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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    We're in the early stages of rebuilding the lean-to extension on the back of our victorian terraced house. Currently the back wall of the old extension matches that of the neighbors, and there's a 1M passage (running along the back of the terrraces) before a retaining wall and 4ft change in level to the garden.

    We're hoping to extend out a bit further into the garden. This means altering the retaining wall and essentially moving our bit of the passageway further into the garden.

    We've had a builder friend over and he says to build off that portion of the retaining wall, which would turn it into the base of the side wall of our extension.

    Instinctively it doesn't feel like a good thing to have earth up against a masonry wall which is the external wall of the extension, but are there ways in which it could work, and the obvious damp problems be alleviated?

    Any thoughts/advice appreciated,
    • CommentTimeSep 29th 2018
    I don't think damp is the first thing to think about.

    First, can you move the passageway? If it's Victorian and been in continuous use then it is a right of way and you will need to apply for permission to move it. Also there's a practical question; there's no reason why your neighbours should give up anything to help you, I suppose, so you will have to find 1 m at each side of your extension to move the passageway further back. Do you have space to do that?

    Second, I'd be very doubtful about building off a retaining wall. It wasn't designed to support a building and probably has very minimal foundations, if any. So I'd plan on knocking it down and starting again, which means you could put your new back wall wherever you want it.

    As regards the damp problem, it isn't really a problem. There are techniques for dealing with it, but they involve digging down the back of the wall, so again you'd do better to knock it down and start again.

    You'd have to knock it down anyway since you need to run the passageway behind it.
    Thanks for the reply. My description wasn't especially clear. It's not a passageway as such, just a space that runs behind the terrace. There's no right of way along it.

    I've knocked the existing retaining wall down, so will be rebuilding with appropriate foundations.

    I intended to put some kind of DPM behind the wall before backfilling, but wasn't sure if this was foolproof, or if anyone had a better solution.

    Thanks in advance.
    • CommentAuthortony
    • CommentTimeSep 30th 2018
    I would keep the retaining wall separate from the building and run a French drain round below floor level.

    You could also check out my basement construction philosophy that does not have damp proofing or membranes in the walls.
    Hi Tony,
    I also wanted to keep them separate, but that would have meant moving the retaining wall into our neighbours garden, and them losing that garden space, to allow for the gap at the corner of the extension. I did check, but understandably they weren't keen on the idea.

    The other option would be to move our external wall in from the boundary (retaining wall), but it's a small property and space is a premium.

    Where do I find information on your basement theory?

    • CommentTimeSep 30th 2018
    Posted By: acobite@yahoo.co.ukMy description wasn't especially clear. It's not a passageway as such, just a space that runs behind the terrace. There's no right of way along it.

    Does the space run across all the houses, and do people use it to get from their house to the end of the terrace? If so and it's been there longer than twenty years then it is a right of way.

    Best to establish it for sure before spending too much money.
    What's your basement theory Tony? Thanks
    • CommentAuthortony
    • CommentTimeOct 2nd 2018
    So long as no ground water is present the movement of moisture in a basement (contrary to popular belief) is from inside it into the ground. This is because the partial vapour pressure of the water vapour in the air in the basement is higher than it is at any point through the wall or floor!

    This comes from the basement development association in the 1980’s and is very basic physics and it works, I live with no waterproofing in my basement
    • CommentTimeOct 2nd 2018
    It's true - think french drain around the perimeter - as long as you have somewhere to drain that out to by gravity.
    • CommentAuthorvord
    • CommentTimeOct 2nd 2018
    I wouldn't build on the retaining wall. Don't like French drains either. I had one at the side of the house and even the gravel that hadn't already been filled with earth at the top was wet once you dug a little way down.

    After removing the French drain to fix some damp problems due to high exterior ground levels I dug a trench around the house with a retaining wall, left it open and covered it with grating.

    I would move the retaining wall and retain the air gap. Otherwise you will be relying on perfect builder detailing and perfect materials and sealants lasting for as long as you hope they will last. Neither will happen.
    • CommentTimeOct 2nd 2018 edited
    Posted By: vordthe gravel ... was wet once you dug a little way down
    How do you mean - like rain-washed wet, or standing water? The first is how it's supposed to work, conducting water downward; the second means blockage of the drain at the bottom.
    • CommentAuthorvord
    • CommentTimeOct 16th 2018 edited
    <blockquote><cite>Posted By: fostertom</cite>How do you mean - like rain-washed wet, or standing water? The first is how it's supposed to work, conducting water downward; the second means blockage of the drain at the bottom.</blockquote>

    Sorry to be slow - I'm not on here much. For me the French drain didn't work at all. Some muppet had built a car park a foot above interior floor levels and put a French drain in against the house. It was draining OK but the walls inside were soaking wet, wood was rotten and plaster had gone green. I replaced the French drain with an open trench with a grating on top and that fixed the trouble.

    My theory is you always want some ventilation outside down to below interior floor level so water can evaporate away and keep the underside of the house dry. Gravel just doesn't allow ventilation. I have an old house and there was time for my parquet to rot and my kitchen walls go green with mould.

    For modern houses you would use modern wonder materials and techniques so don't need to worry about first principles. An older house owner has to go through the pain of replacing 400 year old stuff rotten away by the modern materials once they fail and might tend to be thinking more about what happens when this stuff fails and how do you fix it. Lots of stuff has been forgotten, especially that builders hold to the long tradition of never doing anything especially carefully.
    • CommentAuthorCWatters
    • CommentTimeOct 21st 2018
    Retaining walls over about 1m tall should be properly designed not just left to a builder who claims to have done one before. The existing one is 4ft but how high would the new one be?

    It looks like you need to get a new wall designed that's a combination of retaining wall and basement wall.
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