legs-akimbo, help with rough-cast.

[paddypower]

Legs-akimbo, as your profiled from Ireland I’m hoping you can help..
I have done some renovation to an old cottage in Ireland (Co.Cavan) and need to roughcast some external rendering where a door has been blocked up and windows altered. The original roughcast used ‘ballast?’ up to about 0.75 of an inch. Any tips on how to do it? I assume a scratch coat, then a second coat, then cast a ‘mixture’ onto the wet second coat? Any idea what the mixture would be? It looks like granite chippings and cement mixed together in a slurry then cast. Is it simple as that?

Regards,

derek..

 

[legs-akimbo]

Hi paddy. Are you sure it is not a dry dash firstly, where stones are thrown onto a rendered surface? if it is rough cast then your local quarrie (sand and stone supplier) will have what you need ....the builders providers usually dont carry theese items. Pebbles can be anything from 6mm to 18mm or granite chips from 3mm upwards. The larger the stone the coarser the finished look, permutations are many and you will need to be specific to match the existing but a rule of thumb is 3 parts aggregate to 1 matrix (mortar).
The mix needs to be hurled (thrown) with a good amount of force using a dashing paddle, throw at right angles to the wall to prevent bunching and sliding. The force of the hurl brings the stones out to prominence by the force of impact with just a film of mortar over them. Hard really to describe and not easy to produce an even uniformed texture. Done well rough casting looks superb but a poorly done job will be an eyesore!!! Its a strong gauge also so will not come off easily when cured if you are not happy with it.

Good luck.

 

[paddypower]

Legs-akimbo,
thanks for taking the time to reply.
I must admit to not knowing what roughcast is, just assumed its what is on my cottage because its a very rough texture. From your description I'll give it a go with your recipe. The finished coat is rough angular chips with drips of mortar and gaps having mortar in them. Hard to describe but I'm sure its been hurled on wet. Just one question to clarify, is the wet mixture hurled onto a dry scratch coat or onto a setting second coat?

thanks again