Recommendations for an exterior levelling compound.

Bendy_House

Hi.

Issue is that's there's an uneven concrete area outside a metal building which allows water to pool against this building, and ultimately find its way in.

This concrete area is otherwise very solid, and has vehicles parked on it with no issues. The surface is the typical straight-edged-levelled finish, so nicely ridged/textured - should provide a nice key when jet-washed and cleaned up.

So:
Depth of additional screed to level it out (and provide a slight fall away from the building) will be around 30-ish mm, falling to zero.
Overall area will be around 4x8m.
Existing slab is solid, and little risk of movement.
New surface will need to be able to take vehicular traffic with no issues.

Recommendations, please
Thanks.

plumb1_2

Have you thought about jacking up the building 30mm

stuart45

There are a few that are made for exterior work like Ardex but is going to be expensive at that thickness. Also, if you need a slope self levelling can be a problem. It's cheaper to make your own with sand/cement, SBR and fibres.

Bendy_House

stuart45 said:

There are a few that are made for exterior work like Ardex but is going to be expensive at that thickness. Also, if you need a slope self levelling can be a problem. It's cheaper to make your own with sand/cement, SBR and fibres.

Thanks Stuart. 

We weren't thinking of 'self' levelling, as we'd ultimately want a slight slope away from the building, so the alternative sounds ideal.

More info on the sand-cement mix, please: what type of sand, what ratio, and which 'fibres'?

Thanks

Apodemus

plumb1_2 said:

Have you thought about jacking up the building 30mm

Or a lean-to roof to provide covered parking! 

Apodemus

More seriously, I would have concerns about the long-term frost-hardiness of any screed that thin.

Bendy_House

Apodemus said:

plumb1_2 said:

Have you thought about jacking up the building 30mm

Or a lean-to roof to provide covered parking! 

That's still a plan, as the tree debris landing on the boats and vans is hellish. It wouldn't stop rain getting in and pooling, tho', so the levelling needs doing as a priority.

stuart45

The mix is 3/1 sand/cement. Sharp flooring sand is the best, although it won't run to nothing. The amount of SBR will vary as manufacturers use different strengths. If you are using a Belle mixer, then for 3 buckets of sand and one bucket of cement it will probably need at least 5 litres per mix. As the area will probably take at least one bag of sand, then you are looking at over 10 mixes. Fibres can be bought from the merc, of screwfix. They are optional, and the mix will work without them. 
Is it possible to just make a concrete kerb or run a course of engineering bricks round the building to stop the water getting in?
I must admit that I wouldn't fancy my chances of doing a good job with a screed, although I'm not a floor layer. 
You will need some help. SBR in the mix makes it go off really quick. You have to keep cleaning out the mixer, barrow and tools.
The fall of 30mm in 4000mm is quite small. Generally, 1:80 is a minimum. Having a low fall doesn't allow any room for dips in the finish. It will be a bit tricky to get an accurate fall with a wettish screed. 
I've done falls with concrete, and it does tend to slump a bit, you have to rework it a bit.

Bendy_House

Thanks Stuart.

Two aspects - one is to stop the rain getting in, but the other is just to make that area pleasant to walk on - it's currently foul, splashing in that water and leaves :-(

What's "Fibres can be bought from the merc..."? What are they called, these fibres?

Leon_W

Would it not be easier to channel a drain in along the edge of the building ?  Otherwise you run the risk of the water pooling somewhere else !

stuart45

Concrete & Screed Reinforcement fibres 100g bag (bradfords.co.uk)
Like these Bendy. Can you get a bit more fall on the screed?