External ground height

[kg99p]

1880s sandstone semi in Scotland (the wet bit…). Some issues with rot in the joists of the suspended timber floor which we have fixed.

Home report commented that the external ground height is too high. I guess this may have contributed to historic damp issues. I’m renovating the back garden so have an opportunity to look at this.

My slate DPC is probably about 30mm below the concrete tiled patio area at the back. The ground is probably about 200mm below the internal floor.

Some sources suggest that the ground should be 150mm below the DPC - am I seriously meant to dig down about 200mm for the new patio ? I’ve read about French drains but they need planning permission and I’m not convinced that it was there previously, and so why is one now needed ?

 

[Cubist]

Your key and critical issue is that the DPC is compromised. Damp leaching into the stone above the DPC will continue to rise and will cause the joists and wooden floors indoors to rot. To be sure this does not happen the earth and any other materials that breach the lowest edge of the DPC must be removed and if possible to around 150mm below that bottom edge. You do not necessarily need to reduce the height of the entire patio, a narrow trench may suffice.

I am not familiar with Scottish building regulations but would be surprised if any particular permissions for a French Drain are required. Regardless, without more information concerning the garden layout, proximity to neighbours boundaries and access to any waste water drains it is unclear why any such would be appropriate or required.

 

[a twig]

Calls for a Zebcolm trench methinks:

Thread 'The Zebcolm Trench'

Oct 10, 2022

The Zebcolm trench is so named because I made one, and then malcolm made one. If you make one, let us know and we can elongate the name.

The Problem: ground levels higher on the outside than inside, causing penetrating damp through solid wall, with no possibility of lowering external ground levels.



The Solution: a trench with a retaining wall, topped by mesh grid, which allows air to circulate beside the lower portion of the wall.

First you dig - the rule of thumb is that you can dig to the bottom of the last foundation course. If you need to , you can then dig down away...

• Zebra
• Replies: 45
• Forum: Projects

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[fernicarry]

I am not familiar with Scottish building regulations but would be surprised if any particular permissions for a French Drain are required

I don't think you do, so long as you follow the SuDS guidelines you should be absolutely fine.