Anyone see grout turn black and get holes like this?

JamesW

TFP Expert
Mar 2, 2011
16,520
How old is the pool?

Do you have a septic system?

Is the ground always wet?

Is the problem just in one spot or everywhere?
 

JamesW

TFP Expert
Mar 2, 2011
16,520
Maybe a septic system backup?

If you're getting bacteria migrating through the ground water, it might be saturating the area under the deck and going through the grout.
 

Newdude

Well-known member
Jun 16, 2019
1,100
NY
If I didn’t know any better. And sometimes I don’t, pic #2 (the one with no feet). looks like it was burnt. Like something arc’d and blew the hole while doing so. You got the dark stain and the brown stain at ground zero with fresh grout around the hole. Pic 3 has the blowout hole also. 80 ft is too far for me for sewage. It would possibly stain if it did but it wouldn’t make the holes.
 

JamesW

TFP Expert
Mar 2, 2011
16,520
Bacteria can do some crazy things. The smell is a good indication that it's bacteria.

If you dig a hole outside the deck behind the bad grout, is it black and stinky?

When is the last time that the septic system was serviced?
 
Last edited:
  • Like
Reactions: Newdude

Newdude

Well-known member
Jun 16, 2019
1,100
NY
If it’s sewage related could methane or similar have built up under the patio and blew out the holes once the pressure got high enough ? Maybe those were weak spot to begin with and gave first ?
 

JamesW

TFP Expert
Mar 2, 2011
16,520
Bacteria can create acids that can damage anything susceptible to acid damage.

Many of the foods you eat cause the bacteria in your mouth to produce acids. That's what causes cavities. Not sugar or acidic foods.