Let suppose I have a thick external wall that I want to drill through and need the hole to exit at a specific point on the outside of the wall. The drilling has to be done from the inside. Is there some sort of sensor/detector that can be placed on the outside of the wall and it’s postion detected on the inside thus indicating where to drill?

Ta

Not possible to drill a small pilot hole?

APF

Tape measure?

Tape measure?

this.

Also, partly depending on the size of the hole and type of drill bit you may end up with a much bigger hole on the outside than you start with on the inside. My house seems to sacrifice a whole brick on the exit side as soon as I try and drill a 12mm hole through a wall as the mortar’s rubbish and bricks either hard as nails or made of sand.

Run a live electric cable down the outside of the wall. You’re bound to hit it 😉

Get 2 very very strong magnets?

Yeah, available at Screwfix I believe….

A tape measure is the obvious answer but it’s not always possible to do it easily as you can’t get a good common reference point to measure from.
As mentioned, a pilot hole to use as a reference is the often the best way, I suppose it depends on what the consequences of getting it wrong are.

My house seems to sacrifice a whole brick on the exit side as soon as I try and drill a 12mm hole through a wall as the mortar’s rubbish and bricks either hard as nails or made of sand.

Use a long bit
Drill through to the cavity and then hold the drill very lightly and let it find it’s own exit, if it’s anywhere near a mortar seam it’ll hit it.
seems to work – maybe – 7 or 8 times out of ten.

Try a strong magnet and an iron compass? About £3 all in. Maplin sell small Neodymium magnets and probably a keyring compass too. Get a feel for the behaviour first over a few inches and you’ll soon figure it out. Don’t put them less than 6″ apart or you’ll bugger the compass.

Rub a burger on the wall? I remember watching a documentary, which showed the oil would make the wall transparent.

What kind of finish is on the outside wall? Drilling from the inside often causes damage that’s bigger than the drilled hole. You would normally drill a pilot from the inside (only if necessary), then core drill from the outside…if the hole is that big.

Edit – sorry, I see it’s already been mentioned.

I’m assuming the issue with drilling from outside, is space to get the drill in place.

Really can’t see any other reason for not doing from the outside, otherwise.

Not possible to drill a small pilot hole?

If I wanted a specific exit point I’d go this route ^.

Long, thin drill from the outside, then if you need a bigger hole you could drill from both sides to prevent knock and big chunk out.

OP – If there is such a device, where a sensor is placed on the outside wall, why not drill from the outside in the first place?

Are there any windows on the wall? They’re the best reference points for transferring measurements from outside to in or vice versa.

Don’t use a tape measure. How will you get the height right? Chances are the floor inside the house isn’t on the same level as the floor outside 🙂

I wonder if the issue is that the bit will be emerging a long way off the ground and the OP has no ladder?

there’s usually some common reference point between interior and exterior spaces, pipes, windows, roof, ceiling, floor etc.

Long, thin drill from the outside, then if you need a bigger hole you could drill from both sides to prevent knock and big chunk out.

If thats the case he would just drill the whole thing from the outside I presume. Same with the ‘tape measure’ idea, if he can’t get access to the external wall how can he take a measurement, place a magnet, etc.

I wonder if the issue is that the bit will be emerging a long way off the ground and the OP has no ladder?

there’s usually some common reference point between interior and exterior spaces, pipes, windows, roof, ceiling, floor etc.

Pretty much. The external wall is above the roof of the kitchen extension where cheap (chinese?) slates have been used. Some of the slates broke the last time the roof was used to gain access. Therefore I don’t particularly want anyone standing on the roof.

Spread the load

Roof boards will spread the weight enough to prevent slate breakage.

Is there some sort of sensor/detector that can be placed on the outside of the wall

Even if this existed you’d still have access the outside of the wall to place it surely? If you can do that you may as well drill from the outside

The OP will also need access to seal the hole

Don’t use a tape measure. How will you get the height right? Chances are the floor inside the house isn’t on the same level as the floor outside

That’s not a reason not to use a tape measure.

It’s a reason not to use the floor as a datum point.

If you drilled a hole through the wall, you could use that as a datum point for the tape 🙂

It’s a reason not to use the floor as a datum point.

What would you use as a datum point for the height?

The only thing I can think of is use two points on the window and your intended hole position to make a triangle.

Let suppose I have a thick external wall

Let’s pretend you don’t, does this solve the problem?

What would you use as a datum point for the height?

That would depend on what was available and how accurate it needed to be.

Each situation is different and couldn’t really say what was best without seeing what the situation is.

There is always a way of doing it though, but the floor/ground is generally not a good way.

if you have no immediate access to the outside of the house it’s going to be a bit random where it comes out anyway.

To measure you need access to both sides of the wall…

assuming you can reach the external hole exit point from inside, how about extra long block spirit level?

plumb up a vertical line on outside with chalk, measure back horizontally to e.g. centre of window, then plumb back horizontall and vertically on inside. Bosh.

you always get to cherry pick the best images, Jamie.

Leica Boss

Maybe build some sort of access equipment to avoid touching the roof?

I think the above solutions are over complicating things a bit;

this might help:
http://www.wikihow.com/Use-a-Water-Level

roof boards will still break chinese and spanish slate, its all pap. i have an old gym crash mat i use in these cases, plonk it on the roof, scaffold planks on that bobs your uncle