Foundation sanity check.

[panhapticon]

After being told my first idea was doomed to failure, I've come up with two alternatives, the first comprises a minimal strip and foundation blocks, and the second is a full fill trench. Building will be an unattached outhouse, non-Building Controlled, 3m tall brick-and-block home gym, with no drains or utilities nearby.

Could I have some opinions as to the designs? Does anything strike you as utterly wrong? What would you do differently, if you had a limited budget?

In particular, I'd appreciate some brief answers to the following :

A - the LABC says insulation should be suspended at least 225mm above the bottom of the cavity. How should I do that with batts? Is it safe to sit them on wall ties, or will they sag through over the years?

B - the top ~20 cm of the soil here is quite well drained. Do I need a French drain here, or is a trench of pebbles good enough? My goal is to reduce the water being held against the bricks.

C - for the first option only, would I need to reinforce the footings? The majority of the forces are compressive...

For a full-fill trench, how high should I fill? I'm considering 10cm below ground level, or 225-300mm below DPC. Is that sensible? In accordance with unofficial BC advice, trench will be at least 1m deep.

Insulation upstand - is 7.5cm too much?


I'm strongly leaning towards option 2; it'd be cheaper (additional 6M³ C30 is cheaper than 120 foundation blocks and rebar), easier and quicker (pour-tamp-wait versus laying 120 20kg blocks), and simpler (one pour versus mixer + cement + sand). Are there any good reasons to not trench fill?

Thank you!

 

[noseall]

A - the LABC says insulation should be suspended at least 225mm above the bottom of the cavity. How should I do that with batts? Is it safe to sit them on wall ties, or will they sag through over the years?

Once you have laid the DPC, you then set out your first course of tie wires, upon which the cav' insulation sits.

B - the top ~20 cm of the soil here is quite well drained. Do I need a French drain here, or is a trench of pebbles good enough? My goal is to reduce the water being held against the bricks.

Why? Anything below DPC will be inherently wet.

For a full-fill trench, how high should I fill? I'm considering 10cm below ground level, or 225-300mm below DPC. Is that sensible? In accordance with unofficial BC advice, trench will be at least 1m deep.

The top of your concrete should ALWAYS coincide with a pre-set distance up to DPC level. That said, we strive to pour as much concrete as possible into the trenches, to save on laying masonry. Ideally you need to accommodate any changes in exterior ground level as well as any gullies etc, that sit next to the building. 90% of the time our ideal foundation top is 525mm below DPC.

Insulation upstand - is 7.5cm too much?

Eh? The insulation upstand needs to deal with any unprotected masonry, up to FFL. This could mean putting your inso' upstand around prior to fitting the floor insulation (deep pieces) or after (shallow pieces).

Are there any good reasons to not trench fill?

Drains.