Tempted by office to residential conversions?

More than 217,000 homes were added to England’s housing stock in 2016/17, with nearly 18,000 of those facilitated through office-to-residential permitted development rights, according to figures published by the ONS* in November 2017.

There are plenty of post-war concrete-framed office blocks that are ripe for a revamp. The 3m floor-to-floor heights, great views and relatively open concrete structural frame make them well suited for residential use. But delivering them brings many technical challenges, which can add up to a significant cost per sqm.

Based on our experience of conversions including Centrepoint in London, Trinity House in London and Westpoint in Manchester, issues relating to sound transmission and fire protection are significant challenges for design and construction teams.

Acoustic considerations

Sound transmission is a concern, particularly in 1960s blocks as solid concrete structures tend to allow impact sounds to bridge between dwellings.

Part E for Refurbishment sets a requirement for a minimum 43dB DnTw + Ctr airborne sound insulation between dwellings, lower than for new build but nevertheless making compliance hard to achieve.

By consulting with Siniat’s Technical Support Team specifiers are able to call on experts able to provide solutions and installation details aimed at avoiding these common pitfalls.

Passive fire protection

The concrete structural core in 1960s or 1970s buildings are also unlikely to meet (Part A) Fire Safety requirements for residential use. An upgrade to the concrete columns are often required, to provide additional fire resistance, because either the columns are not deep enough, and/or there may not be sufficient concrete cover over the steel reinforcement; this ensures the concrete keeps its design load and does not collapse. A specialist board, such as Promat’s Promatect ®H is simply fixed direct to the concrete column with non-combustible fixings and decorated as appropriate.

With its traditional concrete slab, retaining horizontal compartmentation in a 1960s or 1970s conversion is also challenging. The floor slab may also need an upgrade because either the slab is not deep enough and/or there may not be sufficient concrete cover over the steel reinforcement. Promatect® H can be fixed directly to the underside of the soffit to ensure fire resistance is maintained through the slab to the flat above.

Similarly, designers should be vigilant of any suspended ceilings built or retrofitted in the office building. Vertical partitions must continue to the underside of the concrete slab and not to a suspended ceiling, where potentially there may be a void between the suspended ceiling and the concrete slab. A retrospective fire stopping solution may be required if a void is found and it is not possible to take the partition up to the underside of the slab.

Fire stopping

Any penetrations made to the original structure – for example, to introduce waste pipes and services – need to be fully sealed. Whilst there are a wide variety of fire stopping products available as individual components, it is essential they form part of an overall passive fire protection system that has been tested to meet the correct fire performance criteria.

Fire stopping is technically very challenging and requires consideration of three key facts:

  • the construction detail of the fire rated wall, floor or ceiling
  • the type of service penetration
  • the performance requirements in terms of fire integrity and insulation, and any additional requirements including the ability to accommodate building movement

This is why fire stopping products should not be specified in isolation and attention needs to be given to all of the technical requirements of the project. Promat has this in-depth understanding of passive fire protection.

National statistics announcement: Housing supply: net additional dwellings, England: 2020 to 2021

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