The Brew Cabin

Hi

I am currently at snagging stage with my contractor who laid my 69 sq m patio some 18 months ago, using sawn Indian sandstone (Harvest Mix from London Stone) and pointing with a Larsen GPM 5mm mortar line. This was a part of a larger extension/renovation project.

Construction of the patio was problematic. The subcontractor tried to get away with a minimal sand and rubble sub-base in parts. Originally spec’d at 100mm, I had to insist on it being redone twice before we were able to get something more like a barely acceptable 70mm; we are on heavy London clay too. Cement was in short supply at the time and there was a heatwave, and it became apparent that they had been applying GPM with a trowel rather than the gun (it was reaching 38 deg at the time), leading to endless cleaning operations after laying.

18 months on, the patio has held up, although various stains and cracks had developed. Selecting the 9 worst affected slabs from various parts of the patio (5 had developed some form of reflective staining and 4 had developed significant cracks) my contractor has lifted and replaced them with surplus slabs, as part of the snagging process, revealing that:

- the reflective staining was due to the use of a form of dot and dab, using tile adhesive on top of the mortar bed

- beneath the cracked slabs, it looks like the mortar bed was not entirely full. Various hairline cracks along some of the GPM lines indicate that there has been a bit of movement in parts of the patio, but the mortar bed revealed hadn’t itself cracked, so I am not sure whether this was caused by moisture in the voids, movement or perhaps tight butting (ie 5mm is at the tight end).

There are 6 or 7 further slabs not yet lifted which have started to develop cracks (at this stage minor, mostly next to T junctions in the GPM line). My contractor is working on the basis that there may be voids beneath the edges and talks of drilling through the GPM to inject quick setting mortar. [Edit: on closer inspection, more like 15 although some pretty minor]

More importantly, there is the longer term issue as to what is going on beneath the rest of the patio. The drying patterns are very variable (not easy to photograph, but I will attach), suggesting that there may well be more void problems out there and various of the slabs show some discolouration.

So down to the questions:

(1) Would the forum recommend running the idea of injecting quick setting mortar under the remaining cracked slabs? Or does it make sense just to go ahead and replace them too. Checking out the material here, as a minimum it seems to me that it would be advisable to get somebody with specialist experience to do this - the lad doing the snagging (not the original subcontractor by the way) has made a reasonable go of laying new slabs but is definitely not a specialist. What do people think?

(2) I am assuming that a well-laid patio should last 20 years plus and am trying to get my mind round whether I have a ticking time bomb on my hands. 18 months in would you expect most of the construction issues to have emerged or is there likely to be a slow burn effect, with future cracking and discolouration possible/likely?

Here is the Flickr album that contains the photos

https://www.flickr.com/photos/sjmawson/ ... 0306549825

I wouldn't class myself as expert but have done plenty . With most paving like yours, we tend to coat the back of the flags with a mix of sbr and cement. Have a read through this wonderful site. All your answers should be there. This forum has died a bit since the owner passed away

Sorry to hear that - what a loss. His personality (and true common sense) shines through all his material. I bought his book recently and was bowled over by just how comprehensive it is - and yet how easy to read and enjoy.

Indian sandstone is a popular choice for outdoor paving due to its natural beauty and durability. However, there can be some challenges and issues associated with using Indian sandstone, including problems with tile adhesive and other concerns. Here are a few common problems and potential solutions:

Tile Adhesive Issues: Indian sandstone can sometimes present difficulties when it comes to tile adhesion. The uneven surface and varying thicknesses of the stones can make it challenging for the adhesive to bond properly.
Solution: It's crucial to use a high-quality, flexible tile adhesive suitable for outdoor applications. Additionally, preparing the sandstone surface properly by cleaning it, removing any loose particles, and ensuring it is dry can improve adhesion. Back-buttering the tiles (applying adhesive to the back of the tiles in addition to the substrate) can also help create a stronger bond.

Staining and Discoloration: Indian sandstone is susceptible to staining and discoloration due to its porous nature. Factors like oil spills, organic matter, and weather conditions can cause unsightly stains.
Solution: Applying a penetrating sealer specifically designed for sandstone can help prevent staining. This sealer creates a protective barrier that repels moisture and prevents stains from penetrating the surface. Regular cleaning and prompt removal of any spills or stains can also help maintain the stone's appearance.

Efflorescence: Efflorescence is the white powdery residue that sometimes appears on the surface of Indian sandstone. It occurs when water-soluble salts within the stone migrate to the surface and crystallize.
Solution: Efflorescence can diminish over time, but you can help alleviate it by using a suitable efflorescence cleaner to remove the residue. Applying a breathable, vapor-permeable sealer can also help reduce efflorescence formation.

Natural Variation: Indian sandstone is known for its natural variation in color, texture, and thickness. While this adds to its charm, it can make achieving a perfectly uniform look challenging.
Solution: Embrace the natural beauty and variation of Indian sandstone. Plan the layout carefully to create a harmonious blend of colors and textures. Sorting and grouping the stones based on their characteristics before installation can help achieve a balanced and visually pleasing result.

Remember, it's always recommended to consult with professionals or experts in stone installation for guidance specific to your project and to ensure proper techniques and products are used.

that is rubbish, as the gaffer would say
all flags should be laid on a full bed
no idea where tile adhesive comes into it?
we are not laying tiles
rip it up and redo is the only correct advice
LLL