Home extension projects can be very heavy on embodied carbon, but there are greener ways to manage the load – and they’re cheaper too (drawing downloadable below)
We hear so many complaints about the number of steels or the size of the footings in domestic refurbishment projects, but how can you do this kind of work without racking up a big bill and a big carbon footprint?
Let’s consider a typical Victorian terraced house (1). The owner wants a dormer, a closet wing extension (2) with a through lounge and to take out the chimney breast.
The usual practice, starting from the top, is to span steels from party wall to party wall and under the chimney (3) and support a new floor between them, totally ignoring the load bearing spine wall and ceiling joists. The dormer is dealt with in the same way, full width steels (4) and probably a moment frame to the back. Further down in the building a through lounge opening is added with a steel lintel. The rear facade is removed at ground and replaced with a steel box frame (6) which also supports the upper closet wing. The back of the back extension is supported on a steel portal (7) also needing to take wind load. Finally, because the site is on clay, 1.5m x 0.6m wide trench footings are dug in around the perimeter spanned by a 200mm slab (8).
There is an alternative. Re-use the existing ceiling joists and carry the loft conversion load down the spine wall. The spine wall has carried the roof load and a loft full of junk for 100 years, so why not? The existing 100mm ceiling joists aren’t strong enough so either double them up with bigger joists, or, for more head room, screw a ply deck to the existing joists, strengthening them with a stressed skin (9).
Prop up the front rafters and add stiffness to the floor by making the framing for the eaves cupboard a ply web girder (10). Use the fold in the roof and a couple of 200 x 50s to carry the ridge (11). Hold the chimney breasts up on a timber cross beam – it doesn’t carry all the load of the chimney because the bricks corbel out. Make the rear of the dormer a portal frame using timbers (13) with a steel elbow for wind restraint.
The through lounge (14) can easily be achieved in timber as the wall above is light and the spans short. Make a timber portal frame for the rear facade (15). In a house like this it will need perhaps three 360 x 75 LVL beams, with a hidden flitch plate – designed to carry just the wind moments (not the dead load moments which would require too many screws) by screwing the plate after the wall load is transferred. The closet wing can be supported on more LVL beams (16) – orienting them in this way means you won’t need to needle and prop the wall, which will save time and cost. The timbers don’t need additional fire protection due to charring resistance and so can be left exposed.
Most Victorian houses have timber ground floors. Properly ventilated and drained they can last forever, so why bother with a heavy slab, just build a suspended timber floor (17). Finally, now your extension is as light as a feather, if you clad the boundary walls in tiles instead of bricks (for example) you can ditch all footings in favour of hand driven oak mini piles (18). A hand shear vane can be used to estimate soil load capacity and a geotechnical engineer can easily calculate the capacity. In normal London clay you might expect a 2m, 100 square pile to carry 2t (a lot relative to a small extension).
The piles can be driven in by the builder (not a piling specialist) using a £300 petrol powered fence post driver by driving a pilot hole with a steel pole first and then pulling it out and driving in the oak one. A big saving on digging and concrete, and really fast!
QUICKER, CHEAPER AND GREENER
I’m convinced that many builders would prefer to build this way. It’s cheaper, its quicker to build and it creates far less disturbance: no digging, no concrete deliveries, no cranes for steels, just the gentle tap, tap, tap of carpenters.
It’s quite difficult to find the data on how much residential extension work is carried out in the UK each year. If I had to put a rough number to it I would say of the 40 houses in my ordinary Willesden street, five have converted their lofts in the last three years (4% annually). If there are say 12m houses in the uk and only 1% do an extension or a loft annually that’s 120,000 lofts and extensions. If each of those reduced the job by 15t of CO2 that would amount to 1.8m tonnes of CO2 – 0.5% of the UK’s total emissions.
Steve Webb is co-founder of Webb Yates Engineers
IN NUMBERS
3t Less steel = 7t Less CO2
50t Less concrete = 7.5t Less CO2
0.5t More timber = 0.6t net sequestered CO2
TOTAL 15t Less CO2