Number of scratch coats with riblath

[craigie10]

Hi all,

My first post here but I've been lurking over the years and found plenty of sage advice.

I've got an external parapet wall c.1925 that has been wet dashed with granite in the usual Scottish manner. About 20 years ago I had the loose copes rebedded (they bed onto a lead flashing that covers the top of the wall) and patches of bossed harling stripped and redone.

Over the years the unpatched dash coat has worsened (probably due to the copes being a bit narrow with insufficient drip-off) so I've stripped off the harling back to the brick for the top 2 foot. The top row of bricks especially have spalled badly and are crumbly so I've been advised to use riblath to get something decent for the new harling to get a hold on.

I'm doing the basecoats myself this time, then a roofer friend is going to wet dash it for me. I've been told that you need to do 2 scratch coats on riblath and I've also read the same on here somewhere (by Micilin I think).

Not being a tradesman (I'm a marine engineer), what's the reasoning behind that? Do you coat over the riblath till you've got a couple of mill over the metal then scratch that, then level it out with the 2nd scratch coat, as I'm told the lath goes quite uneven once it's fixed to the wall.

Also, where the bricks are badly wasted back I was thinking of doubling up the riblath to make it stiffer as there'll be a gap of about 18 inches where I won't be able to get a fixing to the wall.

All help gratefully received, thanks

 

[Micilin]

I think you have the idea from your post.
EDIT. Censor affects what should be 'pr1ck'

Your first scratch coat , a 'prick' coat, firms up the mesh, not essential that it covers all the area or is flat.

Then you are straightening up with at least one scratch coat .

The prick coat stops the lath moving as you rule off, and also stops you pushing too much render through and behind the mesh, if your background is not tight enough either because of substrate or some kind of membrane eg your 18 inches.

The prick coat provides that essential key with just enough pushed through the mesh .

In cases where the bricks are bad, it my be worth putting a scratch coat on them first to bind them , then the rib lath ?

Be careful with scratch coat that you do not rule off back to the ribs as then you have sections of render rather than one piece . Also make sure the riblatthnis the right way round so that you are filling the concave ribs.

 

[craigie10]

Thanks Micilin for your solid advice.

I put the first coat on the riblath yesterday, maybe a bit heavier than you've said, more or less all filled in, can only see the surface of the mesh at a few bits. It's probably got a couple of mill over it with a light scratch but not back to the wire. I'm hoping it will be ok like that, all part of the learning curve.

I'll get the 2nd (scratch) coat on tomorrow, then a final wet dash harling coat sometime next week.

I'm sorting out something for the copes to create a wider drip-off so hopefully it will cure the root problem.

Should have done that 20 years ago!