Footing & oversite Question

[padstar]

I am due to start the footings for the rear extension and have a question before i do.

The ground floor will be constructed from a traditional suspended timber floor using 200x50 joists requiring a 150mm clear void below the joists for ventilation (as per BC requirements). So a minimum of 350mm from top of joist to ground level.

The extension is sitting where the existing patio is. You step down from the house to the patio as shown in the photo below by approximately 400mm. The patio then slopes away from the house.

My question is can i juts cut the footings out of the patio and then dig down and use the existing patio as the concrete cover required to cap the ground or do i need to remove it all back to soil and start again.

I was planning on removing the lot but then thought why bother as i will be saving the muck away, digging and then reinstating a 50mm concrete covering afterwards?

What is the best way to deal with this?

 

[^woody^]

You are supposed to remove the top layer of vegetable matter and then put a barrier down to stop anything growing. You already have stuff growing through the paving. Are you are confident that there are no large seeds waiting to grow there?

 

[padstar]

I had thought about this but assumed that with no light and a drenching in weed killer before hand nothing would survive.

It sounds like its back to the original plan then of pulling it all out and then capping the ground before installing the floor build up. A few more muck away lorries to be ordered.

 

[^woody^]

A thick weed barrier (and gravel) and glyphosate may well do if you think that there is little chance of anything growing through - past history