ᐅ Ventilated or Non-Ventilated Insulated Gable Roof on a Bungalow

Hello forum!

I have a question about the roof construction of a fully sheathed pitched roof on a small weekend bungalow. The roof pitch is quite shallow at 7° (7 degrees), and for durability, it is planned to be covered with bitumen membranes and an EPDM roofing membrane on top. Since the EPDM membrane is completely diffusion-tight, I am concerned about condensation forming due to temperature differences in summer and winter, as I don’t want mold growth in the insulation or rot developing in the timber framing later on. The house will be heated in winter with a central heating system and cooled in the peak summer heat by air conditioning, so it will be fully usable year-round. The house is exposed to direct sunlight in summer with no shading from trees.

It would be great if roofing professionals could share their opinions or experience on whether the proposed construction is appropriate and durable.
The layering of the roof components would be as follows:

• EPDM roofing membrane (fully adhered over the bitumen membrane)
• Bitumen membrane (fully adhered or welded onto the OSB boards using cold adhesive)
• OSB/3 boards with tongue-and-groove connection
• Breathable underlay membrane
• Roof rafters
• Three purlins per roof side (foot, middle, ridge purlins)
• Insulation between rafters
• Vapor barrier membrane
• Drywall boards
• Ceiling plaster
• Paint finish

Is this setup acceptable, or should I add battens on top of the rafters below the OSB boards (that is, above the breathable underlay) to create a ventilation gap between the underlay and the OSB boards? At the eaves, a fine-mesh soffit vent would prevent animals or pests from entering, and at the ridge, a continuously ventilated ridge flashing would allow air circulation.

I have already read that ventilation is essential under diffusion-tight roofing to prevent mold and allow condensation to escape, while others say that ventilation would be unnecessary or even counterproductive in this case. So, what is the correct approach?

I hope the roofing experts here can help.

Thanks in advance!

Hi,

welcome to the forum! With a 7° roof pitch and a vapor-tight EPDM membrane, you definitely need a ventilation gap beneath the OSB boards. Fully adhered bitumen membrane plus EPDM without air circulation will inevitably cause condensation, especially with air conditioning in summer and warm, humid indoor air in winter. A simple batten system installed over the rafters is sufficient, make sure to properly ventilate the eaves and ridge so the air can flow through. Without this, water will eventually penetrate into the insulation and dampen the rafters...

Good luck!

NoName77 schrieb:

I hope the roofing experts here can help me out.

I’m not a roofer! But your roof structure WITHOUT ventilation is called a self-composting (flat) roof. So, see @Nauer ==> would probably work with 5cm (2 inches) battens.

Thank you for your replies!
Is there a minimum requirement for the size of the ventilation gap for this roof pitch (e.g., from/to XX° slope), or is the 5cm (2 inches) just an estimate?

Regarding the construction of the ventilation cavity, I was thinking, as Nauer already mentioned, of battens over the rafters. Beneath the battens would be the underlay membrane, and the OSB boards would be fixed directly onto the battens. Would that be acceptable?
At the air intake at the eaves, a fine-mesh eaves grille would be installed, and at the ridge, a continuous ventilated ridge flashing (also with a mesh to prevent small animals/insects) would be fitted.