# Forum Home Renovation Plastering  Is a notched trowel essential for black blocking?

## joynz

For back blocking, the CSR Red Book instructs that the back of the plaster back block should be covered in cement using a 6mm notched trowel - with the 6mm beads at 20mm centres.  *QUERY*:  Is a notched trowel really necessary? If so, I’ll buy one.  
(History: This is a follow up to a question I posted in 2021 about back blocking a celing crack that appears at the edge of an 80s extension where two plaster sheets were butt-joined -  but not on a framing member. https://www.renovateforum.com/f208/u...laster-130782/ 
I’ve now had a look in the roof space and there is definitely no back blocking. I’m planning to back block with cornice cement and plaster board. (Metrix mentioned that he uses ply and glue - but I didn’t end up finding out what type of glue so I’ll just get some cornice cement and use plasterboard.)

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## METRIX

This is a common problem, the glue I referred to is stud adhesive.
My current place had this problem one long crack along one part of the kitchen, it was not an extension, rather bad gyprock installation, they had put two sheets next to each other in the same line. 
I was knocking down part of the wall where the crack was, and needed to put a new beam in the roof for the new opening.
When I started scraping the old cracked plaster out, there was three sets of paper tape / mesh that came out, obviously been attempted repair a few times unsuccessfully. 
Before I put the beam in, I back blocked with some 6mm ply and stud adhesive, also screwed the ply in, repaired the join and crack hasn't shown again that was a few years ago now.
I suspect the trowel is for the same reason as tiling, when you squash it onto the gyprock the ridges will collapse and give a high coverage of cement over the sheet. 
If you just put a few blobs on the sheet these would be the only sections adhering to the join. 
I would just use a taping knife or similar, put a thin 3mm layer of cement over the entire backblock and squeeze it on, also clean off any dirt etc from the back of the sheet, i usually use a small hand broom, then give it a wipe with a damp rag to get rid of any built up dirt.

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