what foundation for timber extension?

Discussion in 'Builders' Talk' started by Ludwig b, Feb 16, 2021.

  1. Ludwig b

    Ludwig b New Member

    Hi everyone
    my 1930's semi has a little room 3000x5000 mm built on the side that we use as a utility room, wash machine, shoes and all these stuff. it further has a toilet. unfortunately this room, built in the 1950's - 1960's has a single wall only and the concrete floor is just poured on the ground. thats very cold, and its damp. I guess no isolation and no damp prove. I hasn't a decent foundation, there's some concrete poured in a trench, maybe 400mm deep, and the walls are fitted on this slab, no cracks in the walls, none of the internal tiles cracked. I am planning to demolish this room and replace it with an extension the same size made of timber frame, OSB boards, insulation to current standard. I am planning to use this room as a spare room, maybe a room for the kids. the only trouble I have is its foundation. Our soil is clay, no trees nearby. I tried to dig already just for a try but there are lots of rubble buried , I think still from the time the houses were built, it makes digging nearly impossible. as these timber frame extension is not that heavy like a solid brick extension, what would be the requirement for my foundation?
    I can not dig 1 metre deep in this rubble/clay soil. would something like 400 deep x 400 wide suitable for my plan? I can not put the timber building on the existing concrete slab ( no insulation, no damp prove ) even if there are no cracks.
    does anybody have an idea or experience made with this?
     
  2. warcs

    warcs Member

    I'm not an expert but from what you have said it will be subject to being built in accordance with the building regulations, foundations if its a trench it will need to go down to 900mm minimum depending on the shrinkability of the clay and proximity to trees as some species even 15m away affect the depth of foundations, you could use a concrete raft but likely building regs will want calculations from a structural engineer and from what you have said you don't have any idea where to start. Why choose timber frame and not brick/block. what are you going to use on the external face? OSB will not last and look a bit of a dogs dinner. It really is all in the details of how you plan to construct this as to what type of foundation you need.
     
  3. Frutbunn

    Frutbunn Active Member

    It needs B regs, I would accept a raft using an over engineered approach without calcs, though your local authority/private inspector may insist on calcs' with a strip found I would still insist on a minimum depth of 1m and through any fill material. Be aware there maybe fire safety issues regarding timber in proximity to boundaries
     
  4. Ludwig b

    Ludwig b New Member

    OSB boards fully insulated with eps and rendered. I think that the construction will safe me a lot of work compared to solid brickwork and it will be less weight. nothing of this would be a problem, the only issues that I have is the foundation for this. the existing building is 50 or more years old and by that time insulation was far less. it is made from bricks and blocks just standing on a concrete slab. demolish it would be easy but to digging 1 m deep trench will be a challenge, I know where all the left over rubble was left when the build the main house in the 1930's. so I'm just looking for a solution on this matter as I can not access this with a digger
     
  5. FlyByNight

    FlyByNight Active Member

    I have to ask, does it really need Building Regs? If the OP does a "repair" to the floor, then a repair to side1, repair to side 2, repair to back, repair to roof, using similar materials in all case - it cannot be classed as being built. Not suggesting he does not build to appropriate standards, but full demolition and rebuild could be way more problematic. The OP should do a some checking locally to see if it can be achieved.

    Close to me that has been done a few times and in one case the council tried to stop it, but could not.
     
  6. Frutbunn

    Frutbunn Active Member

    Raft foundations probably the best solution.
    If you are within 1m of the boundary you will require 30 minutes fire resistance to both sides of the wall, please note the boundary can be taken as the centreline of a road, path etc
     
  7. Frutbunn

    Frutbunn Active Member

    He's stated its being demolished so it a new extension as far as B Regs are concerned.
     
  8. Ludwig b

    Ludwig b New Member

    §
    thanks mate, I thought of that one as wells it would be a repair only. but in this case I still have my walls standing on the old un-insulated concrete slab. and that is cold like a freezer. could lay an insulation layer of say 75 mm on this and build a new floor but then I have the new room higher than the existing house.
     
  9. FlyByNight

    FlyByNight Active Member


    You missed the point, if a structure is not demolished and "repaired" then BR would not be required often making it easier. There are many cases where BR will not approve the work on a demolition and rebuild even if is the same as before. The OP could demolish and then not be able to rebuild a similar size.


    You could break out the slab under each wall as part of the repair and re do that. Or, repair the full slab leaving the roof on posts - the walls coming down was needed to facilitate the repair.
     
  10. Frutbunn

    Frutbunn Active Member

    I think your getting B Regs confused with planning. There are no cases where B Regs can refuse any building. Regardless of whether he leaves the building up or not, B Regs will still be required even if its juat for the replacement floor
     
  11. FlyByNight

    FlyByNight Active Member

    What if he wishes to put a timber structure along close to a boundary? B Regs can object - however if repair of an existing they cannot.
     
  12. Frutbunn

    Frutbunn Active Member

    No B control can only enforce the B Regs, there's nothing to prevent timber on the boundary providing it complies with Part B (fire safety).
     
  13. FlyByNight

    FlyByNight Active Member

    And that may make it difficult to build - so repairing an existing would not require building control.
     
  14. Frutbunn

    Frutbunn Active Member

    Not particularly, a bit more expensive, but certainly feasible under b regs, - depending on the extent of the repair B Regs may still be necessary, certainly Part l applies to the floor and may also apply to the walls and roof.
     
  15. Ludwig b

    Ludwig b New Member

    It would be more than 1 metre of the boundary, but my main issue is the foundation. I could easy remove the roof and walls that I have and remove the floor tiles and the screed. doing this it will leave with a concrete slab that is ~ 5 inches thick and is poured on the soil, with just a little foundation ( if we can call it a foundation ) of maybe 400 mm depth. this concrete slab is 50 or more 60 years old and has not moved. I will focus on rat foundation design
     

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