Help filling gap over timber wall plate

[adam1894]

Hi all, I need some advice. I have removed all the coving in our main bedroom, it was a modern-ish gyproc coving probably put up in the 80's. I'm planning to put up some cornice more sympathetic to the age of this late Victorian house (although it now looks like this room wouldn't have had any originally).

After removing the coving I discovered original lime plaster behind it that was so crumbly it just turned to dust as soon as you touched it. However there is a bit of a mystery - the outside walls have clearly been stripped back and re-rendered in what appears to be quite a thick cement render (possibly tanking slurry), but this stops at a hard line exactly where the coving started and then the very top part of the wall is original lime plaster. Why did they leave this small section at the top, did they put the coving up before stripping back the wall or how did they leave such a straight line between the old and new plaster, this seems like it would have been a lot of trouble!

Anyway, all the lime plaster essentially fell out and I and had to be removed, now I'm left with this gap at the top of the wall that I need to fill. On one of the walls this gap has exposed the timber wall plate and so I'm not really sure of the best way to fill it. My initial idea was to just use sand and cement but I'm now concerned this wouldn't bond to the timber although it could be pushed up into the gap between the ceiling and the wall somewhat. Clearly originally they just rendered over it so maybe this would still work, however now I'm wondering if I should just cut some plasterboard and stick that in there to fill the gap or if there is a better (or more proper) way?

 

[housegolem]

The size and depth of the gap should mean that whatever you pack in there its going to be fairly secure, whether its lime, gypsum or cement

If the backing was completely wooden, getting a key would be more of a priority (especially with lime). On the areas with large wooden faces in my house the original builders, skimmed on a thin wet layer of lime plaster and then hammered some lathes into place over the top. If you go for lime and had some kicking about it would be worth doing but a bit over the top really. The main goal would be really packing it in there.

More modern methods are available to get a good keys, you can get specialist lime primers which are painted on before plastering. They contain aggregates to give you a key and help with suction.

The gypsum equivalent would be blue grit. And something like SBR mixed with cement for a concrete mix or the classic PVA.