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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorewoo
    • CommentTimeNov 22nd 2021
     
    Hi,
    how do I drill pilot holes when fixing thick insulation board *(140mm) to rafters? The screws don't seem to like going in without pilot holes, wood is quite hard.
    I can find 200mm drill bits, is that the way to go?

    Thanks,
    Euan
  1.  
    I would use self drill screws (and an impact driver if you can get your hands n one)
    • CommentAuthorewoo
    • CommentTimeNov 22nd 2021
     
    thanks Peter, is there a danger it will split the rafters?
    In some cases I'm trying to screw a plaster board screw through plasterboard + PIR board into the rafters. I'm using 120mm drywall screws. Can you get self drill drywall screws?

    I have an impact driver, but it gets really upset trying to drive the screws in! It's a building from 1850 or so, if that's relevant

    thanks
    •  
      CommentAuthorfostertom
    • CommentTimeNov 22nd 2021
     
    And how to start drilling in the right place, and dead perpendicular, when you can't see the rafters.

    OK, you can see them where the next board hasn't been fixed yet, but what about when it's insulation board on top of rafters (or outside of studs), boarding is complete, breather felt laid, and you're trying to do the same trick in screw-fixing downslope battens right through felt, insulation and OSB3 sheathing, to hit the rafters rather than missing and making holes in the (air barrier) OSB3?
  2.  
    Posted By: ewoothanks Peter, is there a danger it will split the rafters?
    In some cases I'm trying to screw a plaster board screw through plasterboard + PIR board into the rafters. I'm using 120mm drywall screws. Can you get self drill drywall screws?

    I have an impact driver, but it gets really upset trying to drive the screws in! It's a building from 1850 or so, if that's relevant

    thanks

    Self drill screws shouldn't split the wood, they are designed to have the right drill head for the thread to avoid splits.

    You can get self drill plasterboard screws - they usually have the fine thread for use in metal but can be used in wood although length and availability might be a challenge.

    I guess your timbers being from 1850s might be a bit harder than the pine as used today.
    •  
      CommentAuthordjh
    • CommentTimeNov 22nd 2021
     
    Posted By: fostertomAnd how to start drilling in the right place, and dead perpendicular, when you can't see the rafters.

    Just mark where the rafters are on the adjacent walls if necessary. Mark the tops of whatever surface is visible and/or pull a string across.
    •  
      CommentAuthorfostertom
    • CommentTimeNov 22nd 2021
     
    Does that work in the scenario of downslope battens on breather felt on OSB3 sheathing onto top of existing rafters, from eave up over ridge to other eave? i.e. when there's little to mark onto?
  3.  
    We left a 1m wide strip of breather membrane off the apex of the ridge, and a long screw sticking out of the apex where the rafters meet, enabling the counter-batten to be lined up with the rafter (to sandwich the membrane and insulation). But we used hammer-in spiral fixings with a handy alignment tool that helps stop the fixing from bending while you get it started. Goes through the counterbatten and the sandwich and into the rafter below.
    Also it was only 100mm insulation thickness over the rafters, the other 100mm was between the rafters.
    As we got near to to the top with the tile battens we removed the 'alignment screws' and slid the top piece of membrane that covered the apex under the counterbattens, then fixed them. Then carried on with tile battens to the top.
  4.  
    Like so
      CB alignment screws.jpg
    •  
      CommentAuthorfostertom
    • CommentTimeNov 22nd 2021
     
    Always clued-up Clooney! thanks. Do you prefer the spirals, and their perp-aid, to long screws? cheaper?
    •  
      CommentAuthordjh
    • CommentTimeNov 22nd 2021
     
    Posted By: fostertomDoes that work in the scenario of downslope battens on breather felt on OSB3 sheathing onto top of existing rafters, from eave up over ridge to other eave? i.e. when there's little to mark onto?

