Sand Blinding Layer

[LA3222]

Yarp..probably a ridiculopolus question but here goes.....

 

The sand blinding layer below my EPS raft - any particular type of sand required here?  I am thinking along the lines of grit sand, bit coarser so better draining but sufficiently fine to level out easy enough??  I would rather ask than make a rookie mistake ?

[stephen margerison]

normally sand is put in foundations, so as to level off the "slab" then the damp proofing goes in, so ordinary building sand is normally used, as you don't want ANY perforations in the DPM if in doubt, ask your building inspector, they are usually very helpful.

[JFDIY]

I've always used Sharp sand, then Wacker it smooth. Building sand will just clump up when compacted.

[Onoff]

Sharp sand here too.

[Russell griffiths]

Have a look at what your local quarry has. 

Mine recently had a few thousand tonnes of pipe bedding sand, it was basically sharp sand but the quality was poor so they sold it off cheap as cable laying sand I think it was about £4 tonne. 

[Oz07]

Guessing eps won't have dpm below  so the only considerations are what is easiest to work with

[LA3222]

Yarp what @Oz07 says.  The blinding layer is sandwiched between the aggregate and the EPS.  The DPM is in between EPS layers so no danger of that being punctured by coarse sand.

 

My query really relates to what @JFDIY mentions.  If the sand is fine will it clump and inhibit the drainage layer under the EPS, therefore a coarser grade is more suitable?

 

Many on here have insulated slabs - just what normally gets used?‍♂️

[Oz07]

Yeh use some kind of a grit sand then like you say got to consider drainage 

[willbish]

I used 2-5mm limestone chips. And used a 3.0m screed bar to level.

Advice I was given is that sand could wash away in heavy rain if there is a period of time before the EPS goes down.

 

[Gone West]

We had granite fines specified for under our insulated slab.

[Barney12]

Ours was MOT1 followed by sharp sand. We used a powered roller to compact the MOT1 to the required specification. The MBC guys did the sand binding layer with nothing more than a few bits of baton to get it level. 

[SuperJohnG]

Ahhh. I'm right at this point...its now getting ridiculous @LA3222 that everything I Google search on here its you thats asked the question. 

 

I've just bought concrete sand (which seems to be sharp sand). I'd seen you just put shuttering in the sand I think. What have others done to screed it flat?. The groundworks boys were thinking I should get some steel poles and then we can lay either side set a high with laser then screed it right across to get it as level as possible. Overkill? (I did tell them I wanted it really flat....

 

[Onoff]

5 hours ago, SuperJohnG said:

Ahhh. I'm right at this point...its now getting ridiculous @LA3222 that everything I Google search on here its you thats asked the question. 

 

I've just bought concrete sand (which seems to be sharp sand). I'd seen you just put shuttering in the sand I think. What have others done to screed it flat?. The groundworks boys were thinking I should get some steel poles and then we can lay either side set a high with laser then screed it right across to get it as level as possible. Overkill? (I did tell them I wanted it really flat....

 

 

Screeding rails are used to get things "dead flat" their suggesting of levelled poles imo is bang on. Level to the poles the infill when you take them out. 

 

I levelled the sand blind in just one room using just a long level as a drag board, the subsequent eps/pir layers had a bit of "bounce" in them. I even laid my eps and went over it with the 5kN Evolution whacker plate. My eventual wet concrete floor was put in using screed rails so it all came right in the end. 

[tonyshouse]

Blinding error!,  the sand percolates through your hardcore and leaves a void! I have seen slabs sink as a result 

 

then there is the thought that vapour barrier should go on the warm side of the insulation so I put dpm on top of the insulation - unlikely but the dpm could pick up condensation on the top or inside and sweat ? 

[LA3222]

Yarp, as @Onoff says.

 

I made a mistake here though, as I didn't think things through properly at first.

 

Needed a 50mm sand layer so I bought 50mm timber battens thinking ill lay them flat and that then gives me 50mm of sand.

 

No.

 

Your hardcore layer will be +/- 1" maybe, so this pushed my batten too high in places. What I then did was rip them all down to 25mm thick. Was able to use a lazer then to pack them out to the required height and any undulations in the hardcore didn't matter.

 

If your sand is damp it will be harder to screed so don't set the rails too far apart, maybe 2m? Use a 'stiff' pole or timber to screed it so it doesn't flex in the middle. If it does, it will ride high in the middle leaving almost imperceptible humps. I did that and couldn't get the EPS to sit right at first, I ended up pulling it all off and redoing the sand.

 

The sand layer will make things easier with the EPS if it is done well.

[LA3222]

A lot of people have used sharp sand without issue. Mine was exposed for four weeks in Jan/Feb 20 when we had something like 3 named storms on the trot, real horrible weather to do the founds in and the sand went nowhere. I thought it might and was concerned at the time but every time I did more work I checked with lazer and no change.

