What height to set extension slab? dpc level or house level?

[andyandy1]

hi

what height should the slab be set for a new extension?
the original house has solid floors downstairs with ashphalt over the top of the slab. This means that the original house floor level is higher than the original slab and dpc level.

on the new extension i can see two options as follows.

1. to build up the extension to dpc and have the face bricks and internal blocks level with the existing house dpc and lay the slab level with this. this will mean that when the extension is knocked through, the original house slab (with ashphalt) will be higher, and so the new slab will need to have self leveling compound or similar installed to make it level with the house. a fair amount of self leveling compound would need to be used to bring this upto level. This area is going to be tiled and so needs to be level.

2. to increase the height of the blocks a little inside the new extension to make them the same height as the original house floor slab with ashphalt ontop. then lay the new slab level with the new blocks which are level with the internal slab. this will mean it is slightly higher than the original house dpc, but level with the internal house floor slab because of the ashphalt.


is method 2 an acceptable way of doing it, or should it be done to existing dpc level and then leveled off after? or is there another way of doing it?

 

[^woody^]

Floors should run level and alter the new DPC and DPM as required

 

[andyandy1]

great, thats what i thought.
thanks for advice.

 

[andyandy1]

another slight complication with a few outcomes....

the house ashphalt is higher at one end that the other.
the full internal width of the extension is 6.5m.
over that length one end of the ashphalt is 16mm higher than the other.
this leave a few ways of laying the new slab.

1. create the new slab perfectly level with the lowest point of the ashphalt in the house. this will mean that the house has a gradient rising upto 16mm higher than the slab at one end. its not necessarily a straight gradient either.
this will then mean that the full extension will need "feathering in" etc which will require quite a bit of work to get 'flat' ready for tiles.

2. create the new slab perfectly level with the highest point on the house. this will then create similar work to above.

3. run the internal blocks of the extension level with the highest point on the house slab as an 'L' shape. blocks will go out from the house 3.2m, then return to the garage wall 6.5m. this will mean that on the end of the 'L' the blocks will be roughly 16mm higher over a length of 3.2m.
I hope ive explained it well so far....
Then slope the slab in 2 directions, mating it up with the house in the other corner. This will mean effectively the slab slopes from 0 to 16mm over 6.5m and from 0 to 16mm over 3.2m in the other direction.
This should then in theorey more or less go with the original house slab but might need a little grinding or leveling after.

4.run the internal blocks of the extension level with the highest point on the house slab out 3.2m. then slope the return which is 6.5m with the slope of the house which is 16mm over the full length.
This will then create an unlevel slab in one direction but will be flat with the house. It will also mean that the firthest point away from the house where the door is going will have an unlevel slab, whereas option 3 would make the slab level where the door is.
this will leave an unlevel slab, but flat with the house.

i hope ive explained that well.
i cant really win which ever way its done, just wondering which is the best or correct way?

 

[^woody^]

Generally, things should be built level, but 16mm is nothing over that distance, and it can be feathered in without being noticed. Or spilt the difference 8mm each end

 

[andyandy1]

if i lay the slab level with the lowest point and then feather in the floor later, is there a cheaper way of doing it than with leveling compound or powdered tile adhesive?

i dont think any sand/cement mix would be strong enough being feathered from 0-16mm and would just crumble and the only thing i can think of is the above.

i could even use cement tile board over some of the area and feather the rest with compound, but its still using expensive boards and adhesive.

thats the reason i was thinking of doing it in the actual slab because compound or adhesive will cost a lot using it for that.