Extractor hood - enlarging hole through wall

Discussion in 'Kitchen Fitters' Talk' started by DavidMSmith, Feb 9, 2019.

  1. DavidMSmith

    DavidMSmith Member

    Hi - we are about to replace an extractor hood with (hopefully) a quieter and more efficient and quieter model as part of a kitchen refurb.

    The existing hood extract uses 100mm dia ducting thro' the cavity wall to outside. The new hood spec is for 150mm. Happy to enlarge the hole and easy to do on the inside (lightweight block) but not as easy on the outside.

    The outer leaf is simulated "random" Cotswold stone i.e hard concrete block with "stone" facing. The house is approx 40 years old. The enlarged hole needs to be in the same place as the existing hole, and no option to move it somewhere else. I can't simply use a core drill as there is no centre for the pilot drill.

    Any suggestions as to how to neatly and easily enlarge the hole? Thoughts are: (a) mark the new hole and drill around the new circumference with conventional ~ 6mm SDS drill to full depth of outer leaf block and gently chisel out - lots of holes required, lots of noise but should be a fairly neat result. (b) find replacement weathered blocks the right size ( pseudo random - 2 required), remove old blocks, insert "new" ones, wait for mortar to set (at least a week) then drill hole with 152mm core drill - should be neat but a lot more time and possibly effort.

    I'm racking my brain to try to think of a reliable way to get some material in the existing hole to act as a guide for the pilot drill in the core drill - could I cut a slightly conical wooden plug and jamb it in the existing hole and drill through that? Once the 152mm core drill has gone in 5mm or so, the pilot drill will have done it's job. Anyone tried that? Think it would work? Any other suggestions gratefully received (other than reduce the duct size down to 100mm - the extractor fan spec is for 150mm duct!)

  2. WillyEckerslike

    WillyEckerslike Screwfix Select

    Cut an appropriate hole in a piece of plywood (or two) the same size as your core drill and fasten that to the wall - into the mortar joints. This will provide a guide for the drill without a central reference.
    Make good the fixings in the mortar afterwards.
    KIAB likes this.
  3. rogerk101

    rogerk101 Screwfix Select

    I've done your suggestion (a) a few times. Yes, it is daunting, but in the end it's nowhere near as much work as the other alternatives. Treat yourself with a few nice new 6mm masonry drill bits and go for it. Chances are it'll be easier than you thought and the wall won't be as hard on the drill bits as you thought and you can just return the unused ones. It'll be done in less than an hour. Don't try to drill your holes so close to each other that the bit drops into another hole ... just whack the small bit of stone between the holes out with a bolster chisel. Good luck.
  4. candoabitofmoststuff

    candoabitofmoststuff Screwfix Select

    You can use a core drill...

    There are two options...
    Either fill in the hole, i.e. "brick it up" using a lightweight block and some sand and cement... tyhen you can come back to it the next day and drill it. (This is what I did a couple of years ago when I had an identical problem... and it worked just fine)

    Then I came across this solution elsewhere on this site, which seems much easier!

    Get some wood, say 3/4" plywood, glue two bits together, and cut the hole you want in that, using a jigsaw. Fix that to the wall over your existing hole. You won't need the pilot hole as the walls of the plywood "template" will be sufficient to keep the core drill from wandering. Once you are a couple of inches into the masonry it should all guide itself.

    Good luck,


  5. candoabitofmoststuff

    candoabitofmoststuff Screwfix Select

    Blimey... all typing at the same time!

  6. DavidMSmith

    DavidMSmith Member

    That's great! Many thanks for the suggestions.

  7. KIAB

    KIAB Super Member

    If your present hood has a 150mm outlet & it's been down sized to 100mm, it will be noiser & inefficent,once you go to 150mm,you'll be surprise at the improvement, & will open up a whole new range of hoods with much higher extraction rates.
    Last edited: Feb 12, 2019
    Wolfie308 and rogerk101 like this.
  8. gas monkey

    gas monkey Well-Known Member

    Hello outside the vent cover will cover any unsightly mess

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