Converting a flat roof to a low pitched roof

Discussion in 'Builders' Talk' started by NT555, Sep 30, 2017.

  1. NT555

    NT555 Member

    Hi All,

    A customer has a rear-of-house lean-to structure (containing a bathroom) which has a leaking flat roof that needs replacing.

    On examination the existing roof is chipboard laying on the ceiling joists. It has zero pitch and the chipboard is now turned into Shredded Wheat. The top covering is shed felt.

    My plan is to turn the flat roof into a low pitched roof, by:
    1. mount a timber wall plate in order to fit hang rafters. (Max pitch is 12°due to a window)
    2. fit 12mm wbp ply sheets
    3. cover with a damp proof membrane like this:
    4. cover with felt or perhaps some fibre roof slates like these:
    5. fit guttering etc
    The lean-to is south facing and not too exposed due to other surrounding structures.

    Any thoughts and recommendations on method & roof coverings would be welcome.

    Many thanks.
  2. KIAB

    KIAB Super Member

    Look at warm roof construction, osb 3 & grp, many benefits, grp will easily last 30 years plus.

  3. NT555

    NT555 Member

    Thanks KIAB,

    Can you point me at a specific product for the top covering and the pre-formed edging?

    Also, is the low pitch a problem?
  4. KIAB

    KIAB Super Member

    Not a problem the low pitch, just use firrings to give you a fall.
    Had grp roof at a previous place,it lasted 25 years,until building was knocked down.

    Got a bug on this forum at the moment,adding S to http,so have remove first bit.


    Don't have to use grp flat, it can do any shape as well.

    I would get someone to do the grp for you,quicker, & they have the experience,had firm in here to do workshop roof 7 years ago,save me a lot of hassle.

    Deleted member 33931 likes this.
  5. Nice info, KIAB.

    NT, as KIAB says, the pitch ain't an issue, but as KIAB also says, DO insulate that roof properly - especially since it's gonna be a bathroom. If you don't, not only will you have contravened regs, but the customer will have hellish long term problems with condensation.

    Lots of info out there on how to lay a felt roof - and that will last well in excess of 20 years.


    Note how they use triangular fillets along the top but also down the sides so that all the run-off ends up in the guttering.
    KIAB likes this.
  6. NT555

    NT555 Member

    Fantastic info chaps, much appreciated..

    Another thought. Does changing a flat roof to a mildly pitched roof need to go through planning permissions?

    Thanks again.
  7. BMC2000

    BMC2000 Active Member

    Rear of house, I wouldn't worry about planning. It is a structural alteration changing from flat to pitch so building application would be needed.
    Deleted member 33931 likes this.
  8. Jord86

    Jord86 Screwfix Select

    As all above have stated, but you want minimum 18mm sheet material for your roof, not 12mm.
  9. Jacob555

    Jacob555 New Member

    I'm also going to replace my roof, and I've found out that I will need to make a roof truss. And I've been suggested to use a roof truss calculator here for this purpose. If you want to convert your flat roof to a low pitched type, you will also need a truss. So, maybe this information will be useful for you.
  10. yorkshireboy

    yorkshireboy Member

    If you're using grp you can use the existing flat roof joists,if they are sound.

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