Drilling joists

[Pat H]

Doing a loft conversion and wondering about joist drilling.
OSG states to drill in the middle of the joist 0.25 to 0.4 of joist length.
I have 6" joists but the bottom 4" will be filled with 100mm celotex so If I drill at half way my cabling will be in the way of the insulation. It makes more sense to drill slightly higher to clear the insulation and leave the cabling in the air gap above. This would leave me just about 50mm from the top of the joist.

 

[Des 56]

I would consult a building inspector before going ahead drilling joists in attics
I would also suspect permission would be firmly rejected

 

[Pat H]

The only thing the joists hold up is the dormer flat roof. But I'll speak to the BC as they are onsite.
I assume folks normally drill as per OSG and the insulation guys chop up their insulation to fit (thereby reducing the building reg requirements for the insulation....)

 

[Leesparkykent]

In a loft conversion you can normally get away without drilling joists as you have dwarf walls or cupboard space around the outside which you can utilise for cable runs or where the ceiling is dropped from the apex.

 

[Pat H]

This is in the dormer flat roof. Fire alarm and light fittings. Lots of joists and noggins. No practical access upwards just across.

 

[Leesparkykent]

This is in the dormer flat roof. Fire alarm and light fittings. Lots of joists and noggins. No practical access upwards just across.

What about using the 150mm zone where wall meets the ceiling and then clipping the cables in to position along the joist above where the insulation is to sit?

 

[HandySparks]

The only thing the joists hold up is the dormer flat roof.

I guess they'd be rafters then, not joists.

 

[Pat H]

The top section of the dormer walls is doubled up 6" joists and on top of that will go 1" celotex insulation.
I can't bury or sink the cable into that joist (and be within 150mm) It would have to be surface mounted and then the 1" insulation board would have to be rebated on the rear to allow the cables to sit in it. That would reduce the insulation effectiveness at a key thermal bridging location.
I can discuss with BC but it seems the OSG doesn't look at the bigger picture. I can see the logic on a 4" ceiling joist with loading on it. But a 6" joist with a flat roof on it.

 

[Pat H]

Ah that's right. Rafters. No mention of that in OSG so all good

 

[davesparks]

Ah that's right. Rafters. No mention of that in OSG so all good

Wrong, you can't drill rafters at all. The information in the osg is just an extract from the building regulations. You need to look in the appropriate building regs document for this, I have a feeling it is part A, but you'll have to check

 

[Pat H]

Just glanced through Part A but couldn't see any thing relevant. Have found a number of Building control documents on line and they state the joist/rafter needs to be calculated re any reduction in strength. But generally it looks like stick to the centre.
I guess that element over rides the insulation limitations it imposes.

 

[buzzlightyear]

Are the the new joist above the old raftes and a space in between .
when new joist go in they will be on a support walls ,building regs guide annex 1.1 on structures

 

[Pat H]

At floor level in the new space the new 6" joists are more than 2" above the old rafters so no issues with cabling.
Its the new dormer ceiling.
The original sloping roof of 4" rafters has had 2* extra added to make 6" but I won't drill anything there as I can access from below or above.
Its only the new dormer roof. Steel at one end so no access and doubled up 6" joists the other that form the top of the dormer wall so no access there. I have no option but to go across.

 

[davesparks]

Are the the new joist above the old raftes and a space in between .
when new joist go in they will be on a support walls ,building regs guide annex 1.1 on structures

Joists above rafters? What on earth are you talking about?

 

[buzzlightyear]

sorry davesparks what was I saying, was using my lg phone last night small writing not my pc.