Opening up holes in joist

[pc1966]

A friend of mine is looking to have an electric shower fitted and has floorboards up already (mostly, long story). Anyway there is a run of 2.5mm T&E already following a sane route that the 10mm T&E for the shower could take, but the holes are just a bit bigger than 2.5mm and certainly not enough for both.

It seems like a bad idea to drill more holes in the beams, so I would prefer to open those holes out to around, say, 22-25mm to take both cables. Most auger or flat-blade drill bits assume no existing hole so the pilot can keep them centred, so I was wondering if anyone has good suggestions for doing opening such holes out in a sensibly controlled manner?

TL;DR drill options when existing hole is present in a wooden beam

 

[nicebutdim]

These things are generally very good at opening up holes in joists and also very fast cutting. Available from several manufacturers and in various sizes.

https://www.amazon.co.uk/Irwin-Blue-Groove-Drill-3Pce/dp/B000Y8IF5K/ref=sr_1_46?keywords=armeg+wood+beaver&qid=1690751408&sprefix=armeg+wood%2Caps%2C96&sr=8-46

 

[brianmoooore]

Conical step drill to start of the hole at the required diameter, then conventional flat bit?
Two hole saws on the same arbour is the conventional way, but wouldn't be ideal for a whole run of joists.

 

[SparkySy]

Could you not if opening up the hole, drill slightly to one side of it encompassing the existing hole? Especially if it's only 10mm and you drill a 25mm.

Or make a template up to clamp to the joist to start the hole, once centered if using the bits listed above or similar once it's started it will follow with no issues.
I have the CK version of the Irwin bits and have found them really good!
Sy

 

[loz2754]

Just drill a separate hole!

 

[nicebutdim]

I honestly see no point in anything more elaborate that wood beaver style bits. Unless the joists are on display, where a slightly rough exit would be undesirable, there's not point in wasting time when these bits will enlarge holes in seconds.

Stick one in an impact driver, aim and pull the trigger. It couldn't be more simple as they'll tear through wood in whatever direction you aim them.

 

[ipf]

It seems like a bad idea to drill more holes in the beams, so I would prefer to open those holes out to around, say, 22-25mm to take both cables.

Can't see any problem with just drilling a larger hole next to the half inch one?
Besides, how easy is it to remove and rewire the 2.5?

 

[brianmoooore]

Just drill a separate hole!

Required to be a minimum of 3 x diameter from each other, which can be a problem if you're working through a single, narrow removed floorboard.

 

[pc1966]

These things are generally very good at opening up holes in joists and also very fast cutting. Available from several manufacturers and in various sizes.

I was looking at the DeWalt set from Toolstation as an option, good to know they work well enough:

DeWalt Tri Flute Spade Bit Extreme 16, 20, 25mm | Toolstation

152mm Impact rated self-feed bit, designed for all types of general wood drilling. 3 cutting edges for up to 6 X faster cutting (vs standard flat bits), combined with Tri-Flute design for optimum chip removal. Ideal for electricians, carpenters, plumbers and other professionals.

www.toolstation.com

 

[pc1966]

Can't see any problem with just drilling a larger hole next to the half inch one?
Besides, how easy is it to remove and rewire the 2.5?

There are two holes already, maybe around 3" apart.

At least one of the 2.5mm runs has to be partly changed anyway as there is a socket to be moved out of the area that will become the en-suite shower, and it might work out easier to use it to pull through a rope and back with it and the10mm along the between-joists segment of the run.

The diagonal corner of the room as the CU in a cupboard, which also needs changing at some point, so if I can reliably get the end of the right 2.5mm there it would be a useful way!

 

[pc1966]

Could you not if opening up the hole, drill slightly to one side of it encompassing the existing hole? Especially if it's only 10mm and you drill a 25mm.

That is a good idea. The existing holes are not centred on the beam, they are about 1/3 down, do going below would be a good idea. The beams are 150 years old so bigger than modern cases, but condition along whole length unknown.

 

[Vortigern]

Interested in whether the existing holes comply with Part P, as in what section of the span of the joist are they in? I was just wondering whether the question was a non-starter due to not being in the right place anyway???

 

[loz2754]

Interested in whether the existing holes comply with Part P, as in what section of the span of the joist are they in? I was just wondering whether the question was a non-starter due to not being in the right place anyway???

Good point, except it is not referred to in part P. Part A relates to maintaining the structure of the building.

 

[Dustydazzler]

If the existing holes are very small say 10-15mm but in the place where I need to pull in new cable then I would pull out the old cables , make them bigger and run in my new cables. for the 10mm shower cable I might be tempted to run in its own hole

 

[pc1966]

Interested in whether the existing holes comply with Part P, as in what section of the span of the joist are they in? I was just wondering whether the question was a non-starter due to not being in the right place anyway???

They are not where they should be in the span, at least following UK guidance (the USA has a different figure for distance from end-support as 3*depth, not 0.25*span), but they have been like that since it was rewired in the 90s. But the beams are big and the holes, even if made larger, are a lot less than the max allowed.

My feeling is the least-worst option is opening one hole enough for the additional cable, but going to look at just how wide it would be needed to get the various T&E through without drama (can try-drill some scrap wood to see).