internal walls on slab -no footings?

[gismo]

hi all,

i've had some plans drawn for a rear 2 storey extension which will be 5m wide and go back 2.5m. the extension will be built between my neighbours external wall(he has already extended) and my garage wall.

the plans show that i will have a strip foundation for the 5m cavity wall that will run between the two existing walls, which i understand.

**Then i am to lay the floor slab between the 4 walls and build the internal side walls on top of the slab with lightweight blockwork.

(there will be no joists coming off these internal walls, they'll just form a cavity between the existing outer walls)

will the slab take the weight?

 

[noseall]

No. The slab will not take the weight.

In most scenarios insulation is fitted beneath the floor slab thus rendering it non-load bearing bar from stud walls etc.

How do you intend carrying the roof?

 

[gismo]

thanks noseall, i didn't think it sounded right. will i be able to lay another strip foundation right next to the neighbours footing on his extension then?

the roof:
the extension is going to be block work coming just above first floor level. the roof of the existing 2 floor house will basicallycarry on coming down at the same angle to meet the new wall, a dormer will be built out from this roof. - hope that makes sense.

 

[simonjay]

thanks noseall, i didn't think it sounded right. will i be able to lay another strip foundation right next to the neighbours footing on his extension then?

the roof:
the extension is going to be block work coming just above first floor level. the roof of the existing 2 floor house will basicallycarry on coming down at the same angle to meet the new wall, a dormer will be built out from this roof. - hope that makes sense.

The BCO was happy for me to dig alongside the neighbours foundations (a single storey party wall) to create a cavity wall against it. I actually had to dig some way below. I dug out the whole trench in one, which was OK since the ground was hard clay and showed no signs of movement. BCO inspected the trench like that and did not comment.
I've heard that sometimes you need a flexible separator between foundations but not sure why - unless it is connected to clay heave in some way.
Simon.

 

[gismo]

thanks alot simon, thats reassuring, i guess all i can do is dig the footings out along the neighbours wall and my new wall all in one go and just see what the BCO say, if they say nothing i'll just proceed and concrete the lot.

thanks for the help

 

[johnheritage]

I've heard that sometimes you need a flexible separator between foundations but not sure why - unless it is connected to clay heave in some way.

THIS IS A GUESS:

The two layers will bond to each other to some extent and one may be moving, placed, loaded or expanding differently to the other.

E.g. you have a rough surface on the slab, pour a new raft, it fills all those tiny gaps, sets and is then mechanically coupled on the long horizontal planes (where the expansion is the problem).

Like them there debonding strips used for walls with movement / expansion.

 

[par1sons]

can the weak concrete infill (goes in the cavity) go level with the ground level and the surface of the concrete slab??