Flat garage roof - ventilation and soffit - help please?

[TonyRPH]

My garage is 22' x 12'.

Three sides are sealed from the elements, however there is an overhang running the length of one side, which ventilates into the garage. (see pic)

This is the side that vents out.



This is another view, showing the width between beams.



And finally, a view from inside with dimensions. (It looks to be bricked up, but it isn't sealed - there is actually quite a gap there.)



Now to the questions!

Ideally, I want to fit a ceiling, along with some insulation.

1) Can I seal this side that is vented, or do I need to fit a ventilated soffit along the length?

2) Do I need to provide any ventilation on the inside, if I fit ceiling boards?

I'm acutely aware of condensation - I previously had some ceiling in, but the insulation I used was far to thick - so it was packed between the ceiling and underside of the roof. I have removed this long ago, fortunately no damage was done.

So can anyone advise on the correct course I should take here?

TIA.

[Skyedriver]

Waiting for an answer to your post as I have a similar situation
Vented at one side not at the other, limited headroom too.
Considering celotec/kingspan resting on timber lathes fixed to the side of the joists wit a 50mm air gap but there is only free ventilation at one side.....

[TonyRPH]

I'm beginning to think that I would need to fit a continuous length of grill / soffit (don't know the proper name for it) the length of the roof outside.

And then just install ceiling boards and insulation, being sure to leave at least a clear 50mm gap between insulation material and underside of roof.

That way it should get adequate ventilation from one side to the other - but I'm hoping somebody knowledgeable will comment.

[roofer]

You will need crossflow to carry moist air away. Warm air will pass through the plasterboard and condensate on the underside of the ply otherwise. Google warm roof construction, heaps of info available.

[TonyRPH]

Thanks.

Found some most useful info here

My only option is to leave it as is at the moment, as I cannot see any easy way of providing cross ventilation as there is a gutter and fascia board on the other side and no provision for a soffit on the gutter side.

Edited by TonyRPH on Sunday 14th August 12:48

[roofer]

Its fairly simple to put vent grilles into a fascia, similiar principle to brick vent for floor venting.

[TonyRPH]

Some further investigation revels that there appears to be another board behind the fascia (I'll let the pictures illustrate what I mean).

Underside of gutter / fascia board.



The roof felt overlaps the other board and drops into the gutter like this:



And lifting that overlapping felt reveals this:



So, could I safely put some vents into that piece, and then allow it to vent behind the felt?

How big would the vent need to be?

Thanks for your help so far.

[roofer]

The timber at the top of the fascia board is the drip batten (usually 2x1.) Its used to nail the drips before they are pulled up and to push the drip into the gutterline. If you take the gutter off, you will be able to core a hole through the fascia and use a circuar soffit vent, 50mm should suffice for each rafter run. I would drill through from the rear if possible to get everything central. Make sure that fascia is actually fixed to rafter ends as sometimes they will run brick right up and fix fascia to that, althoug doubtful going on internal pics.

[TonyRPH]

Thanks - your advice is much appreciated.

[roofer]

[Skyedriver]

My prob is a little different as there is ventilation at one side but the other side is against the side of the house so whilst I can get loads of ventilation in one side, the other side could only get ventilation between joist ends but not to the outside world.....

[roofer]

Any ventilation is better than none, or use inverted insulation on roof surface if you have the space.

[B17NNS]

Google 'warm flat roof' and 'cold flat roof'.

[blackcab]

soffit vents will help to stop condensation and stop the deck and joists rotting as quickyou can always just use air bricks below the drip edge for this.