Use Hemp Lime Plaster for plinth?

[noj]

At the weekend the render 'fell' off the plinth in the dining room along the inside of the external wall (and I rather suspect it may equally fall off the equivalent area in the living room pretty soon). The 'original' was a thick layer of cement, topped with gypsum, black gloss paint and flaking vinyl emulsion over 300mm of rough brickwork and most of the sole plate.

I'm thinking of using a 25mm approx layer of the Mike Wye ready mixed medium hemp lime plaster as a relatively simple solution for a breathable and slightly more insulating covering (probably with a thin lime finish coat followed by limewashing). The brickwork is bone dry.

Any reasons why this may not be the best route to take? Or better alternatives?

Thanks

 

[Zebra]

Excellent idea. It's really good stuff. You can apply it really thick, make nice curvy shapes with it, and it dries really really strong, yet still breathable. As long as the wall is dry. If it is used in an environment which is prone to ongoing damp, then it will rot, as it is made of organic material.
The finish you get on it is not very smooth, as you can see the bits of hemp in it. Some people don't like this, they say it reminds them of woodchip wallpaper. If this is the case, then you can use a topcoat of normal lime plaster over the top. This is what I'm doing with my living room ceilings.

 

[noj]

Exactly what I was hoping to hear!

The manufacturer write-ups make it sound a bit too good to be true in terms of just needing a single coat after a bit of dubbing out of the biggest holes. And less finesse needed to tend it in the drying phase too...

Looks like a purchase is next on my list - and then just a smooth(ish) lime plaster topcoat needed and everyone's happy.

Obviously not a true like-for-like replacement for hard cement - which incidentally takes a lot of encouragement to fall off - but somehow I think our little cottage will be a lot happier.

And it'll patch repair some of the holes in the lath and plaster too. All good.

And I meant Ty-Mawr not Mike Wye for the source

 

[Zebra]

Yes, Ty Mawr is where I got mine. Remember to scratch the hemp coat, diamond pattern, with a lath-scratcher before it dries, to give the top coat something to adhere to.

 

[tobydog]

Obviously not a true like-for-like replacement for hard cement - which incidentally takes a lot of encouragement to fall off - but somehow I think our little cottage will be a lot happier.

I wish mine had just fallen off. You are right, it does take a lot of encouragement and unfortunately it can take too much original brick with it in my case. The bricks were generally in poor condition anyway. I started using an NHL 3.5 3/1 mix on the internal plynths that subsequently dried out, but on the inside of the external plynths, where I knew I would be battling some degree of ongoing damp, I used lime putty in a 3/1 mix for better breathability. The outside of the external plynths is still covered in thick cement with extends from the roof to 3ft below the ground :shock:. Removing this is a job too far. I did have a chance to speak with the earlier owner who committed this 'crime' and he said that he got the builder to do it because of the concerns of damp caused by splashback for the thatch run off! It doesn't happen now and wouldn't have happened then. Forgive them for they know not what they do!.

 

[FamilyWiggs]

Remember to scratch the hemp coat, diamond pattern, with a lath-scratcher before it dries, to give the top coat something to adhere to.

The end of your trowel is a bit cheaper. :wink:

 

[Zebra]

Lath scratchers are cheap! It's just a lath, cut diagonally on the end. If you don't have any laths, you could use a pointy stick.

 

[noj]

New to all this - but the plan is/was to go straight from a medium hemp lime to a lime plaster finish coat - so don't I just need a devil float (short pointy nail) rather than a bigger pointy stick?

 

[Zebra]

No, you wouldn't use a devil float on hemp. When you apply it, don’t over-trowel it and make it too closed, leave it a more open texture. Scratch it to depth 3-4mm with a comb scratcher, or you can make one out of a lath cut at angle. Scratch a diamond pattern. Some reckon however it's best to simply brush it to leave the texture open, instead of scratching it, because scratching it snarls it up too much.
You can do the hemp base coat quite thick, up to 15mm because it has longer fibres in it. It can be 9mm same as sand/lime but may be better thicker. It needs less tending than sand and lime because it doesn't tend to have a problem with shrinkage.
The hemp coat must have “gone off” so you can scratch with your nail but not dent it with your thumb before applying the next coat, can’t say how long this would be, depends on conditions. It's best to put the top coat on while last one still “green” but you can leave it and do it later, you just have to spray it.
If you were then going to apply a float coat, this is when you'd use the devil float. The float coat is normally the second coat, trowel it on and run a straight edge along it to pick up any flat spots or bumps. Then come back and float it up with a wooden float after about 2 hours. If you do it too soon, it’ll move the aggregate about and mess it up, but if you leave it too late it's still OK, you can spray it and do it. Close any cracks and flatten the surface. This is hard work, and should be - in circular motion pressing hard to compact the surface. Then you use the devil float, which has pins 2-3mm through, do it either next day or same day, in circles to provide a key for the top coat.
Then for the lime plaster finish coat you'd use 3:1 silver/kiln dried sand : putty, for a very white and smooth top coat. It would be applied in two thin coats, one hour inbetween, leave one hour, sponge off with normal car sponge, trowel again or leave a sponged finish.
Alternatively, you can dispense with the finish coat, and once you've applied the float coat, then don't devil float it, but use a sponge float (or a normal car sponge) next day, damp. This leaves a rougher finish than applying a finish coat, but closes up any pitted bits. This might or might not suit your property, as it is a more rustic finish. This is what I've done here: medium hemp base coat followed by a float coat, sponged a day later, and no finish coat.
I'm no expert on lime plastering, the above is copied from the notes I took when I went on Ty Mawr's lime plastering course, which I would really recommend.

 

[noj]

Thanks Zebra for the detailed and informative response - all duly noted. I'll have a go and post pictures of my success/failure as appropriate...

I really need to take the course, but have to get the job done before I can reinstate the heating system and there's just not a training slot available until towards the end of September. If it all works then it'll be great. If I have to knock it all back off and start again (having taken the training) then it won't be the end of the world - maybe just a pretty cool autumn and a frosty atmosphere at home

 

[Zebra]

You will learn as you go along. And if a bit goes wrong, then you can repair it, and get the next bit right. It'll be OK I'm sure, and the suppliers of the materials are often very helpful.