Void under patio - how to fill?

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Hi all

Hopefully this makes sense

I have a patio with steps made out of sleepers,

There is a gap along the front edge of one of the steps where cement should be (circled below), so part of the slab is not supported at the front and I am worried it will crack under load. There is also a missing grout joint between the edge of the slab and the sleeper (I presume it has fallen through into the gap as it is unsupported).

I hope this rather crude diagram makes sense
patio void diagram.png


I am wondering what is the best way to fill it. The slab is down securely so I dont really want to try to pull it up.

There is a decent enough gap where the missing grout is so my initial plan is to mix up a repair cement (like https://www.toolstation.com/bostik-cementone-rapid-setting-cement/p87635) with a lot of water to make a slurry and then try to force it down the gap into the void below

What do we reckon?

I also saw people online using expanding foam for similar issues, as some of it is rated to be used outside, but this seems a bit dodgy as potentially I could put too much in and uproot/crack the slab
 
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no expert

but render on wood is the wrong solution and will constantly fail
better with a wood or plastic cover with full protection or air gaps to sleepers for moisture removal
 
I had the exact same problem. There was an overhang with a small gap underneath it and I was forever worried somebody would step on it and either crack the slab or lift it up.

Best solution is to pack it with cement. I did this and left a small lip (maybe 5-10mm) along the edge. When rainwater runs away from the house across the patio, it gets to the edge, runs down the front and drips off that lip onto the border.
 
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Thanks for the replies

As there were no better ideas I went with my original plan of pouring a slurry of the repair cement into the gap and it seems to have worked, it's gone off rock hard and filled up the gap so it seems to me like the slab is no longer unsupported at the front, I can't get a finger in under it any more

I am going to brush in some grout (e.g. Sika FastFix) to complete the job when I get chance and hopefully thats the issue resolved ... fingers crossed
 

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