Pointing is the act of filling in the gaps between paving slabs to form a seal and to pad out any irregularities in the dimensions in the flagstones. This is why wider joints of 10-15mm are prescribed for weathered effect slabs such as Natural Limestone or Old Riven. As with brickwork, pointing is the term generally used with slabs. However some people prefer to use the term grouting which may be confusing as this is usually applied to wall and floor tiles although the principle is of course the same. A mix of cement and an aggregate such as sharp sand or building sand is usually used to point slabs but specially formulated jointing compounds are also available although they come at a premium. However, the downfall of using a wet mortar mix to point your slabs is that inevitably (particularly if your a DIYer), you’ll end up getting some of the mixture on the surface face of the slabs and depending on the type of slab, it can leave a stain, even if you wash it off immediately.
This is why pointing with a dry mortar mix is usually recommended with particularly porous slabs such as manufactured paving and certain natural stone varieties as water is added to the sand-cement mix after it has been embedded between the slabs meaning that the wet water mix is never in contact with the face of the slab. Pointing with a dry mix does however involve brushing the sand-cement mix over the face of the slabs in its dry form with a broom or similar brush and so it is of course vital that the surface of the slabs is completely dry, rain isn’t forecast, and even that there is little humidity in the air. It’s definitely worth checking something like this BBC weather map to check that no rain or cloud cover is expected for the next few hours.
The ideal ratio of sand to cement for the dry mix mortar is 6 parts sharp sand to 1 part of cement so for example 3kg of cement to 18kg of sharp sand. This should be thoroughly mixed until it has a consistent colour throughout which indicates there isn’t a presence of higher levels of cement in one area than another. The mix should then be liberally sprinkled over the surface of the patio and swept with a broom over the slabs so that the mortar mix falls into the joints. Although the joints may appear full after this first wave, due to the somewhat fluffy consistency of a dry mix mortar, the mix will need compacting. A pointing bar is an ideal tool for pressing down the mix although you can improvise with something similarly shaped.
After this, brush in any of the remaining mix on the surface of the slabs to refill the now compacted joints and add more dry mix if necessary to fill them up. Compact this second wave of sand-cement and then brush off any excess dust from the slabs whilst being careful not to disturb the dry mix mortar from the joints. Once the surface of the slabs are clean, its time to apply the water.
The best way to hydrate your dry mix mortar is with a standard garden hose gun set to the mist setting which should shoot tiny droplets of water into the air. This is perfect as it gives relatively consistent coverage as opposed to a single jet of water but also because a powerful hose setting would wash the mortar mix out of the joints and leave ugly splatter marks on your new patio. Direct the hose upwards as vertically as possible so that the water comes straight downwards and there’s no risk of disturbing the dry mix. Once the entire patio has been hydrated equally, we’re done. As you may have gathered, pointing with this method is remarkably easy and very little specialist skills are required whereas neat pointing with a wet mortar mix requires a lot of technique and practice. However the downsides of dry mix pointing are that it can only be done in particularly dry weather conditions and it might not be ideal for flagstones of varying thicknesses such as natural stone as it doesn’t level well around inconsistencies in the slabs.