2nd November 2012

How long should a house last?

The checklist for a new build is growing. Under the weight of increasingly stringent regulations, the industry is conducting scrupulous assessments of every material used, every measurement taken, every decision made. And yet no-one seems to require a minimum lifespan for a building.

This fundamental question seems to have taken a back seat in recent years. The (now withdrawn) BS 7543: 1992 suggested a 60 year design life, which still appears to be the accepted norm, but this was not included in the standard BS ISO 15686 which replaced it.

The recent London Housing Design Guide does not include this either. This Guide was formulated to act as blueprint for all housing to be built in the city: there is a meticulous attention to detail with regards to room sizing, housing density, waste disposal; and yet there is no requirement for the building life.

There does seem still to be a general acceptance of a 60 year lifespan (and indeed the BRE Environmental Profiles are based on a 60 year life expectancy). I would argue that this is a rather arbitrary figure, one that apparently has its origins in a financial model, being used as the time over which financial assets are written down.

There are currently moves to introduce a 50 year reference period for Life Cycle Assessment studies, which would harmonise this thinking across Europe, but surely this is moving in the wrong direction.

Consider our own housing situation in the UK: 39% of houses are over 65 years old. If we continue to build houses at the current depressed rate then any house built today will need to last for over one thousand years. In this context any standard that suggests an acceptable lifespan for new buildings of just 50 years seems odd to say the least.

This is not just a problem for the next generation, however, it has huge significance for both builders and homeowners right now.

The issue was brought home to me by the experience of a colleague who was unable to re-mortgage his house (a lightweight timber frame structure) to build an extension because the lender was not confident that its life expectancy would exceed 20 years.

This was in stark contrast to my own experience: I had just successfully negotiated a re-mortgage to extend my own house – a masonry structure built in 1840. Clearly decisions are being made on the anticipated lifespan of a building but there is no widely accepted method of evaluating this.

For builders the anticipated lifespan will have as significant impact on the way houses are constructed. We are being asked to consider the embodied as well as the operational carbon footprint of new buildings, but you can only do this if it is considered over the lifespan of a building.

A recent NHBC Foundation study compared operational and embodied CO2 in different types of new build houses, using 60 and 120 year life cycles upon which to base the calculations, but nowhere is there any data that supports the assumption that all of the building types will actually last for 120 years..

Making longevity an objective will have a huge impact on the way buildings are designed: the focus of the designers will then be on the performance of the building in use rather than the embodied energy of the original structure. Given how long our houses are going to need to last, this is surely a more sustainable approach.

We urgently need new housing to cope with a growing population. All construction uses resources, but surely the less often buildings are replaced the better. It could not be a sustainable approach to accept that new buildings could need replacing after just 50 years.

Our focus surely must be on building sound, energy efficient structures that will last several lifespans with the energy efficiency built into the fabric. Now might be a very good time to consider a longevity calculator that works in a similar way to SAP calculators and to define a minimum acceptable lifespan for the main structural elements of the building fabric.