render question

[kstronach]

So my dome is insulated ready to mesh and render now but I'm just wondering what is the best mix to use? I was going to use a lime render about 4 to 1 with some sharp sand in the first coat for some extra strength but as I understand it lime lets water pass through it which will obviously let the moisture out and let the oven breath but won't it therefore let it in too?
I could just use a normal cement render but then I think this would crack anyway so letting water in anyway! As Im not going to have a roof over I want the most watertight finish possible any thoughts or ideas?

[cannyfradock]

I'm building an enclosed Pompeii oven at the moment but I am yet to try.....and will do on my next open oven build..... K-rend. It's used on lots of "new build" houses as it's waterproof, flexible and comes in 6 different colours. It comes in a base coat and finish coat. The base coat on house applications has plastic meshing laid into it.......I would use chicken wire.

It comes in 25kg bags and you just add water to mix. It costs about 8 or £10 a bag and you may need 3 bags in total....or more?

Many builders merchants can order it in, but your local branch of "Sheffield insulation" (google them for your local branch) sell it to Joe public.

The biggest problem with the finish render coat is the part where it touches the chimney. If it's not a double lined insulated chimney, then the render will crack from this area. Water eventually gets in and this is the main problem for the exterior rendering cracking. Always try to protect the render from meeting direct heat from an un insulated chimney......a piece of fire-blanket or even a 10mm piece of sponge wrap which can be the removed and filled with fire-mastic etc.

Terry

[kstronach]

hi terry!

thanks for the reply! yea i've seen k rend its on my house actually! if you were to use it on a future oven i'd be interested to see how it goes!

good information on the chimney i hadn't thought of that! so i shall try and do something to combat that . . . the fire mastic sounds like a plan!!

got the mesh on tonight flags round the edges and a scratch coat on! i'll post pics on the build thread anyhow!!

thanks again

keith

[bookemdanno]

Keith, you can add things into the Lime Render to waterproof them up. Old days i think it was Tallow or Linseed Oil.
St.Astier told me that you can use Magnesium Stearate powder, which is what is used to stop pills sticking together! You only need a small cup full per mix.
I got issues with Jack Frost from cracks, just like Terry mentioned, from heat cracks, then water, then Frost, on a Lime Render that was too young for the winter.
I'm tempted by K-Rend as its flexible, waterproof and breathable. I've just got to convince myself to save up for it, but a bag of Hydraulic Lime is £20 odd quid, so if my local Merch or Sheffins at Norwich has it, then it might be the direction i'd take.
Well done on your base render, nice shape!

[kstronach]

Thanks! It wasn't as bad as I thought put it on fairly thick with a couple of buckets of sharp sand in the mix which made it easier!

Yea I thought about adding something to waterproof we use an additive in cement called free flo which is basically a retarder waterproofer and plasticiser in one. Or just waterproofer which does what it says on the tin! However I'm not sure on the compatibility of these products with lime. I am using hydrated lime, I'm not completely sure on the difference to be honest! It may be a strength thing I think you can get hydraulic lime in differing strengths and I think I read something about hydraulic setting underwater, if anyone does know the difference I'd be interested to know! Also I'm going to add horse hair to the final coat ( basically just the old fashioned equivalent of fibres) to bond It together abit about more! And I'm going to SBR the scratch coat before applying the top coat to help it bond better.

Keith

[nixie]

Guess since the lime breathes it could be used as a (non waterproof) top coat for a clay oven.

[bookemdanno]

Hi Keith, i'm intrigued as to what mix and materials you used for your render?

I didn't think that Hydrated (builders) Lime ever really set, and you needed Cement?
well, i suppose it does eventually, but over a long time like Slaked Lime.

Hydraulic Lime sets like cement firstly, then gets stronger over time like a Traditional Lime Putty mortar. Admixes for Cement will not generally be suitable for Lime based renders. It comes in different "strengths", and yes will set underwater, or with the addition of water. Ordinary Portland Cement is another Hydraulic setting powder.

Nixie, i'd say that Hydraulic Lime render is the perfect thing for a weatherproof coating over a Cob Oven. You can get ready mixed waterproofed and bagged Renders from St.Astier.
Look up their website.

[nixie]

cool, will take a look.

[kstronach]

This is what I found about the diffrence between them. . .

What is the difference between hydraulic and hydrated lime?

Hydraulic lime and Hydrated lime are both normally available as powder supplied in 25kg bags.  Both can be used for making mortars, however hydraulic lime has a degree of cementitious set when in contact with water (or atmospheric moisture), it also sets by combining with atmospheric carbon dioxide to some degree.  Hydrated lime does not set in the prescence of water, it sets by combining with carbon dioxide in the atmosphere to turn back into chalk or limestone.

So as you say hyrdaulic lime has an initial set then cures in the air, and hydrated doesnt have that inital set if you will.

For my basecoat I used 2 sharp 2 rendering sand 1 lime and a good half of cement and for the top coat I'm going to use 4 silica 1 lime and again a good half of cement, also been to the merchants this am and got some fibers to add. As for the waterproofing after a few good firings and repairing any inevitable cracks I'm going to use a spray on waterproofing system them a few good coats of flexible waterproof masonry paint and hope that does the job!

Keith

[johnaero]

interested see how turns out as i have yet to do this on mine

[alun]

I just contacted my local (it's all relative) Sheffield Insulation (Barking) to ask about K-Rend. Apparently they only do weber, which I can get from Jewson anyway.