Common Roof Problems and When to Call a Roofer

A roof that is performing well is invisible. You only notice it when something goes wrong — and by the time water appears on a ceiling, the damage has usually been developing for months. The roofing problems covered in this guide are the ones TCM Building's team encounters most frequently when called to properties across Borehamwood, Radlett, Barnet, Edgware, and the wider Hertfordshire area. Each section covers what causes the problem, how to identify it, and what the repair involves.

Missing, Slipped, and Cracked Tiles or Slates

This is the most common call TCM Building receives after a storm. A single missing tile creates an immediate entry point for water, and the damage to the roof deck, felt, and ceiling below can accumulate quickly. Tiles slip when the nibs that hook them onto the battens break, when the battens themselves rot, or when the nails holding them corrode. On older properties in Hertfordshire — particularly those with natural Welsh slate — nail sickness is the primary cause: the original iron nails corrode over 50–80 years, releasing the slates.

A slipped tile is not always visible from the ground. The tell-tale signs from inside the loft are daylight visible through the roof covering, or wet patches on the felt or rafters after rain. From outside, a tile that sits lower than its neighbours, or one that has rotated slightly, indicates a loose or slipped fixing.

Replacing individual tiles or slates costs £150–£350 per tile including scaffolding or access equipment. If multiple tiles are affected across a section of roof, it is more cost-effective to strip and re-tile that section than to replace tiles individually. A full re-tile of a three-bedroom semi-detached house in Hertfordshire costs £8,000–£14,000 for concrete interlocking tiles and £12,000–£20,000 for natural slate.

Failed Flashing at Chimneys, Valleys, and Abutments

Flashing is the waterproof seal between the roof covering and any vertical surface — chimneys, parapet walls, dormer cheeks, and the junction between a lean-to roof and a house wall. It is typically lead, though mortar fillets, aluminium, and proprietary systems are also used. Flashing failure is the second most common cause of roof leaks in Hertfordshire properties, and it is almost always gradual rather than sudden.

Lead flashing cracks or lifts when the mortar that holds it into the masonry joint (the chase) deteriorates. On older properties, the lead itself may have been installed without adequate allowance for thermal movement, causing it to crack along fold lines. Mortar fillets — a common substitute for lead on older properties — crack and separate from the masonry as the building moves seasonally, creating gaps that allow water to track down behind the roof covering.

The water entry point is rarely directly above the ceiling stain. Water that enters through failed chimney flashing typically travels down the rafter and appears on the ceiling several metres away from the chimney. A roofer will trace the path of water ingress rather than simply looking directly above the stain.

Resealing lead flashing with new mortar costs £300–£600. Replacing a full chimney flashing set in lead costs £800–£1,800 depending on the chimney size and access. Valley replacement — where the lead or GRP valley lining between two roof slopes has failed — costs £1,200–£3,000.

Loose and Deteriorated Ridge and Hip Tiles

Ridge tiles sit at the apex of the roof and are bedded in mortar. Over time, the mortar deteriorates through freeze-thaw cycling, and the ridge tiles become loose. A loose ridge tile is a safety hazard — in high winds, it can fall and cause serious injury or damage. It also allows water to enter the ridge, saturating the insulation and causing damp patches on the ceiling below.

The standard repair is to remove the ridge tiles, rake out the old mortar, and re-bed them in fresh mortar. On newer properties, dry-fix ridge systems — which use mechanical fixings rather than mortar — are the preferred solution as they do not deteriorate in the same way. A full ridge repoint on a typical semi-detached house costs £800–£1,500. Replacing a mortar-bedded ridge with a dry-fix system costs £1,500–£3,000.

Hip tiles — which run along the diagonal hip of a hipped roof — are subject to the same deterioration and require the same attention. Properties in Radlett, Potters Bar, and Welwyn Garden City with hipped roofs should have their hip tile bedding inspected every five to seven years.

