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Alter ego 05-02-2010 05:27 PM

Cement Render (Stucco) Question
I am now ready to cement render the outside of my dome on top of the insulation and would like some advise on how thick you made yours and whether you had any problems with cracking due to movement?

I have 100mm of insulation which is covered with chicken wire to hold it down and to hold it in shape. I was thinking of about 20-25mm of cement based render.

I donít know how much "reinforcing" effect the chicken wire will have as it is fairly tight over the insulation and I donít think that much render will penetrate below that. Does it need another cover of chicken wire in the middle of the render to reinforce it?

Once I had completed the render I will colour with oxide and waterproof.
Thank for any suggestions.


Tscarborough 05-02-2010 05:41 PM

Re: Cement Render (Stucco) Question
Actually, your best bet is to include polypropylene fibers and the iron oxide pigments into the last coat of your render. The first coat is called the base coat and should also contain the fiber reinforcing, but no color and is applied from 3/8 to 1/2" depth. The second coat is called the browncoat or scratchcoat and if applied should be troweled on at right angles to the way you applied the base coat at 1/4 to 3/8 inch depth. The second coat is optional but desirable especially if you do not use fiber reinforced render. The final or finish coat is a 1/4 to an 1/8 of an inch thick and should have your iron oxide colorants and fiber reinforcement in the mix.

Alter ego 05-02-2010 05:48 PM

Re: Cement Render (Stucco) Question

Thanks for the info. I hadn't thought of using fibres and this will be much easier. Is a total thickness of 3/4" enough to stop cracking over a flexible base such as the insulation?

Tscarborough 05-02-2010 05:58 PM

Re: Cement Render (Stucco) Question
Well, technically all stucco cracks, the best you can do is to control where and how much it does so. The edges should be cased out with X66 casing bead, and try not to have any right angles. Cove the areas around your entryway, etc.

david s 05-03-2010 11:38 PM

Re: Cement Render (Stucco) Question
Fire your oven until all the moisture has been eliminated before you do the stucco. Moisture left in there can turn to steam and create pressure and leads to cracks.

Alter ego 05-04-2010 04:55 AM

Re: Cement Render (Stucco) Question
My oven has been complete for several months now. I have been too busy using it to get around to finishing it so it should be well and truely dry

gefty 05-05-2010 02:56 PM

Re: Cement Render (Stucco) Question
to stop the cracking something I tried was adding equal amounts of lime 1:1 to cement. Has anyone else tried this?

david s 05-06-2010 01:27 PM

Re: Cement Render (Stucco) Question
Yes, The lime provides some elasticity, but if it cracks anyway, give it a good coat of flexible acrylic stucco.

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