Drone Roof Inspection Report

During the initial high-altitude nadir pass over the property, a slipped tile was identified on the front elevation of the main pitched roof, as annotated by the red arrow marker in the drone imagery. The tile has displaced downward from its correct interlocking position, partially exposing the course beneath and creating an open gap through which wind-driven rainfall can penetrate to the roofing underlay and potentially to the roof structure below.

The main pitched roof is covered in modern grey concrete or composite slate-effect interlocking tiles, which rely on mechanical interlock between adjacent units rather than traditional mortar bedding. Individual tile displacement of this type is typically caused by: failure of the mechanical fixing (nail or clip) holding the tile to the batten; impact damage from a falling object or foot traffic; or thermal/wind cycling causing the interlocking nib to work free over time.

The flat roof deck covering the loft conversion dormer structure is finished in a mineral-surfaced torch-on felt (cap sheet) system. During the inspection, a substantial area of standing water (ponding) was recorded in the lower central portion of the roof deck. The water patch is clearly visible in the close-proximity drone imagery (Fig. 3.2a) and the staining pattern visible in the overhead shot (Fig. 3.2b) indicates that this is a recurrent and persistent condition, not an isolated post-rainfall occurrence.

UK Building Regulations Approved Document H and BS 6229:2018 (Flat Roofs with Continuously Supported Flexible Waterproof Coverings) require that flat roofs are designed and constructed to achieve a minimum drainage fall of 1:80, with a design fall of 1:40 recommended to allow for construction tolerances and long-term deflection. The pooling observed suggests the current falls are below this minimum, indicating either: a design/installation deficiency in the original loft conversion works; subsequent deflection of the roof deck due to loading (the ASHP unit) or structural movement; or a blocked/partially obstructed outlet.

At the junction between the slate-clad dormer wall and the flat membrane roof deck, the lead cover flashing has not been fully dressed (pressed and formed) down onto the surface of the membrane. A correctly installed lead cover flashing at this type of abutment should be stepped or wedge-fixed into a mortar or structural chase in the wall above, with the lower skirt of the lead dressed flat onto the roof membrane surface and then sealed at the edge with a compatible sealant or the membrane lap turned up and bonded over the flashing tail.

The drone imagery clearly shows the lead flashing standing proud of the roof membrane surface — the flashing tail has not been worked down to make contact with the membrane, leaving a void behind it. This junction is a primary water ingress point: wind-driven rainfall will be driven under the raised flashing tail and onto the roof deck at the wall/roof junction, which is one of the most moisture-vulnerable details on any flat roof.

Where the main tile field meets the rendered dormer wall at the verge, the cement mortar pointing that seals the tile edge to the wall face is visibly inadequate. The drone inspection reveals thin, cracked, or in places entirely absent mortar at this junction. On a correctly detailed tile-to-wall verge, the mortar should be applied in two stages: a bed of mortar to support and align the tile at the correct overhang, followed by a pointing fillet that is weathered (angled away from the wall) to direct water away from the junction.

The defect is consistent with a verge that was originally mortared but has experienced significant thermal movement cracking and/or was not correctly formed at the time of the loft conversion works. This is a common finding on relatively new builds where OPC (ordinary Portland cement) mortar has been used without adequate flexibility additives — such mortar is prone to shrinkage cracking within the first few years.

The chimney stack visible within the survey area exhibits a significant level of vegetation growth at its head — most clearly visible in the overhead drone image (Fig. 3.5a). Plant matter, including what appears to be self-seeded vegetation and dense moss, has established itself at the base of the chimney pot and across the flaunching (the sloped mortar collar that surrounds and seals the pot base). This is an unambiguous indicator that the flaunching mortar has cracked and opened to a degree that allows soil, moisture, and seed matter to accumulate.

The establishment of vegetation roots within masonry mortar causes progressive opening of joints through root action. The freeze-thaw cycle then widens these voids further each winter. Open joints at the chimney head allow direct water entry into the masonry, which can track down inside the stack and emerge through the chimney breast within the property. A blocked or partially obstructed chimney pot also presents a risk if the flue is in active use, as combustion gases may not vent correctly.

The swept hip perimeter of the loft conversion dormer is finished with concrete bonnet-type hip tiles, which appear to be mechanically fixed using proprietary dry-fix hip clip fixings — visible as small dark clips at each tile junction. This is an appropriate installation method consistent with BS 5534:2014+A2:2018, which requires mechanical fixing for all hip tiles.

However, at the eaves end of the hip tile run, the drone imagery reveals the presence of pale-coloured foam or foam-filler material in the gap between the bottom hip tile and the adjacent roof tile course. The use of expanding foam as a gap filler in roofing is a non-standard approach that is not recommended by any UK roofing standards. Expanding foam is not UV-stable, degrades over time, absorbs moisture, and does not provide a durable weathertight seal. Its presence suggests either an improvised repair or a shortcut taken during the original installation rather than a proper close-cut tile or mortar closure at the eaves/hip junction.

Additionally, early-stage moss growth is observed on the lower hip tile courses (Fig. 3.6b), which if left untreated will progress to the same level of colonisation seen on the neighbouring property’s roof.

The adjacent property’s original pitched roof (visible to the left in the aerial imagery) shows extensive lichen and moss colonisation across the full tile field, including the ridge, hip, and general slopes. Whilst this roof falls outside the direct scope of the survey, its condition is noted as it represents the trajectory the surveyed property’s tile elements will follow if moss is not managed proactively.

On the surveyed property itself, early-stage moss growth is identified on the lower courses of the swept hip tiles (Section 3.6) and at several tile-to-flat-roof junctions. Moss spores are widespread in the urban North London environment and once established, growth accelerates rapidly on north- and east-facing surfaces.

The wider survey imagery reveals evidence of significant water staining and algae growth on adjacent flat roof surfaces visible within the drone’s field of view. The staining patterns — characterised by dark algae trails and tide-mark residue — are consistent with recurrent ponding, where water is retained on the roof surface for extended periods before evaporating, leaving behind algae colonies and mineral deposits.

Where these flat roofs relate to sections of the surveyed property or its outbuildings, the condition should be considered in the context of the overall drainage strategy. Where they relate to immediately adjacent properties, the observation is noted for completeness as water run-off from neighbouring poorly-drained flat roofs can contribute to boundary wall saturation and moisture issues at party wall junctions.