joist drilling for air flow


5 Apr 2011
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United Kingdom
Hi guys, I'm installing celotex between rafters (and below, but that's later). I have already drilled the top of the normal noggins under gable straps with 16mm holes to allow airflow but the small space between the gable wall and the last truss is a bit tricker as there are also other joists at 90 degrees to the last rafter going through the wall that I guess support the last bit of the roof and fascia beyond the gable wall.

My question is, is it ok for me to stick a couple of 16mm holes in the top half of these joists (joists 95mm) and then cover with celotex so I maintain a 50mm gap between the sarking felt and the topmost side of the celotex in this small 150mm void? If not and I need to reinforce somehow what would be the easiest/best way considering the very confined space and only partial access to these joists?

I know 16mm holes not quite enough for 25mm airflow but better than nothing and I think there is some airflow through the wall around these joists also (it def isn't airtight)

Thanks for any suggestions!
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Noggings are not normal between rafters and if you do then you use noggins 1" smaller than your rafters to get your air flow.
Yeah the noggins are 20mm smaller and that is why I drilled a few extra 16mm and also to cope with some saggy sarking felt reducing that gap smaller. These noggins were put in underneath gable straps as per Structural Engineer drawings.

But what I'm talking about aren't noggins, they are joists that are part of the frame that goes through the gable wall to support the roof overhang over the gable wall. As a result they are the same size as the rafters (no gap above)

I've indicated in red where I plan to drill both the noggins and also these joists that go through the wall

any advice on if this would be safe to do or not and any alternative suggestions would be great! (e.g. could I get some straps to reinforce the joists afterwards)

Also there is a problem around the chimney, where the truss that would otherwise go through the middle of the chimney has been trimmed out and web members removed and connected to the 2 trusses either side - again with full size pieces of timber which will block the airflow from eaves to ridge.
Again I plan to drill through the top 1/4 of the joists/timber to allow the airflow to go around the chimney.

Would this need reinforcing and if so how should I do it (ideally without adding too much underneath the rafters to allow more celotex below)

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