Joist span and where to drill

[very_pointy_ears]

Hi,

I'm aware that if you're going to drill through a joist to feed cables though, you've got to be between 25% and 40% along, no closer than 2 inches from the edge and no closer than 3 diameters from the next hole.

However, where exactly do you measure the 25 - 40% from?

I've got some joists that go across to a supporting wall which is the dividing wall between kitchen and diner. The joists actually overshoot the supporting wall by a foot or two and end up running alongside the joists in the next room.

Do I measure 25% along the supporting span (ie from the supporting wall), or do I measure from the end of the piece of wood? The extra bit that overshoots isn't really doing anything and so I wondered whether it counted.

 

[reggie perrin]

Where there's a drill - there's a way.

 

[baz1968]

http://www.eaststaffsbc.gov.uk/downloads/building_control/guides/07leafletDomesticTimberFloors.doc

 

[ebee]

25 to 40% of span from each end.
The span is measured from support to support (ie the wall).
The 3 diameters apart is from hole centres, therefore leaving 2 dia of meat between holes.

 

[ColJack]

general rule of thumb... drill down the center of the room and keep the holes 6 inches apart and in the center of the joist ( vertically ).. can't go wrong then..

 

[very_pointy_ears]

Where there's a drill - there's a way.

 

[very_pointy_ears]

The span is measured from support to support (ie the wall).

Thanks for clearing that up

The 3 diameters apart is from hole centres, therefore leaving 2 dia of meat between holes.

Ah, I'd been working from the edges of the holes. Still, I guess that's erring on the safer side as it gives them 3 diameters of meat rather than two. Better that than the other way round!

 

[very_pointy_ears]

general rule of thumb... drill down the center of the room and keep the holes 6 inches apart and in the center of the joist ( vertically ).. can't go wrong then..

I don't follow what you mean - surely the centre is not within the 25% - 40% range?

Also, what happens at light fittings hanging from the ceiling in the middle of the room? I've been assuming that I use fittings that have the cable entering near the edge of the backing plate and then allow this to overlap off the edge of the joist, allowing the cable to run down the side of the joist and into the fitting. However, I've also heard of people drilling a smallish hole diagonally upwards into the bottom of the joist and out of the side to provide a path for the cable. What is the preferred method?

 

[very_pointy_ears]

http://www.eaststaffsbc.gov.uk/downloads/building_control/guides/07leafletDomesticTimberFloors.doc

Thanks - I've saved that file - good info

 

[GaryMo]

general rule of thumb... drill down the center of the room and keep the holes 6 inches apart and in the center of the joist ( vertically ).. can't go wrong then..

Surely centre of the room is 50% and out of the 25-40% zone?

 

[GaryMo]

I've been assuming that I use fittings that have the cable entering near the edge of the backing plate and then allow this to overlap off the edge of the joist, allowing the cable to run down the side of the joist and into the fitting. However, I've also heard of people drilling a smallish hole diagonally upwards into the bottom of the joist and out of the side to provide a path for the cable. What is the preferred method?

Personally I clip down the joist vertically at the light fitting position and don't drill a diagonal hole.
Sometimes when first fixing I install a piece of 3x2 between two joists to get dead centre. I'll drill a 20mm hole with a flat bit for the cable route.

 

[equitum]

general rule of thumb... drill down the center of the room and keep the holes 6 inches apart and in the center of the joist ( vertically ).. can't go wrong then..

Surely centre of the room is 50% and out of the 25-40% zone?

The 25-40% area is where to drill, not where to avoid. By making holes in the very centre (horizontaly) of the joist is the WORST place to drill as its the weekest point of the joist !!

 

[ColJack]

it's been a long time since I did any domestic work... I'm mainly industrial..

and the coffee hadn't kicked in yet..

 

[Welshy]

I've been assuming that I use fittings that have the cable entering near the edge of the backing plate and then allow this to overlap off the edge of the joist, allowing the cable to run down the side of the joist and into the fitting. However, I've also heard of people drilling a smallish hole diagonally upwards into the bottom of the joist and out of the side to provide a path for the cable. What is the preferred method?

Personally I clip down the joist vertically at the light fitting position and don't drill a diagonal hole.
Sometimes when first fixing I install a piece of 3x2 between two joists to get dead centre. I'll drill a 20mm hole with a flat bit for the cable route.

Ye olde faithful Noggin.

 

[very_pointy_ears]

Personally I clip down the joist vertically at the light fitting position and don't drill a diagonal hole.
Sometimes when first fixing I install a piece of 3x2 between two joists to get dead centre. I'll drill a 20mm hole with a flat bit for the cable route.

I guess if I want a light fitting to line up with a joist I could fix a bit of 3x2 alongside it and drill into that. Thanks.