How to enlarge hole through Bath stone

[andykent]

I need to enlarge drainage holes through two Bath stone parapets on my house in order to prevent them repeatedly blocking. The task is made more challenging by the hole being in an awkward corner and, because it's at an angle, the channel is about 9" long. Best way to go about this without risking damage to a historic house front? (Not listed, but in a conservation area. And historic, and valuable.)

The more frequently troublesome parapet only carries a 22mm length of copper pipe. The other has a zinc channel about 3.5" x 2.5" (see pic) and is less problematic but I'm aiming to sort both as part of the job of replacing their failed zinc flat roofs. What width should I be aiming for?

 

[bernardgreen]

An alternative to making the hole larger is to fit a filter screen to prevent leaf debris etc from getting into the gully. Make the screen out of two layer of wire mesh with the largest area possible.

A fixed layer of large gauge mesh ( large holes ) that supports a removable layer of smaller gauge. Then when the screen needs cleaning the removable layer can be picked up along with most of the debris. The debris can then be shaken / brushed off and straight into the waste bin



The balck being the filter screen

 

[andykent]

Thanks bernardgreen, interesting and simple suggestion, however access to both flat roofs is difficult (respectively a probable and a definite ladder job). For that reason zero maintenance is the objective, so the opening will need to be comparable to a guttering hopper.

 

[Burnerman]

For the copper pipe one, what's the chances of getting a core drill in there (blocking the hole temporarily so the pilot drill can start.)
For the square hole I'd consider chain drilling to the size you need, maybe finishing off with 10mm holes before getting in there with a long sharp cold chisel.
I guess it all depends on access but a temporary scaffold tower would be very useful here.
John

 

[ladylola]

I've fitted a fair few lead gutters similar to what looks to be in your picture. The outlet or sump we generally tried to make at least 6" deep and if possible as wide. Again it all depended on the building , the amount of rainwater coming through and sometimes other factors. If it was possible to widen the outlet hole it was normally done by hand with a sharp tungsten tipped chisel ( first one off google. http://shop.g-gibson.com/tungsten-carbide-tipped-24-c.asp ) . Frankly trying to get any form of mechanical tool in there is normally a right pain and very awkward .
We fitted an extra outlet into the roof on Calke Abbey and that was pretty much done the same . Some of the new outlet was removed with a drill but the hole was tidied up with the tungsten chisels. Of course putting in another outlet nessatated changing the direction of flow on a number of the lead bays.

 

[andykent]

it was normally done by hand with a sharp tungsten tipped chisel ( first one off google. http://shop.g-gibson.com/tungsten-carbide-tipped-24-c.asp ) .

Thanks - chiselling sounds like the way forward. But do I really need something that tough for sandstone?
And what size and model should I go for?

 

[andykent]

For the copper pipe one, what's the chances of getting a core drill in there

Not sure, could be tricky, but would seem the quickest method if it could be made to work. What kind of core drill would it have to be?

 

[ladylola]

it was normally done by hand with a sharp tungsten tipped chisel ( first one off google. http://shop.g-gibson.com/tungsten-carbide-tipped-24-c.asp ) .

Thanks - chiselling sounds like the way forward. But do I really need something that tough for sandstone?
And what size and model should I go for?

I suppose considering the Egyptians had nothing tougher than copper chisels when they built the pyramids the answer is no not really . We used tungsten tipped tools because they hold their edge and stay sharp for a considerable time. The stores did keep a stock of old chisels that had lost their tips and these were sharpened and were used as disposable tools for the odd small job. Sometimes they would re sharpen again. You could always pick up a couple of cheap wood chisels on a car boot and use them keeping in mind they will be ready for the bin by the time you've finished the job in all likelihood .