Advice re ground levels

[EmilyB]

Ground levels question for you amazing people. We are working our way through all the ways to resolve damp before starting to dig up concrete floors! House is G2 listed, Georgian and earlier. Currently digging up our yard at the back, which has 60 years of lumpy, poorly applied tarmac on it. Ground slopes towards house! Few old cobbles under the tarmac but nothing worth saving. Plan is to hardcore onto the soil, then gravel, with additional French drain alongside the house to combat any water flowing that way. Not using a plastic anti weed membrane under the gravel because I hate them!

My question is: does the gravel ground level have to be 6 inches below the interior floor level? Or is it ok if the soil level is that far below but the hardcore/gravel (all free draining in theory) brings it back up so that it’s only an inch or so?

Would be so grateful for advice on this. Not sure I completely understand it.

 

[Flyfisher]

My understanding is that the '6-inch rule' (I'm not sure if it's a rule of thumb or an actual building reg these days) is mainly about preventing rainwater splashing above the DPC (if there is one) or the interior floor level. Based on this, it's fairly easy to understand that the splashing effect will vary enormously depending on the local ground conditions against the wall.

Thus, concrete is going to be bad news and bare soil would be better (for the same level). Gravel should not splash as much but it will hold water, as you'll discover by removing the top inch or so. In short, if you already have the soil level 6-inches below the interior floor level and really can't lower it any further, I might be tempted to add and inch or two of gravel but I wouldn't be comfortable bringing it up to within an inch of the interior floor level - Especially with the ground sloping towards the house that, as you have noted, will collect water anyway.

Given your already extensive groundworks, is it not possible to lower the ground level a little more (footings permitting) to keep the gravel well below the interior floor level?

As for weed suppressing geotextiles, they might be good for preventing weeds coming up through them, but my expeience of weeds on gravel is that they mostly grow from wind-blown seeds setting on top of the gravel and thus growing within the gravel. Having said that, a geotextile layer can help prevent them from deep-rooting so that they're easier to pull up, but it's still a chore I really dislike . . . as you would deduce from the state of my various patches of gravel around the place!

 

[EmilyB]

My understanding is that the '6-inch rule' (I'm not sure if it's a rule of thumb or an actual building reg these days) is mainly about preventing rainwater splashing above the DPC (if there is one) or the interior floor level. Based on this, it's fairly easy to understand that the splashing effect will vary enormously depending on the local ground conditions against the wall.

Thus, concrete is going to be bad news and bare soil would be better (for the same level). Gravel should not splash as much but it will hold water, as you'll discover by removing the top inch or so. In short, if you already have the soil level 6-inches below the interior floor level and really can't lower it any further, I might be tempted to add and inch or two of gravel but I wouldn't be comfortable bringing it up to within an inch of the interior floor level - Especially with the ground sloping towards the house that, as you have noted, will collect water anyway.

Given your already extensive groundworks, is it not possible to lower the ground level a little more (footings permitting) to keep the gravel well below the interior floor level?

As for weed suppressing geotextiles, they might be good for preventing weeds coming up through them, but my expeience of weeds on gravel is that they mostly grow from wind-blown seeds setting on top of the gravel and thus growing within the gravel. Having said that, a geotextile layer can help prevent them from deep-rooting so that they're easier to pull up, but it's still a chore I really dislike . . . as you would deduce from the state of my various patches of gravel around the place!

Thank you so much for this comprehensive and sensible response! Am going to discuss a further lowering with the builder on Monday. Re weeds: I think you get them whatever you do. Even our lumpy old tarmac had plants growing through it in the end!

 

[stuart45]

My understanding is that the '6-inch rule' (I'm not sure if it's a rule of thumb or an actual building reg these days) is mainly about preventing rainwater splashing above the DPC (if there is one) or the interior floor level. Based on this, it's fairly easy to understand that the splashing effect will vary enormously depending on the local ground conditions against the wall.

It's still a Building reg in general, although in new builds where wheelchair access is required the ground level can be be at DPC level. In these cases some different construction details are needed, such as trays, double DPC's and threshold methods.

CPD 7 2015: Level thresholds and water ingress

This module examines the key regulations concerning the accessibility of building entrances and highlights ways to avoid the main risks. It is sponsored by LABC Warranty

www.building.co.uk