Patio grouting

[Ratty2E]

The cement grouting on our patio is all but dissapeared. It is a raised patio outside our house as the garden slopes slightly away and over the last couple of years the grouting has crumbled and dropped through the gaps and I need to replace it. The problem is the gaps seem very deep and I would like to know how I can stop this happening again. i don't really know how big these gaps under the patio might be so I didn't want to be just pouring cement down there till it fills up.

[TonyV]

Do you know how the patio was constructed originally i.e. what sort of sub base was laid? Are all of the slabs still securely fixed with no movement or rocking. When you say the pointing (thats the normal term for the mortar between the slabs - grouting is for tiles) has dropped though the gaps how deep is it below. Can you push a bit of wire / knitting needle down gap and measure? Finally as the garden falls away, is there any sort of wall at the furthest edge of your patio? Sorry for all the questions and no answers as yet, but above information will help me and others give you the correct answers.

[Ratty2E]

Hi Tony, I am afraid the patio was here long before me so I have no idea what it's base is but the slabs themselves seem very secure. The edge of patio is sat on blocks no wall as such and it's about 8" from the gravel beneath. The depth of the gaps vary, probably due to the varying amounts of pointing (I knew that really) that has fallen through. The deepest seem about 3-4 inches but some seem to run under the slabs too if you see what I mean. Obviously the original pointing was just suspended in the joints with nothing underneath it that's why it's dissapearing. I did think about filling it with pea gravel or grit but would that stay just under the joints or get moved by water/insects or whatever may be lurking under there.

[RATTER15]

Get some bags of builders sand and keep brushing it down the gaps until you cant get any more in, then re-grout the slabs with dry sand and cement.

[TonyV]

ratty - I'm surprised your slabs are still secure, as they seem to be just laid on "spots" of mortar i.e. a bit of mortar under each corner of the slab and one in the middle - not the best method, but if the are still secure then fine. Only solution I can think of is similar to Ratter's, only I would use sharp sand mixed dry with cement 4/5:1 brushed in and tamped down to bottom of slab, then point from there. Reason for addition of cement to the sand is that it will form a mortar with water absorbed from soil and stop migration of the sand - as you appreciate you have some pretty large voids under the slabs. Hope the above helps and makes sense?

[The Builder]

As the others have said, you obviously have voids below the slabs. I think Tony's right in that they're sitting on "dabs". With those voids, the bearing for the dabs is going to weaken.
Since this is a bit unusual, I'm going to stir things up by suggesting something slightly different. I would try filling the voids with slurry instead of dry mix. Ordinary 4 or 5 to 1 sand/cement mix ................... but VERY wet. Wet enough to be able to pour it into the joints. Use a plastic kitchen measuring jug, or a funnel.
As you can guess, slurry is self-levelling, and is likely to find its way around the dabs, and make the whole thing pretty solid.
Pour to the underside of the slabs......... then finish with conventional pointing.......
Or, fill to the top, let it semi-dry, then iron and brush the joints to finish.
If you're not confident about pointing......... you could rent a pointing gun (like a silicone mastic gun, only bigger.)

[TonyV]

The Builder - Appreciate your "outside the box" thinking!!! Going to take a lot of slurry but should give the slabs better support. My thinking was by tamping down the dry mortar mix you should force some under the underside edge of the slabs and provide some additional support. I guess at the end of the day it depends on how much trouble ratty wants to go to?
I even thought about suggesting some judicious use of high density expanding foam - but decided against it.

[Ratty2E]

Thank you all for your responses, it certainly given me a few things to think about. I think it's likely the slabs are layed on spots of mortar so as you say there will be lots of voids, one of the reasons I had discounted brushing a dry mix in. I agree the slurry idea would certainly make for a more secure base but I could end up having to use gallons of the stuff, and it sounds like it could get a bit messy. I didn't know you could get pointing guns, certainly better than my rubble sack 'icing bag' idea, I'll have to check them out. BTW expanding foam did momentarily cross my mind too but I could just imagine lots of foam 'sculptures' popping up all over the patio. Thanks again :o)

[The Builder]

Here you are Ratty ....................

http://www.screwfix.c...ick-mortar-gun/67965#

(Expanding foam might tend to lift the slabs as well )

[TonyV]

The Builder - Have you ever used that type of gun? I have only heard bad reports - either squeezing the water out of the muck - or muck needing to be so runny that you end up with drips every where. Just after your opinion.
Ratty - whatever you decide upon under the slabs - would recommend you do your final pointing wet. Depending on the area it may be worth getting a couple of quotes from Hard Landscapers to solve the problem - just avoid being talked into having slabs relaid.

[The Builder]

I was using one just recently Tony. I was pointing in stainless steel bar that's used to "stitch" across cracks in old stonework. When I first used one, years ago, I had that problem .......... squeezing out the water and leaving the solids behind.
I should have mentioned above that a "fatter" mix is needed. 3 to 1 ......... much fatter than I would ever use normally, but the main thing is to use plenty of plasticiser in the mix. With that, you can reduce the amount of water considerably. If you whip it up really well, you can reduce to about 4 to 1.
A really plastic mix flows so much more easily without the separation.
Using lime instead of some of the cement also makes it flow better eg a 1:1:6 mix (cement, lime, sand)

[TonyV]

Hi Builder - can see if you use some Lime and propriety plasticiser in the mix it will work - but that is knowledge that most DIYers will not have. I was playing devils advocate in this case as did not want to dispute your choice of tool. I just prefer an auger driven machine similar to this which maybe hireable?
http://www.pwm-distri...tml/pointing_gun.html
Anyway without giving ratty to many problems / decisions about mixes to use - will not sidetrack the thread anymore and will leave it there.

[TonyV]

PS - if going for slurry method - make sure there is no way for it to run outside patio - down the front wall or at the edges.