Ive just fitted a new bathroom and am fitting a new extractor. There is an existing hole through the external wall (breezeblock cavity)of 3″ diameter. The ducting is 100mm (4″). However, the ducting runs between the ceiling and upstairs floor, so i have no access to the inner face of the external wall. How do i enlarge the existing hole on the inner skin of the cavity wall. The core drills i have seen do not appear ‘long’ enough to pass through the outside skin, cavity and inner skin.

You drill them skin at a time using extensions in the core drills.

The erbaurer set in screwfix is surprisingly good at this and isnt made of cheese imo.

I assume you have big sds hammer drill or your on a hiding to nothing though

Ive just got a normaly heavy duty hammer drill.

i was thinking whether i could chain drill masonary bits and a cold chisel

You could but you’d have to make a hole big enough in the outside skin to access the in the inner and then make good both skins
Best to invest in the drills trail rat talks about or pay some one who already has some

Can you not just fit a 100mm to 75mm reducer?

Just gone and hired an sds and core drill – i just want the job finished.

Thought about the reduder, but the problem is that i cant access the space in the ceiling ( without cutting a hole in the ceiling) to fit it/connect it up. It has to be connected outside and fed in through the new hole.

Enlarging a 75mm hole to a 100mm hole with a freehand drill. How high of the ground is this?

All sorted, centered the new hole to the side of the old one ( overlapped the holes), fitted new ducting and used expanding foam to fill the unused section of the old hole. I’m glad i used an sds and core drill!!