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What causes tiny grout cracks in fairly new shower stall?

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  • May 5th, 2009 2:26 pm
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What causes tiny grout cracks in fairly new shower stall?

We redid our bathroom (ripped everything out and rebuilt everything) recently and have just started noticing a few tiny hairline cracks in the grout in different places in the shower stall. The grout isn't crumbling at all. Just a few thin, lengthwise cracks between tiles.

We are going to regrout those areas, but are hoping to avoid it happening again. We were wondering if it is likely happening because of shifting or if there was something wrong with the grout?

This reno was done by a family member. Although now retired, he was a building contractor for 30+ years and has done many reno projects for us with great results so I'm not thinking that it was necessarily something that was done wrong. We just want to try to prevent it from continuing. He's away for a few weeks or would ask him.

Thanks for any advice!
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Could be one of or a combination of several things. Is it the floor or walls cracking? Could be the wrong grout used for the size of grout lines. It could also be an unstable subfloor with too much flex if it's the floor. We need way more info to give you a better idea of why it's happening and how to correct it. How was it built and with what materials? What is the subfloor specs? Re-grouting won't help in most cases unless it was the wrong grout to begin with.
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Cracks in your grout are often caused by shrinkage, improper grout choice based on joint width and substrate flexibility/movement. Shrinkage normally occurs when too much water is added during the mixing process. Follow the manufacturer’s instructions carefully when preparing grout. Always choose either a sanded or nonsanded grout based on joint width. Sanded grouts are appropriate for joints at least 1/8” up to 1/2”. Nonsanded grouts should be used when the joint width is less than 1/8”.


Three of the most common methods to prevent cracking grout are:

Do not use a non-sanded grout on joints that are over 1/8" thick. It isn’t designed to fill larger joints.
Do not set tile directly over a flexible surface like plywood. It tends to distort over time and will cause grout to crack. We recommend the installation of WonderBoard
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booblehead wrote:
May 4th, 2009 2:07 pm
Cracks in your grout are often caused by shrinkage, improper grout choice based on joint width and substrate flexibility/movement. Shrinkage normally occurs when too much water is added during the mixing process. Follow the manufacturer
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Hello, do you see anywhere in my post suggesting I would require such credit to be given ?? I am just extracting the solution from the web. Exuse me for posting the info., rather than linking the info.


mcplar wrote:
May 4th, 2009 2:15 pm
could you not just post the link to the site?? and give credit ;)
http://www.custombuildingproducts.com/S ... iy&lang=en
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Thanks all.

Regarding more info, there is a crack in the grout on the wall (along the marble mosaic tile) as well as a crack in the grout along the porcelain floor. If they turn out, I'll try to take pictures to see if it helps. These are just very thin cracks that you really have to look for to see.

As for the materials, all I know is that it was all new install. There is a backing board (not green board - apparently a better board). Hubby is a plumber and installed new piping and the shower pan or liner. I'm not 100% sure of everything since I didn't do the work and unfortunately didn't watch it either. I know for sure there was a liner and concrete laid. The base was made out of concrete and the tiles are a porcelain floor and walls with a marble mosaic border tile used. I'm not sure if non-sanded grout was used or not, but it was the type that the tile store recommended we buy.

Thanks for the advice.
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What's the subfloor structure under the drybed (I assume)? If the floor has flex and it wasn't addressed it's going to continue to crack, especially with the added weight of a drybed shower pan plus body weight. How was the tile installed, mastic or thinset? What's the size of the grout line?

Best case scenario is that the wrong grout was used and you'll be removing it all and re-grouting. Worst case is the structure can't support the weight and it will have to all come out to fix the underlying structure issues....
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CSK'sMom wrote:
May 4th, 2009 2:35 pm
What's the subfloor structure under the drybed (I assume)? If the floor has flex and it wasn't addressed it's going to continue to crack, especially with the added weight of a drybed shower pan plus body weight. How was the tile installed, mastic or thinset? What's the size of the grout line?

