Stone window sill repair

[paul243]

Hi
My daughter has moved into a Victorian terrace.
When we removed a nasty cheap upvc front porch we discovered that the cowboy who fitted it had knocked a large ragged chunk off the end of the stone window sill to make it fit.
Any suggestions please on a method of repair, is there a specialist repair material or is it a case of "building up" layers of mortar and finishing with something else? I can't see a method of constructing a frame to support the repair.
Thanks

 

[oldbutnotdead]

How big a chunk are we talking? (Pics would help).

 

[paul243]

Struggling with pics.A chunk off one end approx 10- 12 inches.Left a jagged hole .The porch fitter decided to smash it instead of designing the porch to fit

 

[Tigercubrider]

Could you use a piece of 18mm timber underneath held up by a couple of posts knocked in sideways?
If it sticks out by 18mm then a vertical one screwed to it would hold the front in while you cast a fresh one and again a couple of supports to the ground

Sticking a small piece of tubing to the first will cast a drip stop and using cooking oil will stop it sticking

A few screws into whatever is left and some wire twisted about may help

You can mix a finer mix of concrete for the outside and leave it for a couple of days

 

[oldbutnotdead]

That was pretty much my plan- drill into the existing cill & put some long screws partway in (or rebar or Helibar if you really want to make sure) then shutter round and cast a new bit. Provided you paint it when you've finished it'll look OK.

 

[paul243]

Thanks
What should I use to fill it, mortar mix or something more specialised?

 

[oldbutnotdead]

Use a concrete mix with fine gravel, make it a bit wet so you don't get too many voids on the faces of the shuttering.

 

[Tigercubrider]

Use a less stony mix for the surface
This video shows a guy "plastering" the outer surface before filling the moulding

As it is a join, wet the existing material before adding cement