Working on an extension with a pitched roof and open to the ridge. Regarding the air gap between the wind barrier and sheathing, there seem to be several schools of thought. The options I'm considering are:
1. 25 mm air gap with openings at the eaves and at the ridge.
2. 25 mm air gap with openings only at the eaves, ventilation above the ridge.
3. 25 mm air gap with openings only at the eaves, ventilation above the ridge - and mold stoppers in each roof compartment.
4. 45 mm air gap with openings only at the eaves, ventilation above the ridge.
Which option is most suitable? The building will be located in Västra Götaland about 70 km from the coast, meaning not very humid air.
To keep the roof height and complexity down, I would prefer option 2. Would this provide adequate ventilation?
As an old builder, I would definitely go with option 1, it's always safer with ridge ventilation. It's not that complicated to fix, there are products for it for all types of roofs. Option 2 probably works... but not with the same margins at all.
Thank you for your answers! I've imagined that ridge ventilation is both complicated and risky to get right if you're not experienced. That's why I've been considering the mold stopper, which seems to be a simpler solution.
The roof will be tongue-and-groove boarding, felt, battens, and corrugated sheet metal. @John Smith, how would you solve the ridge ventilation on such a roof?
I have found the following schematic drawing on how ridge ventilation can be solved. However, I need a more detailed description before I dare to try something similar myself. For example, width of the air gap at the ridge board, how much the ridge plate must overlap, what ridge filler consists of and how it is mounted, what ridge tape is and how these are attached, etc...
I chose option 2 for my extension. Cellulose insulation 340mm, board, 28mm air gap from eave to eave, raw wood decking and standing seam metal roof. 7-degree pitch on the roof. I haven't encountered any problems so far.
I did option 2 on my extension. Cellulose insulation 340mm, board, 28mm air gap from eave to eave, wooden boards, and folded sheet metal roof. 7-degree pitch on the roof. So far, I haven't encountered any problems.
With such a flat roof, ridge ventilation might not be as critical. The roof I'm building has a 22-degree pitch, so it's somewhat in between steep and flat.
I've made a suggestion on how I envision it could be done:
To use both belt and suspenders, I thought you could place a piece of cardboard over the ridge and on top of the battens, as the image shows. This way, the air can circulate out underneath it.
What do you think about this - have I forgotten anything or is it unnecessarily complicated?
The roofing felt is likely to hinder air flow somewhat under the plywood, and over time it will likely "sag" between the battens and further reduce air flow... so that's probably not so good...
Make sure the sheets rise properly (but still with a few cm gap) so you get both good air circulation and no risk of water going backward into the roof. And if a few snowflakes come in, there's nothing to worry about, those drops dry away quickly
Another thing - I haven't drawn a ridge board, should I have one? Isn't the ridge board just there to have something to screw the verge boards into, or does it have another function?
Hello again,
I just found a study that suggests that air gaps are outdated since the original purpose was to keep the roof cold to avoid icicle formation. Thanks to today's well-insulated roofs, this is no longer an issue. An air gap can be beneficial for drying out moisture that has entered the roof if it is diffusion-tight. However, the air gap can also introduce moisture, which is negative. Therefore, there is apparently vapor-permeable roofing membrane that allows moisture to escape even if there is no air gap. Without an air gap, the building height, complexity, and risk of moisture entering ventilation openings would be reduced. Does anyone have experience with constructing roofs in this way?
The ridge board is probably more useful on older tiled roofs; in your case with a metal roof, it should work without it.
Regarding the existence of the air gap or not... again, as an experienced builder, I would never ever skip it; it can never do any harm, but if there is a mistake in the construction, it is a lifesaver...
The study you refer to assumes perfect conditions, no leaks, 100% correctly executed roofing work... sure, but if there is a mistake somewhere, the air gap is there to forgive those mistakes... the study you link to I see as a theoretical paper product with only theoretical value, but we live in reality with all its flaws and advantages