Best case scenario is that the wrong grout was used and you'll be removing it all and re-grouting. Worst case is the structure can't support the weight and it will have to all come out to fix the underlying structure issues....
Thanks for your help CSK'sMom. I just asked my husband and he said the subfloor structure is only concrete with a liner (it's a condo). He said there's no plywood at all underneath - just concrete as that is the construction of the condo floor as well so he said the floor is as strong as it can be. He said when they started from scratch and he was there and guarantees it is properly sloped and levelled. He said thinset was used.

I'd have to measure to see the size of the grout line but I'd estimate 1/8 of an inch although not sure - the floor tiles and mosaic tiles at least were already joined and grout would just have been applied in the predetermined space so we didn't have a choice in size with that.

Thanks again for your advice!
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Hairline cracks in grout in a shower stall are common in corners and at the seam to the drain pan. This is caused by flexing of walls and floors, vibrations, etc. IMO, these two transitions should not be grouted but rather caulked. If these are where your cracks are, you can run a bead of transparent (or opaque) caulking over top of the existing grout or you can remove the grout and go with an opaque caulking.

I've learned the above by making my own mistakes in the past... :o
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CaptSmethwick wrote:
May 4th, 2009 5:11 pm
Hairline cracks in grout in a shower stall are common in corners and at the seam to the drain pan. This is caused by flexing of walls and floors, vibrations, etc. IMO, these two transitions should not be grouted but rather caulked. If these are where your cracks are, you can run a bead of transparent (or opaque) caulking over top of the existing grout or you can remove the grout and go with an opaque caulking.

I've learned the above by making my own mistakes in the past... :o
Thanks. The crack along the mosaic tile is along the seam on the wall of where the mosaic tile starts and the porcelain tile ends. The one on the floor does start along the wall, but comes inward (in the direction towards the drain rather than along the wall). Will keep this tips in mind. Very helpful!
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As one poster has already mentioned, hairline cracks can be just the curing of the grout and shrinking slightly. This can be from not forcing enough into the gaps, by not mixing the grout properly or, it being a bit too watery.
Anyway, I don't believe it to be a problem. There is no way to prevent this. I suggest that you seal the grout with a good silicon sealant. I believe you are supposed to do this anyway. The sealant will penetrate the grout and make it water resistant.
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Pete_Coach wrote:
May 5th, 2009 9:01 am
As one poster has already mentioned, hairline cracks can be just the curing of the grout and shrinking slightly. This can be from not forcing enough into the gaps, by not mixing the grout properly or, it being a bit too watery.
Anyway, I don't believe it to be a problem. There is no way to prevent this. I suggest that you seal the grout with a good silicon sealant. I believe you are supposed to do this anyway. The sealant will penetrate the grout and make it water resistant.
Thank you. Hopefully this is what it was since it is fairly newly done. My husband regrouted today. We had sealed it, but will do it again. We will also use some transparent caulk in the areas suggested by CaptSmethwick.

Hopefully no more problems! Thanks everyone.
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Did you remove the original grout from the whole wall/area first and not just the cracked area? If you didn't you're not going to solve anything as you can't regrout over existing or butt up to existing grout except in an area like a corner that will be siliconed...
Thinking seriously about the 4 S's...Sun, Sand, Surf and ... Booked for Sept in Mexico and booked Samana DR for Jan!
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CSK'sMom wrote:
May 5th, 2009 12:41 pm
Did you remove the original grout from the whole wall/area first and not just the cracked area? If you didn't you're not going to solve anything as you can't regrout over existing or butt up to existing grout except in an area like a corner that will be siliconed...
Yes. Thanks, good advice. He removed it yesterday using a little tool he had that looks somewhat like a screwdriver, but apparently is made for removing grout. He just removed the 2 lines of grout that had a crack in them rather than all the grout in the whole shower (too much work) so hopefully it will be okay.
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Nope it won't be sillysimms. Grout cannt bond or stick to itself like that. It will start crumbling at every grout line where new meets old...

Pete have you ever ripped out an old shower where the grout was cracked? It is a problem as it allows water penetration which allows mold to grow...
Thinking seriously about the 4 S's...Sun, Sand, Surf and ... Booked for Sept in Mexico and booked Samana DR for Jan!
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