The Brew Cabin

Hi, appreciate any insight into these strange patterns appearing 2 months after my Patio Installer fitted Kandla Grey Indian Sandstone slabs in my garden (pics taken before the massive rainfall we've been having). Installed around July 2025, on bed of mortar, still there today. Many thanks.

Hi there sorry to see you're having trouble with your patio. Looking at the patterns on your Kandla Grey Indian Sandstone, this looks like a classic case of "Reflective Staining" caused by the bedding method.
The Cause: It appears the slabs were laid on "dots" or "dabs" of mortar rather than a full mortar bed The moisture and minerals from the mortar are drawing up through the porous stone, creating those dark circular or irregular patches
The Rainfall Factor: While you mentioned the pics were taken before heavy rain, the moisture already trapped in the sub-base or mortar dabs is enough to cause this capillary action
Solution: Unfortunately, since it's coming from underneath, cleaning the surface usually won't fix it permanently. In many cases, the only long-term fix is re-laying them on a full, uniform bed of mortar with a bonding agent (SBR) on the back of the slabs to create a barrier.
Hope this helps clarify what's happening