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Pull the carbon out from under your very sustainable building

Words:
Steve Webb

Steve Webb continues his series on reducing embodied carbon in structures by revealing more sustainable ways to build foundations

A new lower carbon model for foundations. Carbon values total for footing material only, based on 15m by 10m house near trees on shrinkable soil.
A new lower carbon model for foundations. Carbon values total for footing material only, based on 15m by 10m house near trees on shrinkable soil. Credit: Steve Webb

The honey tone of your exposed Douglas fir frame proudly proclaims sustainability and a planet-loving rapprochement with nature, but under the ground lurks a dirty secret: strip footings and a concrete ground slab. 

Consider a beautiful woody 300m² house you have designed. It weighs in at a mere 30t of timber above the ground floor, but the 2.5m deep footings that encircle and traverse the footprint of the building – together with the hefty suspended slab – weigh 325t, giving off 50t of CO2 without even beginning to consider the excavation and muck away. 

A cascading set of carbon choices. Why do footings need to be continuous? Much of this habit derived from the need to support brick cladding. Intermittent footings with beams between (pier and beam construction) will knock out 75% of the foundation concrete. 

Let’s ditch the slab. It’s totally feasible to build a suspended timber floor. Not only is it lighter and quicker, but if properly detailed it will last as long as all the Victorian timber ground floors we are sat on right now. 

Soil drives the footing depth. Footings need to be extended beneath the horizon of moisture movement especially when there are trees nearby. This doesn’t mean that the whole hole needs to be filled to the brim with concrete. Why not build a pad at the bottom of the hole and a brick pier onto it (1950s style)? This not only reduces the material used but a lot of the excavated soil can also be put back down the hole.

These highly conventional moves would bring your carbon footprint down to about 10t.

What else can you do? How about stone pads with stone piers? If you can use a local stone this would reduce the carbon further to about 1.6t, and the stone blocks can be removed and reused in the future. These solutions still involve a lot of spoil, digging and disturbance. Although made of steel, screw piles are removable, up-cyclable and involve little digging. Small screw piles can be hand driven, and can achieve loads of up to 7t. How does the carbon footprint compare? A screw pile may weigh something in the region of 40kg, creating perhaps 100kg of CO2. In a typical clay this is equivalent to a 0.85m strip footing with a weight of 3,500kg and carbon footprint of 500kg. It’s much better (and recyclable), but it’s worse compared to a stone slab 4m deep. 

These are simple and immediately applicable changes that have significant carbon benefits. Solutions like the stone pad and pier involve more complexity and labour: in order to build pads at the bottom of a deep hole, protection is required for the excavation, the stones need to be cut and laid and the hole backfilled. This means that not only are you saving carbon, you are also creating employment. This is the core issue.

The desire in general building practice in wealthy economies is to limit expensive man hours – both in design and execution – and use cheap materials instead. Over-rationalised structures save design and construction time, but with the by-product of maximising carbon. When carbon is taxed properly, labour can be taxed less – perhaps then building will create more employment and less carbon. 

Steve Webb is co-founder of Webb Yates Engineers

Read more of Steve Webb on reducing embodied carbon in structures including on house extensionstowers and design approach. And on using stone

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