OSB sarking & ventilation above breather membrane ?

[sean1933]

Hi all. 

 

I am just about to start boarding out our large garage/workshop roof (inc room in void above) with 11mm OSB sarking  (Required for racking strength) and am struggling with a couple of details that I hope someone can help with.

 

Our buildup is::

 

Natural slate

Batten

Counterbatten

Protect VP400 LR membrane

11mm OSB

195mm rafters fullfill rafter roll

25mm celotex (might leave this off as it is only a garage after all!)

VCL

PB

 

Firstly, as the OSB isnt T&G'd, does it need to be 3mm gapped on all 4 sides or should it be butted up?

 

Do i need a watertight nail tape under the counterbattens or is this overkill in the real world?

 

Ventilation... now this is where I am struggling. Accepting no need for any ventilation below sarking as fullfill - the Q I have is I keep reading about the arguments for and against ventilation above the sarking/membrane - suggestion being a lack of ventilation gives a warmer roof. My question is, is the counterbatten space and natural gaps created by natural slate enough for escape of any moisture, or do I need to ventilate at ridge and eaves? My setup is pretty much as @ProDave 's but he used an over eaves vent.

 

These are the 2 conflicts: Diagram below for example doesnt show any over eaves ventilation, so only eaves vent is spaces between tiles...  help!

 

but @ProDave 's layout does:

 

 

[ProDave]

That picture was my sun room roof which is clad in box profile steel hence the larger and more wide space "battens" to support the roof.

 

The other difference mine is a full blown "warm roof"  I used 100mm wood fibre board as the sarking, which insulates above the rafters. That would be equivalent to you putting your 25mm celotex above the OSB then the membrane then the counter battens etc.

 

I have the over fascia vents and (on all the tiled roofs) a vented dry ridge system.

 

I really don't think omitting the vents would make much difference to heat loss.

[sean1933]

thanks Dave - until I had a lead time issue I was going with 25mm wood fibre but jumped to OSB to save some time and money seeing as its only the garage. Will probably use wood fire on the house though.

 

I think part of my question is around the eaves vents being a method of water escape, where as without them it has to fit though small gaps as will be resting on the tilt fillet or fascia board.

 

It is a big roof (6m to ridge, 10.6m wide) so want to make sure it is done right. Do you think I should just stick the over eaves vents in like yours, a dry ridge system and be done with it? 

It's peanuts in terms of cost but I figured if not ventilating was more energy efficient and possible due to the roof buildup, then why bother ventilating...

 

 

Edited July 2, 2020 by sean1933

[ProDave]

25mm wood fibre does not have much strength. I would not want to use it on it's own as sarking. If you want to use it as extra insulation, put OSB down first.

[sean1933]

Quick couple of Q's  @ProDave  if you dont mind.

 

-Did you tape under the battens to seal any moisture risk from nail penetrations? Cant decide if this is overkill...

-What size eave vent did you use on your main roof 10mm or 25mm? My reading now suggests if it is to be ventilated then a duo roof needs at least 25mm at eaves and 5mm at ridge, but I might be getting confused as sleep is somewhat of a luxury these days so brain is struggling!

-11mm square edged OSB sarking - would you just nail it or is screwing necessary? And should it be gapped or tight?

 

thanks in advance.

Edited July 4, 2020 by sean1933

[ProDave]

No taping of nail holes.  The theory is it is not a problem, unless you pull a nail out and then leave an open hole.

 

The OV10 vents I used are supposed to give a ventilation area equivalent to a continuous 10mm gap. the vents are somewhat thicker than 10mm but I don't know by how much.

 

All my OSB is just nailed. I used a 3mm gap because like you I was not sure.