Feasibility of extending garage by 1m

Discussion in 'Builders' Talk' started by andyscotland, Apr 15, 2018.

  1. andyscotland

    andyscotland Member


    I'm quite far on with planning what was to be a fairly straightforward garage conversion with small utility room extension. My wife has now asked about the feasibility of also extending backwards by 1m.

    I have a structural engineer lined up but he's on holiday and is expecting to come back in a couple of weeks, finalise the designs and get the building warrant in. I'd like to get a sense of how difficult / expensive the extra metre might be first, so I can try to rule it in/out before he's back rather than delay the project.

    The rear wall is 100mm blockwork, and the roof over the back section is low pitched (about 10 degrees) with splayed 50 x 150 rafters all taken from a circular steel bracket on the rear corner of the house. The new extra bit is hatched blue in the attached drawing.

    I was already planning to strip, insulate and re-cover the roof in EDPM so making the surface a bit larger isn't a huge deal. I assume to get the extra depth I'd need to remove the existing rafters and rear wall, build a new wall further back, and then install new rafters. I'm thinking the current rafters are already quite long (especially across the corner to corner diagonal) so we'd maybe need to change that roof structure to something completely different?

    We are going for a vaulted ceiling up towards the fixings to the house to maximise height so I'd want to keep a consistent pitch over the whole rear area rather than just adding a lower flat bit onto the end.

    And of course I'll need new foundations, slab, etc. The extra area is just under 5 square metres.

    Any rough advice on whether this is at the "doable for around the usual cost of a square metre of extension" or "complete nightmare to change the roof like that" or even "you'd be better just demolishing the garage and building from scratch" end of the scale?

    Thanks very much in advance,


    Attached Files:

  2. stevie22

    stevie22 Active Member

    Should be doable without too much grief technically and shouldn't be silly money but not sure of planning north of the border.

    I would run a new rafter down into then corners so the side slopes stay as is and only the back slope flattens. You're using flat roof covering so no problem there,

    Some interesting angles to cut and you may need to sister some of the rafters: your SE will advise
  3. KIAB

    KIAB Well-Known Member

    Habitable room, perfect opportunity to change to a warm roof design (superior to a cold roof) & GRP roofing
  4. andyscotland

    andyscotland Member

    @stevie22 thanks very much for that. Planning-wise we're OK so really just depends on the structural changes - good to have an opinion ahead of the SE getting back.

    @KIAB absolutely, was already planning to strip the old corrugated roof covering and change to a warm roof : prefer the look of EDPM to GRP unless there's a technical reason to go for GRP?

  5. KIAB

    KIAB Well-Known Member

    GRP any colour you like, hard wearing, had it at previous place 25 years, not leaked, still as good as the day it was laid.
  6. andyscotland

    andyscotland Member

    @KIAB thanks, I'll give it another look. Our neighbours had what they described as a GRP roof which has all manner of joints and seams and I'm not keen on the finish at all, so I'd ruled it out on that basis. But from a google now theirs doesn't look anything like what anyone sells as GRP, it's more like a plasticy membrane, so I'll maybe reconsider it. Cheers for the tip.
  7. KIAB

    KIAB Well-Known Member

    It's all down to proper preparation of the deck,great thing with grp, you can do any shape.

    Sadly there are some right cowboys out there doing grp roofing, but there are some excellent firms as well.





  8. andyscotland

    andyscotland Member

    Yeah I just saw those on the topseal site today, a totally different thing to what next door has! I'll definitely explore it. I do like the clean lines around the upstands, even EDPM has a joint there. Really appreciate the tip.
  9. KIAB

    KIAB Well-Known Member

    And epdm can tear, neighbour had window cleaner put his ladder on his epdm roof,ended up with a few tears in it.

    And if you damage grp( unlikely, I've dropped several roof tiles on mine by accident, no damage), it's easy to repair.
    andyscotland likes this.

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