Drilling-through-the-wall poser

[Lobster]

Would appreciate the group's collective wisdom on this one!

I need to fit an outside light next to the front door, on an old and none-too-stable pebble-dashed, 9" solid brick wall. I have to drill the hole from the inside, as the internal position of the hole is critical (and also the inside is fully decorated, so potential mess needs to be minimised). The SDS drill bit will emerge on the outside somewhere within a 2" square, which is fine, but I know that the emerging bit will inevitably knock off an area of the pebbles-dash render about 10 times that big.

Now if this was a flat brick wall, I'd simply place a large block of wood flat against the 2" area, wedge a length of 3x2 between that and the ground, then get SWMBO (bless 'er) to park her butt on the 3x2 to hold the wooden block firmly against the wall, to prevent the drill from doing the damage.

However, that isn't a go-er as the wall is coarsely pebble-dashed (ie not fine Tyrolean render), so the block wouldn't provide even support for it.

Any ideas? I was wondering about somehow placing clingfilm or grease over the area, then trying to cast something like mortar or car body filler in situ temporarily, to provide a flat pad for my wooden block to bear against... seems awful complicated though. I can try really, really gentle drilling of course, but from previous experience with this old render I'm pretty sure that won't cut the mustard.

Thanks! David

[Tony Williams]

What about carefully cutting out that 2" square of render first?

[dom]

Would expanded polystyrene, if applied with sufficient force, take the shape of the surface?

[Alan]

In message , Lobster wrote

Something such as (glazing) putty between the wall and the wooden block?

[Andrew Gabriel]

I would start by drilling a very thin hole through, which tends to produce much less collateral damage at break-through. I have long 6mm and 8mm bits I use for this. Then drill part way through from each side using the final size bit required (or for large holes, going up in stages).

Also, always try without hammer action first, and if you happen to hit a mortar line, it will glide through with even less collateral damage. However, don't continue without hammer action if you aren't making easy progress, as you'll knacker the bit. You could try turning hammer action off as you get near to breaking through too. Also, you often don't need any hammer action for widening the pilot hole out.

Pebbledash can vary enormously in its hardness and how well it's bonded to the wall. Some 20 years ago, I had British Gas knock a hole through a 9" pebbledash wall for a gas heater flue. It took perhaps 5 minutes to get through the 9" of solid brick, and probably another 20-30 minutes to get through the pebbledash. OTOH, on houses in a different road, their pebbledash has been falling off all by itself.

[meow2222]

I had a similar situation not long ago. Drilling thru from the inside is not an attractive option if the render is harder than the bricks, and drilling from the outside has its problems also. In the end I went thru from the outside first, and the hole landed in the right place. If it doesnt its easy to fill and spot paint, and drill from the inside to meet the hole in the middle.

Why is the inside hole location critical? Can you not do something to make it less critical?

I think at some point you've just got to line it up as best you can and drill. Polystyrene sure wouldnt be tough enough to support the render. Cement mortar on clingfilm would be, but you'd have to clamp that in place pretty damn hard for it to be effective support.

Dont omit the gloop over the outer hole, else water may very slowly saturate the brickwork.

NT

[Dave Liquorice]

Only if you go at the last 1" like a bull in a china shop. Apply mininal push to the drill, enough to stop it really rattling about but only just and let it do the work. Major spalling only occurs when lots of energy is being transfered into the brick work, the harder you hit something (ie push an SDS drill) the more energy is transfered.

A small pilot hole, as already suggested, is probably a good move. It also enables you to know how thick the wall really is and thus how far the main drill has got.

[Lobster]

Many thanks for all the responses: as I 'bit the bullet' last night I thought I'd report back with the outcome...

In the end I taped a 12" square of aluminium foil to the wall to keep the wall clean, then slapped a lump of putty over the proposed exit point, moulding it to the pebbledash and leaving a flat surface to bear against.

Got some very strange looks from the neighbours at this point.

Applied a 6" square of 18mm MDF to the putty, wedged in place by leaning a 6' length of 3x2 against it. SWMBO now climbs up the 3x2 and perches daintily on a small step I nailed on about halfway up.

The neighbours' curtain-twitching is going into overdrive now...

Drilled from the inside with a 10mm bit, switching off SDS at the end and progressing only verey slowly - bit emerged with no break-out damage at all to the render: Re-sult!!!

Thanks again for all the valuable advice.

[google]

Using a water level you can refine the exit point to somewhere along a

2" line, rather than being in a 2" square. Then you just have to refine the horizontal measurement and you can accurately drill from outside. Starting a hole in exactly the correct place on pebbledash is going to be fun.

MBQ