Building on sites in coastal locations

Many people aspire to have a home with a sea view. Less desirable is a home with water ingress and corroded lintels and balconies. Homes built within coastal areas present greater risks of future claims and require additional risk management. From 1 April 2023, our Technical Operations Team have updated how they manage the technical risks of such sites. They will ask you for additional technical information when you register a site with us that is in a coastal location - that is, one within 500m of the coastline.

In the article below we summarise some of the most common defects and how these costly and disruptive issues could be prevented.

Identification of ‘coastal’ sites

Identification of more exposed ‘coastal’ sites is not as straightforward as it might seem. Currently, NHBC Standards define costal sites as those within 500m of the shoreline. However, homes adjacent to large estuaries can also be exposed to high winds and storms coming in from the sea. Additionally, we are currently taking a detailed look at our claims data on homes slightly further inland, as it seems that these may also be experiencing similar issues.

BS 8104:1992 ‘Code of practice for assessing exposure of walls to winddriven rain’ provides a method for assessing exposure of walls to winddriven rain which gives a more precise understanding of exposure conditions on a particular site. Local knowledge and experience should also be taken into account, particularly where the site is exposed to open water. Even when a coastal site is defined as being in a sheltered or moderate location, occasional exposure to storm conditions can test the building’s design, detailing and construction, exposing weaknesses and leading to failure.

A prudent approach may therefore be to treat any coastal site as though it is in a very severely exposed location and to use construction materials, systems and detailing appropriate for these conditions.

Detailed guidance can be found in NHBC Standards, but the following key issues should be considered when constructing homes in coastal locations:

Doors and windows

Making the most of the sea views, homes in coastal locations often incorporate large doors and windows. All doors and windows should be classified in accordance with BS 6375 ‘Performance of windows and doors’. This requires determining, through test and specification, whether they will suitably resist air and water penetration, and wind pressure specifically for the location where they are installed.

Weaknesses can occur where windows are joined together horizontally or vertically to form faceted, ribbon or stacked windows. Weathertightness of the interfaces between windows is just as important as the windows themselves, and joints should not be reliant on sealant alone for weathertightness. Properly designed joints incorporating gaskets or seals should be used. Larger stacked widows are considered as curtain walling, and the guidance in Chapter 6.9 ‘Curtain walling and cladding’ applies.

Sills with drip details should project the external face of the wall and sufficiently shed water away from the building. BS 8213-4:2016 ‘Windows and doors. Code of practice for the survey and installation of windows and external doorsets’ says ‘The size of the sill should be such that there is a minimum overhang of at least 25 mm from the face of the building’; however, manufacturers may have their own guidance for more exposed locations. Curved walls below faceted windows can reduce this dimension, so the sill may need to be oversized to account for this.

DPCs and cavity trays around openings should be suitably robust. These details are critical to the performance of the building and are prone to poor workmanship. The devil is in the detail, so their installation needs close supervision to ensure that a good standard is maintained.

Ironmongery should be manufactured from suitably durable materials to withstand the more highly corrosive coastal environments. BS EN 1670:2007 ‘Building hardware. Corrosion resistance. Requirements and test methods’ provides guidance for hardware and its durability.

Detailing of flat roof coverings

There is a trend for homes in coastal locations to adopt a Mediterranean-style design, incorporating flat roofs, often with balconies and interfaces, with other building elements. Again, the detailing is critical. NHBC has dealt with claims related to water ingress through flat roofs with the following issues identified:

• Poor parapet coping details, often using metal copings with no drip detail (NHBC Standards require a minimum 40mm drip), cavity tray or DPC beneath
• Poorly designed balcony details where the supports penetrate the building façade
• Building elements penetrating or fixed through the waterproof membrane
• Poor detailing between different parts of the building, for example at abutments and/or between adjacent roofs.

Curtain walling and cladding systems

Similar to windows, cladding should be tested to ensure that it can sufficiently withstand air and water pressure. Key issues to consider include:

• The system should be certified for use in the given exposure zone
• Where the cladding is not the primary weather-resisting element, the structure behind should be made weathertight in accordance with recognised standards
• Test samples should accurately represent the proposed end use, taking into account the support structure, fixing arrangement, damp proof details and interfaces
• Sealant joints should be designed in accordance with BS 6213:2000 ‘Guide to selection of constructional sealants’ and ISO 11600 ‘Building construction – Jointing products – Classification and requirements for sealants’; simply stating ‘mastic joint’ on the drawing is not acceptable
• Metal components should be manufactured using suitably durablem materials or protected to withstand higher corrosive environments.

The recommendations in this article represent good practice that could be applied to all homes, but it is particularly relevant where homes are located near the coast. In these conditions, the building is more exposed, which, without proper consideration, can result in corrosion of metal components and water ingress. There’s an array of technical guidance that can be applied that will help prevent these costly and disruptive claims, which are currently all too frequent.