause it to sag even further over time. Furthermore, standing water degrades old roofing felt rapidly, meaning the material in that center dip is likely already compromised and letting moisture into your structural timbers. The Permanent Fix: To solve this properly, the old felt and decking need to come off so the joists can be inspected. A roofer will then 'fir out' the joists (adding tapered timber strips) to create a proper, continuous slope toward your guttering. While a full strip-back sounds more expensive upfront, upgrading to a modern EPDM rubber or high-performance fiberglass (GRP) roof is the only way to get rid of the puddle permanently and secure a fresh 20+ year guarantee. A quick patch job on a sagging roof will only buy you a few months before the water finds its way into your ceiling."

Water pooling (ponding) on a flat roof indicates inadequate drainage or low spots. The general industry rule of thumb in the UK is that standing water remaining for more than 48 hours requires attention. It is typically caused by insufficient roof pitch, structural sagging, blocked outlets, or compressed insulation