Loft Conversions and The Party Wall etc Act 1996
Many people will wish to expand and enlarge their property for additional space, and going upwards seems to be popular, whether one is dealing with a residential property or a commercial office development. Pre-planning is important to establish which loads have to be transferred from the roof to the remaining structure below, in a safe manner, with the minimum of disruption.
There will be times when The Party Wall etc Act 1996 is triggered by such works, and professional advice might be needed here. If the party wall, whether to a terraced, semi-detached, or other property, is cut into, amended or raised, then notices may need to be served on adjoining owners. This includes firebreak walls such as in roof spaces, which provide a fire barrier between different properties. Loft conversions may require changing or adapting the timber rafters and cross-beams, termed ‘purlins’, and installing new beams and supports. The loads from those beams need to be transferred to the structure in a manner that avoids concentrated loads which might otherwise adversely affect an older masonry structure.
Before work starts an engineer, or surveyor will need to assess the loads to be transferred through the structure from the loft conversion layout. This may require inspecting lower floors of the property, to ascertain the condition of walls, chimneys, and masonry, and perhaps to arrange a schedule of condition identifying structural features, cracks, etc. Schedules of condition, based on a record of what is there and amplified with photographs, will assist the owner and adjoining owners if there is a problem later on, but are not specifically mentioned in the Party Wall etc Act 1996.
One question for the engineer is whether the increased loading at upper level say, in forming a new additional floor, can be sustained adequately through the structure and foundations below. This might require inspection at ground floor level or basement level, and even excavating a small trial hole to ascertain the existing foundations.
Each case will be different. If the loading is not exceeding around ten percent of the overall original loading, then there should be little problem in the existing foundations taking the loads. But, if the building is fragile, and there is evidence of cracking of masonry around some physical features, extra care needs to be taken. Making substantial alterations in the roof structure and removing certain supports, may result in spread of the roof, with adverse consequences.
It is for this reason why the existing roof layout needs to be understood structurally, by the designer, before any alterations are made. Those who take the time and trouble to trace the members of the existing roof structure, and identify how loads are transferred, can be confident that this is time well spent in avoiding problems later on. ‘Know your roof’ is a good starting point.
Existing upper level ceiling joists are usually inadequate for imposed floor loading, so will have to be replaced or upgraded to ensure joists are of sufficient depth and load-carrying capacity for the new rooms and areas. Sometimes, if levels are not properly calculated within the design, the ends of steelwork can be seen projecting through the external roof, often encased in lead for weatherproofing. This can often be avoided with pre-planning and careful checks on levels.
Where loading from the new layout, perhaps incorporating a mansard roof or similar, exceeds ten percent of the original loadings on foundations, then detailed structural calculations and investigations will usually be required. Are the existing bricks in the walls sufficiently strong to take the loads, or not ? If not, then the loads need to spread across a larger area of masonry to avoid overstressing the structure, or perhaps a pier rebuilt in some cases. Access to other tenancies below the roof level for inspections of the structure, can prove problematical at times. Use of lightweight structural solutions might be helpful here, to reduce loading. The foundations and the subsoil below may need close examination.
Raising the party wall for the alteration is permitted subject to the requirements of the Act. Light wells can sometimes give access to areas for trial excavation pits, to further explore subsoil and foundations, so should be considered. Engineers will want to avoid overloading the structure and foundations. Underpinning of existing foundations is one way to strengthen the existing structure, but is a complex procedure which needs project managing by a professional. However, in most situations, for loft conversion underpinning would not be required. Planners may require the roof line to be maintained for aesthetic reasons, and impose strict conditions.
Concentrated loads from new structural beams can be spread over several metres of structure below, through a forty-five degree line. Pockets may have to be cut into the wall via concrete padstones for supporting steel beams. The padstone area will need to spread the loads to below approx. 0.4 Newtons per square millimetre to protect older masonry below, which is roughly the crushing strength of older masonry. The engineer will be aware of this.
Percussive tools should not be used when cutting into the party wall. Vibration from them can cause damage. Disc cutters are better in this respect if care is used.
Then, there is a practical issue of the length of the new supporting beam :- Beams will be longer than the span you need to cover and one can install ‘nested-channels’ either side, to slide these within the steels and the beam bearings. This is like installing a mini beam within the steel beam, and overcomes the problem of installation. Normal tolerance bolts, if used to splice a steel beam, may result in deflection, due to the holes in the steelwork, so needs to be addressed, to avoid any sagging or deflection when the final loads are applied to the beam. Splices with steel plates can be used in some cases to connect sections together.
Neil Thind of Arun Associates shows Nick Huband, (a forensic building engineer from William J Marshall and Partners), a picture of damage to finishes in a neighbouring property, in the video. This is where the end of a new steel beam installed by the building owner’s contractor was actually impacting the party wall, damaging masonry and plasterwork, due to lack of care. It might be because the steel beam was a bit too long in length, or it was carelessly swung in and hit the party wall brickwork. The contractor and his subcontractors needs to be both competent and careful, during the installation of structural supports.
Cutting into flues and chimney breasts should be avoided where possible, but sometimes it is not possible to avoid this. The Building Regulations need to be adhered to in respect of both structural work and fire-resistance to prevent fire spread from property to property, i.e. using approved fire barriers. Where this is a party structure, the party wall surveyors will see there is provision made for any such necessary work.
Flues need to be tested before works are carried out and sealed off where work is about to commence. This needs to be done in such a way, that flues can be properly traced, eg with ping-pong balls or coloured smoke pellets. No flues should be sealed-off without ensuring the user / occupier is made aware, for safety reasons.
In many tenanted or leased properties, there will need to be much coordination and sharing of information to enable the works to be planned and implemented. All relevant Party Wall notices and awards will need to be in place before any relevant work commences.
The above is general guidance only. You are advised to obtain professional advice before work commences or is designed, and to
obtain all necessary statutory and other approvals and licences in good time. Note also, that the Construction (Design and Management) Regulations 2015 may apply, and these regulations also apply to domestic works, affecting clients, designers and contractors.
N.B. This blog was prepared on behalf of Arun Associates in conjunction with William J Marshall & Partners, to support a video on the subject matter.