Skylight Roof Ventilation Details
When framing for skylights, it’s often difficult to properly vent the cavities created between each rafter since skylights require double horizontal top and bottom, 2x blocking between each rafter. Wood blocking will restrict the flow of air from the bottom of the eave to the top of the ridge. This will create a dead air space which is not recommended for a vaulted roof. As most experienced framers know, dead air spaces can trap moisture which in turn can cause mold build up. This can cause wood to decay which can ultimately lead to leaks and costly repairs.
Recently I built a greenhouse with a north facing insulated vaulted roof where (2) operable 4 x 4 skylights were installed. I asked the building inspector how he recommended venting the cavities below and above the skylight frame. Goes to show you… some inspectors just don’t know all the tricks.
He told me to drill holes into the sides of each adjacent rafter so that air can pass from the blocked cavity to the adjoining cavity which ran clear all the way to the ridge.
Provide an Air Path – Eaves to Ridge!
Although this suggestion is do-able and would pass an inspection, I just felt a little uncertain about drilling multiple holes into the adjacent rafters. This would no doubt weaken the rafter and only a certain size hole is permitted. There’s a ratio to adhere to diameter size to rafter dimensions.I did an online search on how to effectively vent blocked cavities for skylight framing.
After viewing a few Google searches I found a site that provided an excellent solution. This method certainly made a lot more sense as opposed to drilling holes and it also made for a cleaner application
It essentially calls for notching the top edge of the rafter, with a 6″ wide x 1″ deep slot at the top and bottom of the skylight on the rafter, directly below the lower blocking and directly after the top blocking of the skylight frame. See illustration>
After studying and observing the locations of moisture, mold, ice dams, condensation stains, and other clues from various skylight roof applications, notching an air passage is one of the solutions mentioned. There’s nothing wrong with 1″ holes and if its suit you. do both!
Skylight Roof Frame detail
Most skylight roof frame details require double 2x rafter on each vertical side of the skylight. When both rafters are nailed together, creating a notch is a lot harder than cutting the notch from a single rafter. It”s best to cut one, and then mark the notch outline onto the other rafter, before nailing the rafters together.
Roof Ventilation for Vaulted Ceilings
These interior vaulted ceilings and roofs, with or without skylights, as mentioned earlier, are more critical and require a continuous vent cavity between each rafter for continuous air flow. A continuous vented air flow barrier extends the life of any wood roof structure.
If you”re looking for more…. begin by reading roof ventilation information ……lots of good info…
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