Cold deck, noggin size for ventilation

[Wils81]

Building a garden room, cold deck, span is just over 4.2m, used 8x2 joists. Roof will dual slope sideways, from 40mm in the centre to zero both sides,,should I be putting in 1 or 2 rows of noggins and am I right in saying they should be slimmer than the joists to aid airflow, so 6x2 or 7x2 for noggins. TIA

 

[noseall]

Building a garden room, cold deck, span is just over 4.2m, used 8x2 joists. Roof will dual slope sideways, from 40mm in the centre to zero both sides,,should I be putting in 1 or 2 rows of noggins and am I right in saying they should be slimmer than the joists to aid airflow, so 6x2 or 7x2 for noggins. TIA

If I'm reading it right - just add varying thickening strips to the tops of the joists, to facilitate falls.

 

[Wils81]

Apologies, the joists run side to side because of the shape of the room and span (longer front to back). Question is more about if noggins need to be smaller so as not to block through ventilation. Firrings will raise middle section by 40mm as falling from centre to both sides, so if only needing 1 set of noggins at mid span, air has a through route but if needing 2 sets of noggins at thirds then the firrings are lower and not sure there is sufficient room for flow.

 

[Dereekoo]

1 set at mid span and should extend to at least 3/4 depth of joist as per TRADA guidance

 

[Wils81]

If I'm reading it right - just add varying thickening strips to the tops of the joists, to facilitate falls.

However, your post has intrigued me, so if I want to slope front to back and perpendicular to the joists, it would be fine fixing wise? Length would be 6m with overhangs, so if I'm right I could get a firring that length and 100mm to zero and cut our between the joists once fixed, so as correct size firring is perched on joist?

 

[noseall]

However, your post has intrigued me, so if I want to slope front to back and perpendicular to the joists, it would be fine fixing wise? Length would be 6m with overhangs, so if I'm right I could get a firring that length and 100mm to zero and cut our between the joists once fixed, so as correct size firring is perched on joist?

If we want to slope a joist/roof front to back - we add a furring, like everyone else.

If we wanted to slope front to back and side to side - we'd say add a batten to the centre joist and slightly lesser thicknesses to zero on those working away from the centre - prior to adding the furring.

 

[Wils81]

The joists are running side to side not front to back because of the span, so I'm talking about to slope from front to back and perpendicular to the joists. Wasn't sure if affectively fixing a furring strip every 400mm onto perpendicular joist would be sufficient fixing wise

 

[Wils81]

In fact disregard running front to back, furring would be too big.

So I'll run one row of noggins mid span, 1 inch thinner at 7x2 to aid air flow. One last question, would I need to use t&g for roof boards or will my 400mm centre joists have enough fixing points to use straight edge boards