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Standards Extra Closed or mothballed sites The current downturn in construction activity is resulting in a number of sites and plots being closed or mothballed. Past experience shows that where this has not been done correctly, it has resulted in substantial future claims costs for both builders/developers and NHBC. At any stage of construction it is vitally important that appropriate precautions are taken to ensure the durability and likely in-service performance of materials, products and systems is not impaired by the process of being closed or mothballed. To ensure potential issues are fully considered builders/developers should produce a practical site closure plan. The plan should consider the following aspects: Foundations and substructure Where plots are closed just after the foundations have been installed action should be taken to provide protection to ensure that rain, ground water, drying weather and frost cannot adversely affect the foundations. Where specialist foundations and ground treatment have been employed and left exposed the designer should be consulted for advice. Weatherproofing Where possible, dwellings should be made fully weathertight. Where this is not achievable cavities should be closed to avoid water and debris accumulating andtoprotectanycavityfillalreadyin place. The protection of exposed timbers including all types of sheathing, plywood, flooring elements, joists, and roof members is also very important. Particular attention should be given to timber frame construction and the possible problems associated with increased moisture content of the 4 A mothballed site timber over long periods. Temporary sealing of openings in the envelope will be required for both weatherproofing and security. Additional items to be considered include the provision of temporary sealing to prevent infestation by insects and rodents. Ventilation Where plots are closed it is very important to ensure adequate ventilation is provided to prevent possible fungal attack and mould growth to floors, walls, roofs, ceilings, fixtures and fitments. UV degradation Materials that may be adversely affected by the effects of solar UV should be protected to avoid premature degradation. Materials could include dpm’s, dpc’s and roofing felts (including vapour permeable underlays) which would normally be covered. Drainage Plots that have completed roof coverings in place should also have all rainwater guttering and downpipes completed and connected to a suitable outfall to prevent the possibility of damage caused by excess water running over external walls etc. Site security You must be mindful of any health and safety risks presented by the vacant site. The Principal Contractor maintains a liability for the safety of anyone on the site regardless of whether or not they are lawfully there. Because of this, it is important to leave thesiteinassafeastateaspossible as well as taking measures to prevent access to the site. Fencing should be continuous around the site and panels should be of the anti-climb type and double clipped to make access difficult. Where the risk of intruders is high a solid hoarding with furthersecuritymeasuresmaybe appropriate. On site you should make sure any structures are safe and if possible remove all scaffolding from the site. If scaffolding remains make sure all ladders are removed, or locked out of sight. No excavations and manholes should be left open. Securely covering manholes and filling excavations is ideal, but if this is not possible, openings should be adequately fenced off. Ensure any plant is removed from site or locked securely in the compound if removal isn’t practical. Ideally materials left on site should be kept to a minimum, whatever is left on site should be stacked safely within a storage compound or locked in storage containers. Even when the site has been mothballed it is still the Principal Contractors responsibility and regular checksmustbemadetoensurethe perimeter is secure and to check for any signs of trespass or damage. Storage It is important that materials and products are correctly stored in an appropriate manner to ensure their durability and design life is not compromised and to ensure Health and Safety and security issues mentioned above are addressed. Inspection NHBC staff (normally Building Inspectors) will advise builders/developers of the precautions to be taken when sites and plots are closed or mothballed as detailed above. These are practical measures to mitigate damage to plots at all stages of construction ensuring that when work restarts damage is avoided or at the very least kept to a minimum. ACTION Where sites and plots are closed or mothballed an appropriate closure plan, taking account of the above, should be developed and implemented by builders/developers to reduce the risk of additional cost when work re-commences.
Issue 43 December 2008 Gable walls - the importance of restraint When the wind blows parallel to gable walls it generates negative pressures on them. If the gables are not adequately built and connected to the roof structure these negative pressures can ‘suck-out’ the gable walls resulting in catastrophic failure. This has occurred on a number of sites recently during high winds and has highlighted the importance for gables to be correctly constructed and restrained. The common practice of using trussed rafters leads to a situation where none of the roof self-weight (traditionally from purlins and binders) passes into the gable wall. With this form of construction the lateral restraint to the gable has to be provided by adequate strapping at the verge (or by a gable ladder) and at ceiling level if required (see strap location diagram). The restraint must be correctly detailed and installed. As well as providing lateral restraint to gable walls, strapping also contributes to the robustness of the building and reduces the sensitivity of the building to disproportionate collapse in the event of an accident. Restraint straps therefore provide a critical function. The following diagrams show an acceptable method of satisfying building regulations for residential dwellings not exceeding 3 storeys. Where these dwellings exceed 3 storeys and have been designed in accordance with BS5628-1:2005 (Code of practice for the use of masonry) the connections to gable walls should be provided at intervals of not more that 1.25m centres for all storeys or as specifically required by the design engineer. Effective strapping at ceiling level Effective strapping at verge Strap location Ensure that: Collapsed gable wall ACTION The straps are installed and fixed fully in accordance with the building regulations guidance and in accordance with manufacturer’s and designer’s requirements. Rafters are not notched to make the straps flush with the rafter. The strap goes under the rafter and over ceiling joists. At the verge the strap turndown should be on a substantial piece of blockwork, preferably the centre of an uncut block. The strap should turn down vertically a minimum of 100mm and be tight against the face of the inner leaf wall. Packing should be provided, at the strap position, between the inner leaf and the first truss. Noggings (dwangs) are provided and set horizontal unless the strap has pre-formed twist to line it up with the roof slope. A high level of workmanship is maintained throughout the construction. 5
Page 1 and 2: NHBC’s technical newsletter Stand
Page 3: Issue 42 September 2008 Problems Re
Page 7 and 8: Issue 43 December 2008 Secure escap
Page 9 and 10: Sustainability Extra 3 December 200
Page 11 and 12: NHBC Foundation The NHBC Foundation

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