Flat Roof Failures

Flat roofs — on garages, extensions, bay windows, and some 1960s–70s houses — have a shorter lifespan than pitched roofs and require more frequent maintenance. The three most common flat roof materials in Hertfordshire's housing stock are felt, GRP (fibreglass), and lead, each with different failure modes.

Felt Flat Roofs

Mineral-surfaced felt roofs typically last 10–20 years. They fail through UV degradation, which makes the felt brittle and causes it to crack along fold lines at upstands and drips. Ponding water — where the roof has insufficient fall to drain properly — accelerates deterioration significantly. A felt roof that is blistering, cracking, or showing mineral granule loss in the gutters is approaching the end of its serviceable life. Patching a failing felt roof delays the inevitable: a full replacement is the correct solution once the felt has reached this stage.

GRP (Fibreglass) Flat Roofs

GRP flat roofs are the current standard for new flat roof installations and have a lifespan of 25–30 years when correctly installed. The most common failure mode is delamination at the upstand — where the GRP meets the vertical wall — caused by inadequate bonding during installation or by movement in the substrate. Cracks in the GRP surface are rare on a correctly installed roof but can occur if the decking substrate moves or if the GRP was applied in cold conditions. A cracked GRP roof can be repaired with a GRP patch if the damage is localised; widespread cracking indicates a systemic installation problem and requires full replacement.

Lead Flat Roofs

Lead is the most durable flat roof material, with a lifespan of 50–100 years when correctly installed. Failure typically occurs when lead has been installed in bays that are too long — causing fatigue cracking from thermal movement — or when the lead has been dressed too tightly against an upstand, preventing it from moving. Splits along the centre of a lead bay are the classic sign of fatigue failure. These can be repaired by welding or patching if the damage is limited, but widespread fatigue cracking requires re-leading.

Moss, Lichen, and Algae Growth

Moss and lichen growth on roof tiles is common in Hertfordshire, particularly on north-facing slopes that receive limited sunlight and on properties near trees. Moss itself does not damage tiles directly, but it retains moisture against the tile surface, accelerating freeze-thaw deterioration and potentially lifting tiles as it grows beneath them. Lichen is harder to remove than moss and its root-like structures (rhizines) can penetrate the surface of softer tiles, causing pitting.

The correct treatment is biocide application followed by soft washing — not high-pressure washing, which can damage tile surfaces and force water under tiles. A biocide treatment kills the moss and lichen; the dead growth then washes off naturally over the following months. A moss treatment on a typical semi-detached house costs £400–£800. Fitting a zinc or copper strip along the ridge — which releases ions that inhibit moss growth — costs £300–£600 and provides long-term protection.

Blocked and Failing Gutters

Gutters are not part of the roof structure, but blocked or failing gutters cause more water damage to Hertfordshire properties than any other single maintenance failure. An overflowing gutter saturates the wall below, leading to penetrating damp, wet rot in window frames, and — in solid-wall properties — internal damp patches. Cast iron gutters on older Hertfordshire properties corrode at the joints and at the back of the gutter where it sits against the fascia, often for years before the overflow becomes visible.

Gutters should be cleared of debris every autumn. A gutter that overflows despite being clear of debris has either failed joints, incorrect fall, or insufficient capacity for the roof area it serves. Gutter replacement on a typical semi-detached house costs £600–£1,200 for uPVC and £1,500–£3,000 for cast iron or aluminium.

Roof Repair Costs in Hertfordshire (2026)

Costs reflect 2026 tender prices for Hertfordshire and North London. Figures exclude VAT. Scaffolding costs (£700–£2,000 depending on height and access) are additional where required.

When to Call a Roofer Immediately

For non-emergency situations — moss growth, minor flashing deterioration, ridge tile movement that has not yet caused a leak — a planned inspection and repair within four to eight weeks is appropriate. TCM Building's roofing team covers Hertfordshire and North London and carries out emergency call-outs within 24 hours for active water ingress.

Frequently Asked Questions