Well that’s the $64,000 question!
The problem is this – there is no one solution because there are several different causes. So first of all you need to establish what the problem is. Then you can try to come up with a remedy.
So What Are The Main Causes Of Grout Discolouring?
Mould needs permanent moisture to take root so if you can see the beginning of a problem your grout is failing. The things that cause grout to fail are:
- the wrong grout has been used
- poor application of the grout with gaps and holes present
- cracking due to expansion and contraction of the backing board/wall
- cracking due to bath or shower tray movement
- a leak elsewhere that has caused the backing board to fail
We will deal with each of the different scenarios listed above in turn.
But please bear in mind that tiles can cover up major water damage for years so this is not a definitive guide. The only way to know for sure what the extent of the problem is would be to rip off the tiles and start again – preferably with bathroom wall panels rather than tiles, as these are completely waterproof and use no grout (as seen in the photo above – these are panels not tiles).
Has The Wrong Grout Been Used?
The composition of the grout is very important although many bathrooms get tiled using grout that has one main quality – it is cheap.
Not all grout is waterproof. Not all grout is flexible. The ones that are tend to cost a bit more. But using anything else is a false economy and could end up costing you more (in fact it could end up costing you hundreds or thousands of pounds if it leads to long term water damage, which we get to see very often).
If your tiles are displaying signs of mould everywhere, not just in the shower area, then there is a good chance that your grout is porous and is harbouring moisture.
Has The Grout Been Applied Poorly?
This is hard to ascertain but you might notice very random, isolated sections where mould is starting to appear. Have a close look to see if you can see little pin-holes at these locations or even bigger gaps. If the problems has persisted for years these patches might have grown as the substrate behind the tiles becomes saturated.
Cracking Due To Expansion & Contraction
Most grout is not flexible at all, so it just does not cope with any movement – however slight.
If your tiles have been fitted to a wall that expands and contracts at a different rate to your tiles then the grout will struggle to cope and will start to crack. This should affect all of the surfaces equally if the walls are made from the same material.
It is particularly noticeable if the tiles have been applied to a wooden surface (plywood, MDF etc) so unless you use very specialised grout and adhesive this is going to be a permanent problem.
Solution To These First 3 Problems:
To start with you need to rake out your old, faulty grout. Make sure you get out as much as possible in case the mould has taken root deep in the grout. There is a specialist grout tool available for a few pounds that should make this task easier.
Re-apply a waterproof, flexible grout in its place. This will ensure you guard against the same thing happening again.
Apply a grout sealer, if recommended by the grout manufacturer, after the grout has fully dried. This will add further protection to the grout and prevent water getting into the grout.
Grout Cracking Due To Bath Or Shower Tray Movement
This is the most common problem with tiles and grout. All of the tiles around the bottom of the wall near the bath or shower tray will have mouldy grout. The silicone will also display signs of mould.
It could be the movement has caused the tile grout to crack and allow water to get behind the silicone or it could be the silicone sealant pulled away due to the movement and cause water to get behind the tiles and into the grout.
Either way this is a serious problem that is not easy to fix.
Solution – If you have access to the underside of the bath or tray, try everything you can to support the underside to prevent movement. Bricks, blocks, battens, timber can all be used to lend support and try to eliminate the problem. You then need to cut away the sealant with a very sharp knife, the you will need to rake out and re-grout as per the previous solution above, and re-apply the silicone. Make sure the surfaces are spotlessly clean and dry before applying new silicone – re-grouting results in a lot of fine dust on the tiles surface so this needs to be removed fully.
We have an article with more information on this subject here
Leaks That Have Caused The Backing Board To Fail
Leaks from supply pipes and mixer valves can, and do, happen over time. If this is an issue you will usually see tiles becoming loose as well as the grout failing. This is indicative of a serious problem and needs investigating.
On a masonry wall the damage might be quite isoltaed but on a plasterboard wall it could be very extensive as it has a habit of acting like a spoge and soaking up large amounts of water.
If the damage can be inspected, is not extensive and the leak can be stopped, the best solution is to remove loose tiles and panel over the lot. But this is only recommended if you are absolutely sure of the extent of the problem. Alternatively, remove all of the tiles to enable a good look at the wall. You can then replace the plasterboard (drywall) if required and re-tile or fit wall panels instead.
Can You Put New Grout Over Old Grout?
Not usually as the old grout should already be filling the gap between the tiles fully. It is always best to rake out the old grout to remove the mould and apply a good quality replacement grout, as detailed above.
Can I Paint The Grout?
You can paint the grout but the results will be variable and will not eliminate the root of the problem. The mould will still be there underneath the paint and could re-appear.
There are various grout pens and tile paints on the market. You can always give them a go – even if they fail you will be no worse off.
If they do fail then you need to look to a more permanent solution as detailed previously such as raking out the faulty grout and re-applying a higher specification material.