How to raise shower floor 80mm? Screed or subframe and sheet?

bigkid

Hi guys,

Due to an unfortunate plumbing situation, my plumber could not lower the waste trap for the new shower any further into the floor, so the shower floor will have to be raised approx 80mm, possibly up to 100mm to get the right fall to a tiled strip drain.

This will all be behind a brick hob and nib walls, so was wondering what is the best way to do this?

Using a 80-100mm thick screeded slab? Or should I build a timber subfloor frame and sheet with villaboard?

If I should frame it:

1. What timber would you use given the timber will be sitting on a second story concrete slab? Is rising damp an issue in a second story slab?
2. What layout should I use for the timber? A standard box/ladder frame configuration?
3. How would you fix the timber frame to the slab?
4. How would I ensure the villaboard sheeting has the correct fall to the drain?
5. What thickness villaboard should I use for sheeting?

Cheers for any advice.

JacobZ

Hi @bigkid,

While there is certainly a lower limit to the thickness of a screed bed, as far as I know, and as far as my research goes, there doesn't seem to be an upper limit. In saying that, according to this Dunlop Trade Technical Bulletin, "The drying time of screeds is one millimetre of screed thickness per day to reach an acceptable moisture content figure (as defined in AS1884-2021…)", so to have a screed bed that thick, you may have to wait a very long time to reach an acceptable moisture content figure before applying your waterproofing.

Looking at the alternative, the timber frame and fibre cement flooring would certainly have things completed more quickly, but you are running the risk of timber rotting if moisture makes its way into that subframe.

So, I would be weighing up the pros and cons in terms of time to completion against potential future issues. Screed will not rot like timber if moisture issues arise, but you could be waiting up to 100 days before it reaches an acceptable moisture content. Timber would be much quicker to install, but there is also a greater risk of flooring failure.

Long term, I think screed would be a better option, but it could take a long time to dry considering the thickness. For this reason, I think it would be worthwhile speaking with either a tiler, or water proofer who has experience with screeds to get their thoughts on the likelihood of issues with a screed bed that thick. If they, from experience, believe it wouldn't take an extraordinary amount of time to dry, I'd go with screed. If their advice is that it would likely take that full amount of time to dry, then the timber subframe would likely be a better option.

If you did have to use a timber subframe, then I would suggest you should use H3 framing timber as it would be better suited to cope with moisture if it does come across it.

The layout for the framing would be relatively to the flooring material. They will have an installation guide that determines how things need to be laid out to comply with their installation guidelines. Typically, this will mean a frame that has joists at 450mm centres with framing around the full perimeter of the space. This can also mean adding blocking under every sheet join to ensure the sheets are fully supported around their perimeter.

You could fix the frame to the floor using angle brackets, masonry anchors and screws. They would all need to be stainless steel.

Unfortunately, Villaboard is not a flooring material and it is not suitable for this application. You would have to use https://www.bunnings.com.au/james-hardie-2700-x-600-x-19mm-hardie-secura-flooring-sheet_p0851522 which is a fibre cement flooring material that is suitable for use in wet areas.

If you are using a strip drain against a wall, meaning the slope would follow a single plane, then you could simply create the fall with your framing. If you are working with multiple planes, then you would have to lay your flooring flat, then create the falls using screed. The approach all depends on the location of the drain and how many falls you have.

I hope I have laid things out clearly enough to understand. I think it would likely be best to speak to a professional for their guidance on this one, but this should hopefully help to clarify things a bit.

Jacob