Forum: Drainage
Topic: Help! ground level and dpc
started by: HollyPip

Posted by HollyPip on 03 July 2019,17:06
I would really appreciate some advice as to how best to lower the ground level around my property. I have had differing advice from surveyors, builders and architects, and am very much "drowning in information, but starving for knowledge".
We have large concrete pavings surrounding our house, including a large patio area. The gap between the external wall insulation and the pavers varies around the house - At best they are a couple of cms below the DPC, at worst they are touching the external wall insulation which is at the level of the DPC, thus bridging is very likely. We do have some issues with damp. Options we have been presented with:

Lower and move all paving slabs so they are 150mm below dpc and away from wall insulation. Create gravel border between slabs and wall.

Lower and move slabs as above, but create french drain between slabs and wall consisting of trench, mesh and gravel.

Use an angle grinder to cut slabs or move them so they are 150mm away from wall insulation and create french drain as described above. The top of the gravel in the French drain will be less than 30 mm from dpc - will splashback still be a problem?

Just lower paving slabs to 150mm below dpc.

I don't know what the best option is. What I do know is that in my work, french drains tend to cause more problems than they solve, but this relates to historic churches which don't have dpcs.

Any advice greatly appreciated, can also provide pics if this would help?



Posted by Tony McC on 11 July 2019,10:03
Sorry I'm late in responding but if you had had the sort of week I've just had, you'd understand!

All of your proposed strategies have their merits (though I despise the term 'French Drain' - it means too many different things to different people), and it's hard to say what would be most appropriate when I'm not familiar with the site.

The best option is to reduce the level of the paving to at least 150mm delow DPC. The proposed gravel 'splash strip' is a bit of a nonsense - they always are! If the paving is 150mm below, then no need for a splash strip at all, but some folk sem to think they help somehow - no-one's ever been able to successfully explain what it is they actually help, but at least they do no harm (unless small, pesky kids transfer the gravel from the spash strip to a nearby gully, a favourite childhood pastime, it seems!)

The ill-defined "French Drains" have their uses, most commonly in areas with high water tables and imperilled DPCs, but there are far better and more technologically advanaced methiods available to us nowadays.

Could you get a photie or two of the troublesome area?

Posted by HollyPip on 14 July 2019,20:44
Thank you for your reply – no need to apologise, I am just grateful that the website and forum exist.
Photos are attached – as you can see the gap between the EWI and pavings does vary, but I think that is more to do with the pavings than the level of the EWI. When we lifted a slab at the edge of the patio the gap between the EWI (dpc) and ground was 4 inches, whilst at the front of the house it’s about 2.5 inches. Apparently, we do have a ‘french drain’ along one side, though it is just made up of about 2 inches of gravel on a flat cement base.  Our neighbours garden is already about 6 inches higher than ours, and on the opposite side there is a gravel lane, which is also slightly higher than our garden.  I have discovered that we can get a product to protect the EWI from splashback.
We are next to a tidal river, no history of flooding but I have been warned that the water table can be high at certain times of the year.  We have no drainage issues anywhere in the garden that we are aware of.  It’s predominately clay soil.
Thanks again for your help.

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Posted by Tony McC on 18 July 2019,09:30
I would agree that the main problem is the level of the paving.

So: lift and re-lay at a lower level is the best option. Failing that, I would suggest a min 200mm wide splash strip, dug to a depth that is at least 300mm below DPC/EWI, and then backfilled with clean 10-14mm gravel around a 80mm Ø perforated pipe connected to one of those drainage hoppers, something along the lines of the < Drained Channel Arrangment > shown on this linked page from the main website.

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