Extending sloping driveway slab to create base for timber framed side extension

[DamoZ]

Hi all,

I want to build a timber framed room on the side of my house, mainly for storage initally but may make it habitable later on. There’s an exising concrete driveway slab 200mm thick that I would like to use as a base although I’d need to extend it so that it meets the house as there is currently a pathway betwen the driveway and the house. The main issue I have is that the slab drops almost 200mm from front to back so has about a 5-6% downward slope.

I was planning to buid a 150mm high block permeter and build the timber frame on top of that with a DPM in between. I’m not really sure about the best way to go about extending and levelling it.



Option 1 – Extend the slab along the slope of the eisting slab: Drill 10mm diameter 150mm deep holes spaced about 300mm apart horiontally into the side of the exisitng slab and insert rebar dowels then tie reenforcing mesh to these and pour concrete (on sub base of hardcore & type 1 MOT so that I end up with an extended version of my existing slab. On top of this build a level perimeter wall and then use hardcore and type 1 compacted down twith sharp sand top layer to make a level internal subfloor. Then add DPM, celotex and top with either T&G chipboard or a double stageered layer of 12mm hardwood pywood



Option 2: As option one except to level the internal floor I would lay celotex on the sloping slab and then build furring joists to create a level floor that way



Option 3: Extend and level the slab with concrete. This seems like it’s going to use a lot of concrete however because the far end of the slab with then have about 400mm of concrete while the front end only has 200mm. Is the concrete likely to crack under it’s own sheer weight with theis option.



OPtion 4: Any other suggestios?



I’ve attached some photos of the area so you can see what I mean.



Thanks!

 

[noseall]

Hi all,

I want to build a timber framed room on the side of my house, mainly for storage initally but may make it habitable later on

Lose the slab. It will create more problems than it solves.

 

[DamoZ]

Lose the slab. It will create more problems than it solves.

Ok, and then replace it with another slab? What sort of problems will it cause later on? And if Idon't make it habitable how should I comtinue with my current plan to extend the exisiting slab?

 

[noseall]

Ok, and then replace it with another slab? What sort of problems will it cause later on?

It’s very difficult to mitigate damp ingress when the slab is close to the surface (rain/weather) and if the footprint of the slab is bigger than the structure above.

And if Idon't make it habitable how should I comtinue with my current plan to extend the exisiting slab?

Prep similar to existing, ie remove soil and dark growing matter, lay compacted hardcore, shutter and concrete. You could drill in some 200mm rebar dowels if you want to.

 

[DamoZ]

Ok thanks. I was going to lay a perimeter wall around it 150mm high then DPM covering the concrete and up the insides of the perimeter wall and over the top of it with the timber sole plate on top of that. Then use some heavy duty galvanised steel straps to join the outside of the perimeter wall to the timber sole plate so as not to pierce the DPM. I guess I could also paint a few coats of blackjack type stuff on to the concrete and inner side/top of the perimeter wall to stop any damp coming through? As the existing slab is sloping down I guess some drainage holes at the lower end wouldn't be a bad idea just so that any water that did get in could still get out?

 

[noseall]

Ok thanks. I was going to lay a perimeter wall around it 150mm high then DPM covering the concrete and up the insides of the perimeter wall and over the top of it with the timber sole plate on top of that.

Sensible. If you have room for a wee bit of PIR it would help - 75mm PIR/75mm screed.