Roughcast render repair - best approach?

[Cloudscape]

The roughcast was done with hot lime mortar originally. There are some older successful patches where windows have changed shape.

There's a rather noticeable chunk of roughcast render missing from the front of my house. It's about 18x18in. Sad to say, this damage was deliberate. The previous owner had a funny half hour and thought they'd remove the roughcast completely (until stopped by the neighbours - partly on aesthetic/historical grounds and partly because the roughcast extends across the front of three houses).

The wood is a lot more solid than it looks. There's currently cement mortar round the edges of the render to seal it.

 

[Cloudscape]

I know how the roughcast was done, but I can't see myself literally throwing a mix of lime, sand and gravel at the wall.

How sticky can a mix be? Would I need to support the repair in order to stop gravity taking over?

I assume I should remove the cement.

What should I do to prepare the surface to ensure good adhesion? Should I work in layers?

 

[Gervase]

Do it in layers - take off the cement fillets and gently undercut the existing render, then give the area a good wetting and dub it out with a sticky 1:3 lime mortar laid on no thicker than your finger. When it's starting to firm up, compact it down well and give it a good scratching to provide a key for the scratch coat a few days' later, which is more of the same, again firmed up and then scratched well to give a key. A further coat can allow the render to be brought up to follow the contours of the existing render, but only to about half an inch shy of the surface.
Again, compact that down and scratch it - more lightly this time - wait a few days and then apply the finish coat in 1:2.5 lime and sand. When that has started to go firm but is still damp and 'green' (meaning you can make a mark in it with your fingernail), compact it, then spray it with water and apply the roughcast. Try to get aggregate that matches the size of what you've got and mix it in a sloppy slurry with a 1:1 mix of lime and sand, then flick it on firmly with a harling trowel (or a little coal shovel, which does the job just as well). You can feather in the new with the old with a wet brush, but until you've got a few coats of limewash on top you'll not match the patch exactly with the existing roughcast.
The harling is a messy business, so make sure you've taped heavy-duty polythene over the window and put down a polytarp on the ground, so that what falls down can be put back in the mix and used again.

 

[Cloudscape]

Thankyou very much, Gervase.

The harling process should give my neighbours some entertainment.