plasterboard ceiling

[benzine]

I am renovating my bathroom & want to repair the ceiling. One section of the ceiling is about 8mm higher than the rest of the ceiling. It is about 450mm diameter & 8mm in the middle going to 0mm at the edge. How deep can jointing compound be put on a ceiling? Is 8mm deep asking for trouble? What other options should I consider?

Cheers

[rod@plasterbrok]

I am renovating my bathroom & want to repair the ceiling. One section of the ceiling is about 8mm higher than the rest of the ceiling. It is about 450mm diameter & 8mm in the middle going to 0mm at the edge. How deep can jointing compound be put on a ceiling? Is 8mm deep asking for trouble? What other options should I consider?

Cheers

What is the reason for the differnece in height?

It sounds like someone has done a poor repair on a ceiling vent hole.

I would be tempted to cut out the effected area and replace the section with new plasterboard installed correctly. Or better still for the cost of a couple of sheets and some cornice it may better to replace the lot.

Many bathroom ceilings are painted in a satin finish which plaster layed on thick does not adhere to very well. If you decieded to go down the path of fixing the original ceiling you would need to scratch up the painted surface pretty well to give the plaster something to key to. I would use cornice adhesive as a base coat and pre mixed jointing compound for a finish coat. Base coat is ok too except it takes longer to set.

I would NOT try to do the job with just 3 coats 8mm is a lot to fill but can be done. 1st I would get a 4ft level and see exactly how much fill is required to get a level ceiling. Then apply a very thick coat over the deepest area of filling, then use the level to screed it off. This may take several attempts to get a reasonable flat area. Dont worry about this process being a smooth finsh it is just to get a level base to start.

If you have any high spots they can be SCRAPED back flat don't sand. Scrape it while the plaster is still wet BUT SET.

Once you have a reasonable flat area apply thinner coats until the area is smooth, scrape back trowell marks between coats.

A final thin coat to pre-mixed top coat over the area and a light sand and you a done.

Having said all that my preference is to either cut out the effected area or replace the ceiling.

Cheers Rod

[ausdesign]

Rod you make a point of 'scraping' back the fill. Is that to provide a better surface for the next coat to adhere to.

[rod@plasterbrok]

No, it hasn't anything to do with adherance. It is to take off any high points left by trowell marks, overfilling or swelling as the plaster sets.

By scraping back as soon as the plaster has set hard enough to scrape without it peeling, you will fill any little pin holes etc as you scrape.

Scraping back if far better and qicker than sanding between coats as I have seen some DIY people do.

Getting rid of trowell marks or high points is important. If left and you top coat over them you will get a build up at that point. When you sand down the trowell mark or high point will show through the top coat meaning you would need to sand that area flat. The problem is that the base coat is harder that top coat. What happens is that the top coat surrounding the high point is sanded away leaving the high point worse that before.

If that happens it is best to srape it back flat rather than sanding and give it a thin top coat over that area then sand.

I see it often where plasterers are to lazy to scrape back joins etc between coats and the problem described above is left. This is one thing I look for when interviewing plasterers, it tells me if they know what constitutes a good job or not. (We interview our potential contractors on the job they are working on where ever possible, it tell us a lot)

Cheers Rod

[ausdesign]

Well explained & very informative.

[rod@plasterbrok]

Interesingly I went to a job today, and one of our regular contractors who typically does a very good job had a new employee doing the sanding, the job was riddled with examples of the above.

The guy had over sanded top coat gone bumpy base coat left! He now has to scrape back all high points in the base coat (mostly trowell marks not scraped back), then re - top coat every join.

Cheers Rod

[benzine]

Thanks Rod (I was hoping for a response from you!!!) I think I will replace the ceiling with new plasterboard & cornice.

Thanks.

[Joister]

bit of a late reply but if it helps what Rod is saying is a bit like upside rendering - when rendering a wall you often go back over the surface with a straight edge once it has set but still 'moist' to scree off the high areas and top up any low areas (guess same principle but using different compounds whilst with rendering still using your rendering mix)