Do i need ventilation in replacement tile roof

[marsaday]

I am reroofing a pitch on a house i rent out.  I am taking off the old tiles and will sell them.  I am using spanish slate to replace.

 

So i will fit 50mm insulation between the rafter and leave a 25mm air gap.  Then fit tyvek supro breathing membrane, then lats, then tiles.

 

Do i need to put some ventilation in ?  If so do i do it across (from side to side), or from ridge to eaves.  I am just not sure what to do.  I will be reusing the current ridge tiles, but could buy 2 breathing ones, if this type of ventilation is needed.

 

 

[Russell griffiths]

Why are you fitting insulation between the rafters ?

have you got a room in the roof. 

[Cpd]

2 hours ago, Russell griffiths said:

have you got a room in the roof. 

Is so 50mm between the rafters will be pitiful If that’s all there is going to be. 

When I did my roof I put 4 vents at the top where there was a big uninsulated area (Triangle ) below the ridge that all the rafters were open to (50mm air gap below the sarking and above the 190mm of insulation) and the vents worked well as I got lots of air movement up between the rafters in the gap. I also had soffit vents at the base of each rafter to help get air in. All in all it’s worked great and the roof is bone dry. 

 

[marsaday]

Why ?  Because there is a room in there.

 

I have added 50mm 6 yrs ago on the underside of the old lath/plster ceiling.  So now i'm doing the roof i want to add 50mm more.  It would be more, but the rafter depth is 75mm, so i need to leave a 25mm air gap.  Not ideal i know, but if i get some air vents in then it should be ok.

 

Someone on facebook said to put 2 slate vents at the bottom and 2 at the top.  Seems pretty straight forward to me.