The short answer: if your flat roof is leaking in one place and the covering is otherwise sound, a local repair is usually enough. If the covering has failed across the whole area but the structure underneath is dry and solid, full replacement is rarely the only option: a liquid-applied overlay can refurbish the roof without stripping it. Replacement is normally reserved for roofs where the deck or insulation is saturated or structurally damaged.
The right choice depends on three things: how widespread the failure is, the condition of what sits under the covering, and how long you need the roof to last. Here is how to work through each.
When a repair is enough
A repair makes sense when the problem is local and the rest of the covering has life left in it. Typical cases:
- A single puncture or split, for example from dropped tools or foot traffic
- One failed lap or detail, often around an outlet, rooflight or upstand
- Flashing that has come away at a wall or chimney
A competent repair on an otherwise healthy roof can buy years. The catch is the phrase “otherwise healthy”. Felt and similar coverings age across their whole surface at roughly the same rate, so a leak caused by general ageing rather than a one-off accident is usually the first of many. If you are patching the same roof for the second or third season, the covering is telling you something.
When replacement is normally advised
Full replacement, with the old covering stripped off, is the right call when the problem goes deeper than the surface:
- The deck is rotten, sagging or structurally unsound
- Insulation is saturated and holding water (a survey or core sample confirms this)
- The build-up needs to change, for example to meet current insulation requirements during a wider refurbishment
Replacement deals with everything at once, but it is the most disruptive and expensive route: strip-off, skips, days or weeks of the building open to the weather, and the cost of an entirely new build-up.
The third option: overlay refurbishment
Between patch repairs and a full re-roof sits the option many building owners are never told about: refurbishing the existing roof with a liquid-applied system.
HYDRONYLON® is a liquid-applied roof waterproofing system that is applied directly over the existing covering. The roof is surveyed, cleaned and primed, local defects are made good, and the liquid is then applied with mesh reinforcement. It cures into a seamless waterproof membrane formed in place around every outlet, upstand and penetration, with no laps or joints to fail later.
An overlay is viable when:
- The leaks come from the covering, not the structure: the deck is sound and the insulation is dry
- The existing covering is well adhered, with no large areas of loose or spongy material
- The falls are broadly acceptable, so water is not permanently ponding deep across the roof
Because the existing covering stays in place, there is no strip-off, no skips and no days with the building exposed. Refurbishment typically costs significantly less than a full re-roof and takes days rather than weeks. The system holds European Technical Assessment ETA-23/0735, with 10 years of confirmed durability, and installations completed by HYDRONYLON Approved Contractors are backed by a 10-year product guarantee issued by HYDRONYLON LTD to the Approved Contractor.
How to decide: a five-minute checklist
- How many leaks, and since when? One leak after a storm points to repair. Several leaks appearing over months point to a covering at the end of its life.
- What is under the covering? If you do not know, get a survey before spending anything. Wet insulation or a damaged deck changes the answer completely.
- How old is the covering? Felt roofs commonly fail from around 15 to 20 years. Patching a 20-year-old felt roof is paying twice.
- Is water ponding? Shallow, temporary ponding is manageable. Deep standing water that never drains needs the falls addressed, whatever route you choose.
- How long do you need it to last? If the answer is “another decade or more”, compare the cost of repeated repairs against a guaranteed refurbishment, not just against a re-roof.
Next step
If your flat roof is leaking, start with a survey rather than a quote. A HYDRONYLON Approved Contractor will tell you honestly which of the three routes your roof actually needs.