Geo-fabric under slabs

[GaryN]

Tony,
I've read your page on geo-fabrics and you mention the geo-fabric can be laid between the sub-grade and bedding when laying slabs. I tend to use a bedding mix of 6 parts sharp sand to 1 part cement (wet). I'm currently doing a job where I'll be laying flags on clay. I've got some TDP115 left over from a previous job and was wondering whether to lay this over the sub-grade and,
1. Would it be okay to use the wet mix directly on top of the fabric and,
2. Would the fabric be of much benefit in strengthening the sub-grade?
Regards
Gary

[84-1093879891]

The short answers are...
1) - Yes
2) - No

When you use a cement-bound bedding layer, the argument that a geo-fabric eliminatesd weeds becomes redundant, as they wouldn't/shouldn't be able to get through a cement-bound bed in the first place.

So, what other reason is there to use a geo-fabric? If the sub-grade is iffy in some way (wet/peaty/marshy/ loose or made-up ground) then a geo-fabric would be a boon when laying the bedding, but its efficacy once the bedding has hardened is questionable. However, you're laying over clay, which is nornally form and stable, so what purpose does the membrane serve?

I'm sure I've said this elsewhere on the site, and more than once in this forum, but I really don't have anything against using membranes, as long as their use is justified. However, I am concerned that there seems to be a trend, partly inspired by the eejits on the daytime garden makeover shows, that a membrane is "essential" on EVERY paving project. Are they on commission from the manufacturers?

It's iunlikely that unnecessary use uf a membrane would be detrimental to a paving project, but I fail to see the sense in using summat that's simply not needed, just because that's how they did it on Garden Bodgers.

f you think the membrane is necessary, Gary, then go for it - but from the bit of info I have about your job, I'd be leaving that roll of TDP in the back of the van and save it for a job where it was actually needed. :)

[GaryN]

Tony,
You've convinced me, the TDP will remain in the shed!!
Many thanks
Gary

[106-1093880414]

I'm laying flags for a garden path in a stepping stone layout with deco gravel inbetween the individual slabs. Its also a clay sub grade and I've decided to set use membrane under the slabs and gravel but only to stop weeds growing up through the gravel areas.

[jamarsden]

We are in the first stages of replacing our patio. We have removed the old flags, which came up with the base they were bedded in and we are back to bare, solid earth which we are now digging lower and levelling (the old flags were only 4cm below the DPC). As we are putting down areas of flags with gravel in between, should we use a membrane over the whole area to suppress weeds? The plan is to put down 2 inches of sand as a base and I'm not sure if the membrane, if used, should go under the sand or over it? We were going to use the 5 blobs technique but having read the relevant pages, it looks like we should use a solid mortar bed under the flags. Is this right?

[84-1093879891]

I can only repeat what I said in response to the earlier posts - you can use a membrane if you really must, but it's not essential, and you shouldn't fool yourself into believing a membrane will prevent any weeds from establishing themselves. True; a membrane will stop them growing up from the soil beneath, but with 50-100mm of gravel covering between the flags, there's plenty room for detritus and dust to accumulate and provide an opportunity for the weeds to grow into the surfacing.

For this type of project, my preferred solution is to prep the ground and then cover the lot with a cement-bound bedding layer - the same old 10:1 mix that I keep spouting on about. You can put a membrane underneath the bedding if you want, but I'd save me money for summat a bit more worthwhile.

The flags can then be bedded onto the 10:1 mix, with the mix brought up around the edges to hold the flags firmly in place, and then dress the intervening gaps with the chosen gravel, pressing a thin layer into the still-plastic bedding mix before top-dressing to finish flush with the flags.

This method has a number of advantages, not least of which is that the gravel is only 50mm deep, and therefore much easier to walk across than 100mm. The bound bedding layer performs the same task as the membrane in preventing weeds sprouting from the sub-grade.

The only potential drawback is drainage - on a large area, some provsion needs to be made to allow any surface water to either be drained into a convenient gully, or to find its way into the sub-grade. The layout and size of the project will determine the most appropriate drainage method.

[jamarsden]

So, just to check, I put a 50mm 10:1 layer across the whole site, then a further 25mm where the slabs will go? (so the gravel in the gaps will be 50mm deep?) Sorry to be so blonde and thanks for the help!

[84-1093879891]

No - you put 50mm of the 10:1 mix over the entire site a bit at a time and lay the flags directly onto it, with no additional bedding. The gravel will then be the same depth as the flags.

Don't spread the cement-bound bedding over the entire site in one go, or it will have gorn orff by the time you get to the far end with your flags!

I doubt this has anything to do with hair colour. My beloved Mrs Taz is a blonde and she's.....no: you're probably right! ;)

[danensis]

I knocked up a quick base for my beehives, using a 50mm bed of mortar and laying 18" suqre concrete flags directly on top. At the edges, I brough the mix up level with the top of the flags.

At the weekend, I was digging up a dock that had seeded itself at the edge, and the whole corner flag lifted up with the weed. What did I do wrong?

[84-1093879891]

You allowed Diock to seed itself in your garden. That stuff will grow where water can't get!

BTW, it's not that red-veined Bloody Dock (that's its real name, not just me being vernacular) that Mabbett was distributing around the gardens of England a few years back, is it?

[danensis]

Everything seeds in my garden. I'm thinking of applying for the national bindweed collection. Latest addition seems to be couch grass, which we never had before, but has suddenly appeared everywhere.

Oops bit of thread drift here methinks.