    Sorry, I don't understand the problem. Are we inside or outside the house for starters? Not that it makes too much difference. There's always something next to whatever you're doing and generally you can always stick a bit of masking tape to whatever it is and put a pencil mark on that. And you can stretch a bit of string between two pencil marks, using more bits of masking tape to hold it in place wherever necessary.

    Or use screws etc as Dominic did.
    •  
      CommentAuthorfostertom
    • CommentTimeNov 22nd 2021
     
    Posted By: djhAre we inside or outside the house for starters?
    Outside, at a ridge that's fully clad both faces first with OSB then insulation board then breather felt.

    It's asking a lot of a builder to faultlessly follow every step - cover one face with OSB, mark it while rafters still visible on the other face, clad the other face, then do same with the insulation board, perping the OSB pencil mark onto the board top face before cladding the other face, then same, marking on the breather felt. Nothing else there to mark onto. Forgetting at any step means no marks at downslope battening time.

    There must be a way to detect the rafter at the time of battening, perhaps by a long true-perp drilling up from below - but then have to seal the hole in the airtight OSB.
  5.  
    Posted By: fostertomIt's asking a lot of a builder


    Therein lies the problem.

    I still regularly forget to mark things up now that we are still going on the inside, position of studs for plasterboarding, pipes, cables etc. I’m dreading a couple of things like filling the first floor ufh pipes and when the electrician comes back for final fix!

    Posted By: fostertomDo you prefer the spirals,


    I believe less thermal bridging. Not sure about price or speed, haven’t worked it out.
  6.  
    I think the OP (Euan) is working on the inside, fixing plasterboard through insulation?

    We did it a few different ways but probably if doing it again we'd install half the thickness of insulation over the rafters, then fix chunky horizontal timbers spaced every 600mm over the top of that, with coach screws through into alternate rafters. Fit the rest of the insulation between these, then plasterboard onto them with short pb screws, VCL and wiring positioned according to taste.
  7.  
    Posted By: WillInAberdeenI think the OP (Euan) is working on the inside, fixing plasterboard through insulation?


    Yes that is my understanding also. I was only responding to Tom's question about the "warm roof" scenario and locating the rafters from the top.

    Old timbers can be hard to get a fixing in, we kept most of the first floor joists and have had to drill through some for pipe/cable runs; they are bullet-hard even though they are pine. I don't think you could hammer a helical fixing in to them without it bending. The floorboard screws went in fine though (Spax).
  8.  
    Should also have said that timbers throughout our 1800s house (and our previous one) were never spaced evenly to meet the edges of metric plasterboards or timber sheets, not a surprise! Adding new timbers on a regular spacing, reduced faffing time cutting sheets down.

    We had help from a joiner who was keen to use short pb screws because he could use his collated gun, much quicker for his time.
  9.  
    Hi ewoo,

    we do the following for the scenario I think you're describing (working inside a roof space or coombed/inclined ceilings), which is a bit of many of the things noted already:

    - mark the top and bottom of the rafter positions on floor/adjacent surfaces
    - Insulation board placed onto the rafters, and held in place temporarily with either long, large headed nails, like giant felt nail, or a long screw (hammering rafters may well dislodge slates, and certainly debris in your eyes)
    - transfer the marked rafter positions onto the face of the insulation, top to bottom
    - long drill to find the rafter, and a couple of test holes each side, to get a bit of certainty you're in the centre of the rafter
    - 4x1 timber branders/straps over the top of the insulation, fixed into the rafters, pre-drilled to stop screw-wander as it passes through the insulation
    - the branders are run horizontally at 600mm centres, with vertical infills at 1200mm centres to give your grid for plasterboard. If it's just an attic storage space, then don't bother with pbrd
    - push nozzle of foam gun between the boards, drawing it along the inside of the joints, and filling, small controlled amounts. Likewise fill the drilled test holes made earlier to find the rafters.

    Don't know if that helps at all, but it works for the projects where we've needed to use it.
    •  
      CommentAuthorfostertom
    • CommentTime7 days ago
     
    Sounds like the voice of experience. Good that you foam between boards - by no means industry practice.
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