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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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General: Hip to Gable Conversion?

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  1.  
    Currently looking at a detached bungalow with hipped roof, slate tiles.

    In order for the property to work for us as a family we would either need to go into the attic and add 2 bedrooms upstairs, possibly turning into a gable roof and adding dormers or we add a side extension...

    What sort of method would be cheaper/better in the long run?

    The dormer conversion would potentially give us decent sea views also

    The bungalow itself is on a footprint of12m x 12m, the existing ridge line is only 2.5m central to the property. The length of each hip is about 7.8m

    Does anyone know what the usable roof area space would be using a quick calculation?
  2.  
    I think to work this out we either need to know the pitch of the roofslope or the vertical height of the ridge above the eaves.
  3.  
    It won't be 45 degrees because they would meet at a central point like a pyramid, so it must be less than 45
  4.  
    I think it might be about 38 degrees
  5.  
    Also depending on the ceiling height you want. At 2m my rough guess is a hip-to-gable conversion would give you a room 12m by 6.8m (external)

    see attached working out, obviously these are just lines so you need to allow for the thickness of the structure.
    Starting with a 12 x 12 box on the left for the roof plan.
      the roof question.jpg
  6.  
    Adding the dormer to the left give you this VERY ROUGH
    I took off 300mm for the structure and allowed an extra 300mm for the PD regs

    "(bb) the edge of the enlargement closest to the eaves of the original roof is, so far as
    practicable, not less than 0.2 metres from the eaves, measured along the roof
    slope from the outside edge of the eaves;"
      dormer added.jpg
  7.  
    Also depending on when the house was built, you could try this Permitted Development route:

    Class AA - enlargement of a dwellinghouse by construction of additional storeys
    Permitted development
    AA. The enlargement of a dwellinghouse consisting of the construction ofâ€â€
    (a) up to two additional storeys, where the existing dwellinghouse consists of two or more
    storeys; or
    (b) one additional storey, where the existing dwellinghouse consists of one storey,
    immediately above the topmost storey of the dwellinghouse, together with any engineering
    operations reasonably necessary for the purpose of that construction.

    Development not permitted
    AA.1. Development is not permitted by Class AA ifâ€â€
    (c) the dwellinghouse was constructed before 1st July 1948 or after 28th October 2018;


    there may be other restrictions that apply, also you need Prior Approval from the Council, so plenty of reasons to say no.
  8.  
    Over here the useable space of an attic conversion is counted from a finished floor level (FFL) to an eaves height of 1.9M, so from this point to the same point on the other side gives you your useable width. Of course space further into the eaves can be used (cupboards etc.) but doesn't count as useable space. So you would have to do the measurements. The amount of useable space is very dependent upon the angle of the roof.

    It is unlikely that the ceiling joists will be sufficient for a floor so this adds expense and also raises the FFL. Proper insulation will either go on the outside of the rafters, this requires the roof stripping and raises the roof line which may impact planning or if done inside it lowers the eaves ceiling which reduces the usable area.

    By the time you have put in new floor joists and insulated from the inside you can loose a surprising amount of useable space. Also consider the staircase, - this will cost about 4M2 of ground floor area which has to be considered. (Spiral stairs in my experience don't save M2 but might fit where others don't but IMO they are a pain).

    if the ceiling timbers are structural (i.e. needed to stop the rafters spreading the walls) then they can be no higher than 1/3 down the rafter length. But you will probably need a SE to sign off anyway so they will confirm dimensions - meanwhile this will give you an idea about what you can get.
    • CommentAuthorcjard
    • CommentTimeJun 14th 2023
     
    Is it a trussed roof?
  10.  
    Some excellent thoughts for consideration, I guess given the size I'll need to engage with an SE

    The trusses appear to be stubbed eaves

    We have put an offer in so if it goes ahead week 3 be back with queries

    Thanks all

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