Float and Set - NHL 3.5

[spotpush]

Hi,

I'm planning to apply a scratch coat to a room which is back to brick. Can someone tell me how many bags (25kg) of Lime (NHL3.5) and Sand (sharp/builders), based on a 1:3 ratio, 10mm thickness to cover an area of 34.8m2 (L:4.5m x W:3.1m x H2.5m)?

Thanks...

 

[a twig]

Lots of useful calculators for such things here:

https://www.lime-mortars.co.uk/calculators

 

[spotpush]

Thanks but I've used them and many others. However, I'm not getting the answer I'm looking for. I'm planning to mix it myself i.e. 1:3 Lime:Sand and not purchase pre-mixed bags. I believe it will be cheaper and go further, which is what I'm trying to compare.

The area is 38m2, 20mm base coat and 4mm finish coat. According to the calculator, via the link, it says I need 46 (25kg) bags for the Base Coat and 10 (25kg) for the finishing coat. This comes in at just shy of £1000 just for the lime plaster. How is this broken down into x number of NHL 3.5 and x number of Sand so I can calculate mixing it myself and understand quantities for each material

 

[a twig]

Right so then you just break down those total bags of render from the calculator down into sand and NHL by using the ratio of your mix (3:1 in this case)

 

[a twig]

So 46 25kg bags of render becomes 11.5 bags NHL to 34.5 bags of sand

 

[a twig]

Or call it 48 bags and then the maths becomes really easy, 12 NHL to 36 sand

If you get the sand delivered in bulk bags rather than 25kg sacks you shouldn’t be looking at much more than £120.

Even if you’re being scalped £15 a bag for NHL then that’s max £180.

Call it £300 all in - will be a lot of mixing though

 

[spotpush]

Thanks for the input a twig. Can you share your working out? How did you get from "46 25kg bags of render becomes 11.5 bags NHL to 34.5 bags of sand"

Ok, so 46 / 4 = 11.5 (NHL) and 34.5 left over for sand. How do you know the pre-mixed plaster is 3:1? 3:1 is the ratio i want to use...

I'd say £20 per bag of NHL (including VAT) and at least £75 delivery for me. Sand cost is the same as what i got.

Still, significantly cheaper than buying pre-mixed, if you're prepared to put in a bit of effort mixing it all up....

 

[Stevers]

Mixing by weight (as you should) a 3:1 mix has four parts, three of which are sand and one NHL. Happily everything is in 25kg bags simplifying the maths.

Thus dividing your total ready mix no of bags by 4 gives you your NHL no. of bags:
46/4 = 11.5 bags (NHL)

and three times your NHL bags is your sand bags:
3*11.5 = 34.5 bags (Sand)

Unless I've missed something!

 

[spotpush]

Yes, went thorough it and figure it out. However, how do you know the pre-mix uses a 3:1 ratio?

 

[Stevers]

The pre-mix might actually be wrong so I don't think it's relevant. For lime or NHL 3:1 is pretty much where it's at for everything other than finishing plaster.

I bought some Conserv brand 'finishing plaster' that was more like 10:1 (a white powder was in small separate bag in a bag of otherwise clean fine sand), and that was a poor effort even at 3:1 as a finishing plaster. Would have been better at 2:1 or even 1.5:1. In the end I skimmed it (with filling knife) with straight NHL.

 

[spotpush]

The pre-mix might actually be wrong so I don't think it's relevant. For lime or NHL 3:1 is pretty much where it's at for everything other than finishing plaster.

I bought some Conserv brand 'finishing plaster' that was more like 10:1 (a white powder was in small separate bag in a bag of otherwise clean fine sand), and that was a poor effort even at 3:1 as a finishing plaster. Would have been better at 2:1 or even 1.5:1. In the end I skimmed it (with filling knife) with straight NHL.

Ok. So then i have to ignore the recommendation of 46 bags since it's based on this pre-mixed stuff and the ratio may be totally out. Which takes me back to the beginning: how many bags, in today, of NHL 3.5 and how many bags in total of sand (which will be 3 times the number of NHL) do I need to cover 38m2....

If NHL 3.5 isn't where it is at for for finishing plaster where is it at? and what do you mean by "straight NHL", just NHL and water, no sand or a 1:1 mix?

 

[Stevers]

The 46 bags seems a good place to start as any, on the basis that the finished render should have the same volume as 3:1 with the lime matrix in both cases filling the gaps between the sand granules to the same extent.

If I was doing my chimney breast again I'd cut the NHL with hydrated lime (50:50) and try 1.5:1 as a finishing coat. As it was I think I cut the NHL 3.5 with Hydrated Lime 50:50 and used it like Polyfilla to give my 3:1 finish coat a much less 'sandy' and more 'polished' look. For internal work at least, NHL 3.5 cut with Hydrated Lime 50:50 to give a 'feebly' hydraulic mix (NHL 2) is something worth considering - the best (or worst) of both worlds!

 

[spotpush]

I suppose so but there must a a simpler, more accurate way of calculating this. Haven't people been doing it for over 100 years? Without knowing the mix ratio of the premix it's hard to say with any certainly how much of each I need.

I don't see the value in mixing the two since it will detract from why I'm using lime in the first place.

 

[spotpush]

38 m² at 10 mm thickness (1:3 lime:sand ratio + 10% wastage)

Total plaster volume: 38 m²×0.010 m=0.38 m³
Lime (1 part): 0.38\4 = 0.095m³ * 1400kg/m³ = 133kg (10% wastage: 133 * 1.10 = 146.3kg
Sand (3 parts): 0.285m³ * 1600kg/m³ = 456kg (10% wastage: 456 * 1.10 = 501.6kg

Convert to 25 kg bags

Lime: 146.3÷25≈5.9 bags → round up to 6 bags
Sand: 501.6÷25≈20.1 bags → round up to 21 bags

So 6 bags of Lime and 21 bags of Sand per 10mm of thickness with 10% wastage (12 Lime and 42 Sand for 20mm plus 5 Lime and 6 bags sand for 4mm finishing coat)

Shouldn't be this complicated should it....

 

[a twig]

Most trades I know will do it the other way round - either from doing the sums or bitter experience they will know how much coverage a certain mix gives.

Making up an example - say 1 bag NHL plus 3 sand gives 5m2

They will multiply up from there to the area you are rendering to work out what is needed. The only thing they really care about in advance is how many bags of NHL are needed for the job because that’s expensive and takes longer to get hold of.

Sand wise they will just get a bulk bag delivered and if it looks like running out before the job is done, get another one delivered. Most builders merchants will do sand same day or next day delivered.

 

[larry]

This is definitely a queston for AI.....

 

[larry]

The 46 bags seems a good place to start as any, on the basis that the finished render should have the same volume as 3:1 with the lime matrix in both cases filling the gaps between the sand granules to the same extent.

If I was doing my chimney breast again I'd cut the NHL with hydrated lime (50:50) and try 1.5:1 as a finishing coat. As it was I think I cut the NHL 3.5 with Hydrated Lime 50:50 and used it like Polyfilla to give my 3:1 finish coat a much less 'sandy' and more 'polished' look. For internal work at least, NHL 3.5 cut with Hydrated Lime 50:50 to give a 'feebly' hydraulic mix (NHL 2) is something worth considering - the best (or worst) of both worlds!

Also had success with a 50/50 mix of hydrated and NHL. Including with a perlite mix for a bit of insulation (replaced 80% of the sand with perlite). Perlite is very light so you have to mix by volume, however.....