1 Mar 2016
Reaction score
United Kingdom
I know this gets asked a lot but I couldn't find a thread discussing the exact points I'm confused about.

I'm refurbing a small (1.4 x 2.4m) shower room. The fitter has proposed the following for the subfloor:

- 18mm moisture resistant chipboard screwed to the joists at 250mm centres, with PVA applied to the tongue and groove joins but no other priming

- 6mm Hardiebacker board on top of that, screwed into the chipboard

- Tiles on top with flexible adhesive

My questions are:

1. Everyone seems to hate MR chipboard and recommend WBP ply instead. But if you have 6mm Hardiebacker board on top to stiffen and (semi)waterproof it, do you still gain a significant amount from the WBP? I've seen other threads suggesting that Hardiebacker over chipboard / WBP is worse than just a thicker layer of WBP but I couldn't work out why.

2. Should the chipboard be primed if it's under the Hardiebacker? Fitter insists not. If it is primed, on which sides? Underside and edges (it's ground floor with a not-very-dry cellar space below it)? All over? Any primer should be SBR rather than PVA based, right? Any issues with using PVA at the chipboard tongue and groove joins if it's under Hardiebacker?

I'm intending to be in this house for at least 20 years so want to take a long term view on this.

Any tips much appreciated!
Sponsored Links
If you go to the bottom of this page to similar threads, and to the bottom of each page of the similar threads you will find answers to your post.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links