Lowering External Ground Levels - How Low?

[GravesendDan]

Hi,
New poster who has been browsing for a few weeks learning lots of stuff! Recently purchased a ground floor flat with high external ground levels causing slight damp problems. Advised to lower the ground levels by the local conservation officer who recommended 150mm below floor level - this would almost be below the bottom brick of the foundations!

Idea outside is to have block paving with a slight slope away from the property but the question is how low should this start - I don't want to disturb the foundations obviously so your advice is sought!

I've attached a rough picture showing a cross-sectional area of the wall - the black bricks are existing bricks and the red ones are proposed so where should the block paving be? Or should I have something else between the wall and the paving?

 

[Penners]

Any hard, impervious material that you place adjacent to the wall is likely to make the situation worse, (a) by preventing evaporation of ground water at the base of the wall, and (b) by causing rain-bounce against the wall.

Lowering the ground level against the wall is obviously a good solution, but on no account should you dig lower than the lowest part of the foundations. In theory, you can dig down to the level of the bottom brick of the foundations so long as you then don't cut away at any more than a 45 degree angle sloping away from the wall. Frankly, being a bit of a belt-and-braces type, I wouldn't take the trench this low. I think I would probably leave the soil against the bottom course of bricks undisturbed.

 

[MatthewC]

Hi GravesendDan

Welcome to the forum.

Can we have a few details of your house? Sample questions: Is it brick, timber or stone? Lime mortar or cement? How high are the current external levels? What sort of floor do you have inside (tiles on earth/tiles on concrete/suspended wooden etc)? How do you know how deep the foundations are?

My 150 year old house is limestone with lime mortar; I thought that the foundations were quite shallow but when we dug out beside the house, we found that the foundations were a good deal deeper than we thought. Had I known this, it would have eliminated one worry about the hole we dug indoors for the limecrete floor! Outside is now just about 150 mm down from the floor level; and it has made a huge difference; beforehand it was level with, if not even some way above the floor (clay tiles) and really damp inside.

Matthew

 

[GravesendDan]

Hi Matthew and Penners.

I have an 1840's building built from bricks with originally lime mortar but looks like concrete pointing. I also have external render up to a height of 1m which again looks concrete - when the path is lowered the idea is to re-render with lime.

I have suspended wooden floors internally and sub floor ventilation has been a big issue - more air bricks will be going in this summer when discussed with the other leaseholders (it's shared freehold).

Ground levels are at internal floor height at the front of the property and about 200mm higher at rear. As far as I can tell the bottom bricks of the footings internally are 160mm below the bottom of the joists. I suspect/hope there might be more as I have recently found a small hole into the original cellar - which having poked my camera phone through shows a brick arch full of rubble.

Would the recommendation be to have a small section of gravel adjacent to the wall then?

Thanks for the replies!

Edit to add: My posts are being vetted before they appear so there might be a delay in my replies appearing

 

[Penners]

Would the recommendation be to have a small section of gravel adjacent to the wall then?

That's the most usually recommended solution for dealing with a damp wall, although you would do well to leave your trench open for as long as you can to allow good air circulation to give the bricks a good drying out.

Removal of the cement render and replacement with lime mortar is a good plan. Once again, try to give the wall a good period of drying out after removal and before re-rendering. If you can arrange for this drying period to be during the summer, that would be best.

 

[MatthewC]

I have suspended wooden floors internally .... Ground levels are at internal floor height at the front of the property and about 200mm higher at rear....

I would suggest that you are ready for the discovery of rotten joists and, possibly, the floorboard ends as well. In my house some Victorian idiot plastered the walls right down to the floor and nested the floorboard ends against the plaster, then added skirting board. With the ground level being the same, the whole lot was damp and so rotted. I lifted one bit of skirting board complete and then watched it snap in the centre under its own weight! The I found that the boards were all different widths (and sometimes different thicknesses) so I had a lot of trouble re-using the good floorboards and it all needed a lot of work to sort it.

The 200mm difference at the rear of your house worries me... get digging!

 

[GravesendDan]

Ahhh yes the rear bedroom currently doesn't have a floor as when I took up the yucky laminate flooring that previous owner had installed my foot promptly went through the floor as it had rotted through. The floor was put in 25 years ago when the flat conversion was carried out so the original floor is long gone unfortunately - but this does make it less hassle to repair.

The attached photo is the path at the rear with a retaining wall holding back most of the garden. Looks like someone has plonked a new path on top of the old one rather than take the old one up - both air-bricks at the back are lower than the path at the moment so some new bricks will be put in when the path is finally lowered.

The question I still have is how low should the new levels be ... especially as the footings are so shallow.