Sloping garage floor concrete slab question

[GlennSpray68]

Hi all ,

I hope someone can help with the following questions i have .

My situation is as follows , I have recently had patio doors fitted to replace my up and over garage door , they are 200mm high on 3 courses of bricks up with another 3 course concrete block wall tied to the first course on the inside of my garage . the garage is attached to the side of my 1960's semi 28ft long X 7 ft wide. I am wanting to level the floor along the full length of the garage and fit insulation under 18mm plywood . It is not going to be used as a Habitable room just a util and workshop.

My options are to build a suspended woooden floor with joists etc to counter the slope or pour a new concrete slab on top of the existing one where it slopes from 2.4m to the front of the garage where my new doors are . the fall is 200mm at the doors .

my questions are as follows .

If i order ready mixed concrete (C25)and fill the slope back to the level floor on top of the old sloping slab I am concerned about where the concrete gets thinner and meets the existing concrete slab at the level point in the floor . Is there a way to overcome this to prevent the thinnest point of the concrete cracking ?

the existing slab has no dpm below it so I am going to fit a new one below the new concrete and on top of the old slab.

Would i need to fit timber batterns say 4X1" around the new pour to obtain the new levels and would the dpm go behind these or in front ?

Lastly I've seen in posts you fit the insulation below the new slab , but would i be better fitting it on top as i plan to do in the rest of the garage .

Sorry for all the questions and many ,many thanks in advance for any help offered

Glenn

 

[GlennSpray68]

Hi Again forgot to ask these questions .

1. do I need to rebar the slab ?

2. Can I fit insulation 25mm celotex around the perimeter rather than the 4X1 timber boards

3. Will the slab need an expansion gap ?

Many thanks again
Glenn

 

[theprinceofdarkness]

You don't need rebar. Just why are you fitting the 4 X 1" plank? There is no expansion material in the original base so you don't need it. All of the concrete floor will be at the temperature of the ground for its whole life. Expansion joints are only used when a slab is heated by underfloor heating.
As for DPCs, don't put one under the new concrete and one over the old, it will never work. Lay the concrete, then treat the whole slab as one.
What are you doing/have done about the DPC in the walls and under your new patio doors, because the floor DPC has to be sealed to this. I would be tempted to use "Synathaprufe" (other rubber membranes are available), slap it over the floor and paint it up the walls to cover the DPC by an inch or so. Will need at least three coats, but the most difficult thing is the wait between the coats.
I would be tempted to a, build a reference level across your patio doors, say a line of bricks laid conventionally and a second course, laid on edge, then you can shutter front to back.
The original garage floor can take a weight of a car, so solid concrete is a bit OTT (200 mm thick!), fill the void with old hard core and a bit of concrete to bind it all together and to cover it to give a smooth surface.
Frank

 

[GlennSpray68]

Hi Frank Thanks for you response it was very enlightening .

To answer your queries , external wall is only a single skin so I thought to insulate it fit vapour barrier then dry line it . the reason for the 200mm concrete is because that is the dept that at the doors end bring the floor level via the slope of the the floor to the doors base. the door has a dpc on top of the bricks .

I am a bit confused about what you mean about laying the bricks (the 2 lines of brick are already in place )

I thought it would be easier and quicker just to pour the concrete rather than messing about buying separate hardcore and leveling it all.

thanks for all your help

Glenn

 

[theprinceofdarkness]

As I understood it your front finished level is 200mm higher then the old floor, so if you built a little "wall" right up against the new front wall, then you can then fill the void and shutter it flat using your new little "wall" and the original floor as your levels.
Concrete, its 2.4m wide and 2.4m deep = 5.8 sq. m, times 100mm = .58 Cu m. That's over a half of a builder bag of ballast, so its not a trivial task, even with a cement mixer. More to the point the base is good, so any old bricks or rubble will support the new floor, just with some cement to bind it so its stable.
Frank

 

[geraldthehamster]

Concrete, its 2.4m wide and 2.4m deep = 5.8 sq. m, times 100mm = .58 Cu m. That's over a half of a builder bag of ballast, so its not a trivial task, even with a cement mixer.

According to my calculations, that's at least a tonne of ballast, so at least a full bulk bag.

For what it's worth, the last time I mixed that much concrete, using a mixer, barrowing, pouring and tamping, working alone, it took me four hours.

Cheers
Richard