Sanding concrete slab before 100mm insulation?

[ashthekid]

My previous contractor did a bad job at making sure the concrete slab was perfect laid everywhere so I'm considering getting some sort of sizeable sander mower looking machine to shave off the bad parts. A diamond sander is recommended I'm told.

Is this normal or should I not really be worrying about this because the insulation celotex will likely iron out those imperfections? I guess I'm just paranoid about it affecting things down the line with final finished floor stone moving over time.

[MikeSharp01]

Much cheaper to shape the insulation than the concrete or just have it screeded. 

[ashthekid]

I'm restricted by door and bifold levels with regards to putting additional screed onto it.

Somebody said sand in the the bad areas. there are only 2 areas where it dips a little and then about 5 what I would call lumpy areas that need sanding.

It's about 200sqm so would be quite a bit costly to screed as well.

[MikeSharp01]

I would still look at doing with the insulation. what is the max level above expected zero, what is the raise area as % of total and what happens above the insulation (build up)?

[ashthekid]

I have about 185mm to FFL. 
100mm PIR, membrane, UFH pipework, liquid screed of approx 40-50mm, 5mm matting, 15mm adhesive, 15mm stone floor tile.

 

I don’t know % raise area to be honest. 

[JohnMo]

I had a couple of high spot on my finished floor.  Make sure you use a grinder with extract to connect to hoover.  Grinding makes lots of it.  Get plenty of bags as they get through them quite quickly.  Don't use a Dyson as they fail with building dust.

 

Get a decent mask and goggles.

[jayc89]

5 minutes ago, JohnMo said:

I had a couple of high spot on my finished floor.  Make sure you use a grinder with extract to connect to hoover.  Grinding makes lots of it.  Get plenty of bags as they get through them quite quickly.  Don't use a Dyson as they fail with building dust.

 

Get a decent mask and goggles.

 

+1 to this. Our tiler didn't and he was having to come out the house every 15 minutes it was so bad. You'd think he'd know...

[ashthekid]

A builder came and looked today and said don't bother, just simply lay the insulation down on top without any sanding, the screed will get rid of any imperfections anyway. Said to just use very good gaffer tape to seal the insulation tightly together.

[Bonner]

I have a similar issue, low spots in the slab. Probably 20mm low over a couple of fairly large areas (gradual transition). I know because of the puddles in winter. I expect the insulation will be pushed down by the weight of screed. My concern is that I will have 20mm thicker flo-screed in places, will it shrink more and need levelling to suit large tiles?

[Nickfromwales]

23 minutes ago, Bonner said:

I have a similar issue, low spots in the slab. Probably 20mm low over a couple of fairly large areas (gradual transition). I know because of the puddles in winter. I expect the insulation will be pushed down by the weight of screed. My concern is that I will have 20mm thicker flo-screed in places, will it shrink more and need levelling to suit large tiles?

Your PIR will not conform to that, you deffo need to get either some SLC or some sharp sand and ‘blind’ those shallows.

[Mr Punter]

13 hours ago, Bonner said:

I have a similar issue, low spots in the slab. Probably 20mm low over a couple of fairly large areas (gradual transition). I know because of the puddles in winter. I expect the insulation will be pushed down by the weight of screed. My concern is that I will have 20mm thicker flo-screed in places, will it shrink more and need levelling to suit large tiles?

 

I very much doubt it is 20mm.  Try stacking some pound coins in the deepest part of the puddle.  A pound coin is 2.8mm thick, so the puddle would need to overtop a stack of 7.

[Bonner]

7 hours ago, Mr Punter said:

 

I very much doubt it is 20mm.  Try stacking some pound coins in the deepest part of the puddle.  A pound coin is 2.8mm thick, so the puddle would need to overtop a stack of 7.

Puddles dried up now the roof is on, I will get a level and measure properly.

[Thorfun]

+1 to the comments that state just a sand blinding to level off the troughs. we didn't have many but it's what we did and seems to have worked nicely. as long as the insulation doesn't rock  then it's my understanding that you'll be fine.