Rebar / Mesh / Building Cavity Wall On Lintels etc.!

[tbrain]

All,

I am currently in the process of building a side & rear extension on my house, and having dug the footings am about to order in the concrete prior to the arrival of the sub-contractors...

To set the scene...

Down the side of the house, my extension will sit about 6 inches (to allow for drainage) from the neighbouring property. He has requested that I do not mass fill the entire trench (as originally planned), but shutter off around his footings and drop in three seperate blocks of concrete either side and inbetween his chimney breasts on which lintels wil sit that will span over the footings for his two chimney breasts. The cavity wall will then be built on these lintels. The end result is that there should be no weight at all sitting on his footings.

Questions:

i) Do you recommend that I reinforce these footings with rebar / mesh? They will be approx 2000mm L x 600mm W x 300mm H. At what intervals would you place the rebar / mesh?
ii) The man from the council obviously wants calcs for the weight that will bear on the lintels. Rather than go out and get a structural engineer, I was hoping that I could just over engineer it based on there being an average weight bearing per m2 of wall as I have the load span tables for the concrete lintels that I'll be buying? Is this possible, or should I just go and find a structural engineer now?!

 

[scarypants]

building controls are likely to request calcs and if you just assume it is okay to go over board with reinforcement they will want to know what you based your own calculation on?

 

[noseall]

Visualising your scene, i guess he has a wall with a pair of external chimney breasts jutting out from his building.

So, if you are building to within 150mm of his chimney breasts, why are you having to segregate your footings? Surely there is room for both yours and his butted, or at least room for some differential barrier, i.e. 25mm polystyrene?

If it were me, i would be tempted to bring my footing in an inch or two so that i could build it conventionally. Is room, or the lack of it, at a premium.

Is it a gable wall you are building? If not, 6" does not leave any room for soffit and gutter overhang.

 

[tbrain]

Yes, it is a gable wall & he does have a pair of external chimney breasts jutting out from his building. Unfortunately, his footings protrude 12 inches & the friendly council man is stating that the cavity wall has to be built in the middle (not on the edge) of any concrete footings. As you state, room is at a premium - it would just be a double room making full use of the space. Losing even 6-8 inches makes a significant difference

6" for soffit and gutter overhang was an estimate based on 114mm guttering. In your view, what room needs to be left for the soffits & guttering?

Also, I'm guessing that as the question of lintel load hasn't been answered by anyone that I should just go to a structural engineer?


Cheers, and thanks for your help!

 

[noseall]

6" for soffit and gutter overhang was an estimate based on 114mm guttering. In your view, what room needs to be left for the soffits & guttering?

It depends what you are matching up to, i.e. existing eaves/soffit detail.

A typical soffit overhangs the brickwork by about 9" plus say 5" to accommodate the gutter and brackets. Total - 14".

It is possible to build without soffit if that is the style of your existing building, i.e. brick corbel or oversailer with galv' rise 'n' fall gutter brackets at the eaves detail. This method would still overhang the building but could be constructed within the 6" overhang mark.

 

[Shytalkz]

Hi tbrain: anything spanning at foundation level will require a pukka design for the LA.

Walls do not have to be built in the centre of footings; again a simple calculation will demonstrate that, for trenchfill, a wall sat on the edge of a 450 footing will be fine.