how to calculate foundation depths

[Dodgy-diver]

can anyone help me please , ive spent ages online and cant get a accurate answer

 

I have a 1920s house ( red brick) I plan on putting a timber framed single storey extension on the back 5.5m x 3m 

 

there is no trees for 20 meters at least . no drains running through , 

 

can anyone advise me please 

 

my assumptions so far from finished floor I do down 100mm for screed , 100mm for insulation and 50mm for sand and damp proof membrane 

so that means ill need a little brick wall 250mm high on top of the foundations , but how deep on top of this fo I need to dig down please 

 

thank so much for any help 

[Russell griffiths]

That’s a bit to simplified. 

If you only had 250 of brickwork you would see the foundation concrete at ground level, you would normally have 3-4 courses of bricks/ blocks below ground level. 

 

Your floor is a bit simplified as well, you haven’t got a structural element in there, unless you are mixing up the words screed with concrete, screed is not structural so you cannot just put that on top of insulation. 

 

You also cannot put the insulation on the membrane on the floor, you would did down to firm ground and add 150mm of stone heavily compacted then the sand then the membrane then your floor structure. 

 

As far as foundations go you will probably go into the ground at least a metre unless you are on some sort of solid ground. 

 

You need to do some section drawings to a scale so we can point out the various build ups. 

[Dodgy-diver]

my issue is that the external ground height is only going to be 3 inches lower than the floor in the extension due to the existing patio that is already in place and the floor in the house already 

thanks 

[Russell griffiths]

30 minutes ago, Dodgy-diver said:

my issue is that the external ground height is only going to be 3 inches lower than the floor in the extension due to the existing patio that is already in place and the floor in the house already 

thanks 

That’s not really what you want, your damp course level should be 150mm above ground height as usual will get water splash bouncing above your damp course. 

 

[Dodgy-diver]

with out digging the entire patio up thats not possible  the patio is huge with a slight fall as my garden slopes towards the house . does the damp course have to be below door step level ? I had planned a timber frame building but 150mm from outside to finished floor is just not possible 

 

do I have any other options ? thanks 

[Onoff]

1 hour ago, Dodgy-diver said:

my issue is that the external ground height is only going to be 3 inches lower than the floor in the extension due to the existing patio that is already in place and the floor in the house already 

thanks 

 

You can step your damp course to achieve 150mm DPM above the external ground level. A French drain is an option too if you can't make 150mm.