Footings for a garden room

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31 May 2013
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Oxfordshire
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United Kingdom
Hi,

Apologies that similar questions have been asked loads of times, but I can't find an answer to this particular scenario.

The plan this year is to build a wooden-framed garden room for crafting. The dimensions dictated by the plot available are 9m x 2.8m. Although timber framed, it will be insulated all round with 90mm / 100mm Celotex, be clad in something decent such as Cedral Lap or HardiePlank, and be fitted with uPVC double-glazed windows and door.

I've prepared a rough bill of materials rounding everything up and it should weigh no more than 5 tonnes all in.

The plot is level but sits on Oxford clay, and conventional wisdom dictates taking the footings down at least 900mm, possibly 1000mm. The house was built in 1971 and the footings go down 600mm. When we extended in 2014 building control insisted on 1000mm, and 2000mm in one corner due to the proximity of a tree.

The garden room won't be near any trees and it will be next to a 45m x 1.2m dry stone wall which has stood without any footings to speak of for at least 50 years.

Here's the plan.
  • Dig footings as a 300mm wide x 450mm deep trench and fill with 300mm of concrete.

  • Bring up the level with a single course of high density concrete blocks topped with a single course of engineering bricks. Include 3 vent bricks in each of the long sides.

    This will put the DPC at 150mm above ground level.

  • Remove the turf within the perimeter and cover the ground with a damp-proof membrane and weight / peg down.

  • Top the brick course with DPC and sit the timber frame on that, secured against lateral movement with screws.

  • Build a suspended floor off the timber wall frames leaving at least 100mm gap above ground, and insulate with Celotex, being careful not to block the air vents.
Does this sound sensible?

My only concern is failure of the footings along the 9m length due to the clay shrinking / heaving with the seasons. Should I be thinking about including some rebar?

Most people seem to build these things on piers or ground screws, which is an attractive proposition, but I'd need to shutter as we've already had trouble with badgers burrowing under a shed on piers. By the time I've done that I may as well put in "proper" footings.

Nomis
 
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