[willbish]

I used limestone chips as it was what had been specified.

Bought a 3.0m aluminium length which has come in handy many times since throughout the build.

Timber set level on type1, worked beautifully

 

 

[tonyshouse]

Looks nice, sand when dry flows like a liquid, like in an hour glass 

[Onoff]

People often use Type 1 for instance which is I think 40mm to dust. That stops the sharp sand sinking through. I made own Type 1/2 by grading assorted hardcore through a 40mm mesh (proper mission). I then slung assorted half bags of cement, plaster, assorted sands and ballast etc on top, as much to get rid of it as anything else. The ref spots were wooden pegs to height.

 

 

Looked lovely when it was done!

 

 

[Onoff]

16 minutes ago, tonyshouse said:

Looks nice, sand when dry flows like a liquid, like in an hour glass 

 

If the sand is under the DPM won't it remain "damp" thus less likely to flow?

[tonyshouse]

No! It will dry out as the house is warmer than the ground, moisture will be pressed away due to differences in partial vapour pressures and will become bone dry after a year 

[Pete]

50 minutes ago, willbish said:

I used limestone chips as it was what had been specified.

Bought a 3.0m aluminium length which has come in handy many times since throughout the build.

Timber set level on type1, worked beautifully

 

 

This is what we did with our Passive slab. I ordered a full load and used the rest as pipe bedding

[SuperJohnG]

1 hour ago, tonyshouse said:

Blinding error!,  the sand percolates through your hardcore and leaves a void! I have seen slabs sink as a result 

 

then there is the thought that vapour barrier should go on the warm side of the insulation so I put dpm on top of the insulation - unlikely but the dpm could pick up condensation on the top or inside and sweat ? 

@tonyshouse using sand blinding seems to be the norm - I've no idea how you'd do it otherwise.

My DPM is placed on top of the sand blinding layer and then EPS on top of that. I can't tell if you are saying that's the wrong way. Its a detail by the SE and the insulation supplier which seems right to me. 

 

1 hour ago, LA3222 said:

If your sand is damp it will be harder to screed so don't set the rails too far apart, maybe 2m? Use a 'stiff' pole or timber to screed it so it doesn't flex in the middle. If it does, it will ride high in the middle leaving almost imperceptible humps. I did that and couldn't get the EPS to sit right at first, I ended up pulling it all off and redoing the sand.

 

The sand layer will make things easier with the EPS if it is done well.

My sand is sitting out n the wet just now - we hand planned to try and screed the full 6m width, but seems this might be a mistake. Did you just screed the sand or did you then compact it down? (I thought it was compacted but now thinking about it, wouldn't make sense). 

 

 

1 hour ago, LA3222 said:

A lot of people have used sharp sand without issue. Mine was exposed for four weeks in Jan/Feb 20 when we had something like 3 named storms on the trot, real horrible weather to do the founds in and the sand went nowhere. I thought it might and was concerned at the time but every time I did more work I checked with lazer and no change.

I was worried I'd have to get it covered straight away, but that good to hear. I was worried the rain would mess with it and the wind. Can you walk on it after it's down? it would be better to leave it as then I can put DPM down the day I am going to lay the profiles.  

 

54 minutes ago, willbish said:

I used limestone chips as it was what had been specified.

Bought a 3.0m aluminium length which has come in handy many times since throughout the build.

Timber set level on type1, worked beautifully

Thanks - seems easier, were these compacted? 

 

19 minutes ago, Onoff said:

People often use Type 1 for instance which is I think 40mm to dust. That stops the sharp sand sinking through.

I have type 1. Type was specced, but checked with Tanners and they said I could use type 1. I can't see anyway the sand can pass own through especially after we have compacted down the type 1. 

[Onoff]

26 minutes ago, SuperJohnG said:

I have type 1. Type was specced, but checked with Tanners and they said I could use type 1. I can't see anyway the sand can pass own through especially after we have compacted down the type 1. 

 

The sand won't sink. If it does there's going to be and awful lot of people's slabs on here sinking! ?

[LA3222]

A well compacted surface of type one will stop the sand flowing issue.

 

A lot of builds on here have same foundation type and used sand, they all seem to still be standing?‍♂️ when I trimmed all the perimeter eps off the sand under it all was still there and damp. That's after 10 months - my experience is albeit limited so I may be proven wrong but I can only comment on what I've seen done on here and with my own two eyes on my build.

 

Ref getting your dpm down John, if you can then I would as belts and braces. The sand isn't going to be trashed overnight but if you can get it down sooner rather than later its one less thing to be concerned about.

 

If the sand was dry then yes, you could screed the full width easily. Wet sand is a different beast and just harder to manipulate.

 

I did wack it after, didn't really move it much tbh.

Edited February 3, 2021 by LA3222