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Venting around a skylight

2x10 rafters, sheathing is already on (remodel)

How do I vent skylights? I don't want to drill or notch the rafters too much (especially at the top obviously).

I will be adding 2x4s perpendicular and underneath the rafters and using 14" of cellulose.

Asked by Anonymous
Posted Aug 9, 2010 3:46 PM ET
Edited Aug 9, 2010 4:37 PM ET


5 Answers

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Good that you started another thread. Piggybacking on another, unrelated thread just makes everything unnecessarily confusing.

Is this a cathedral ceiling/roof? If so, then you need continuous vent channels from soffit to ridge that can withstand the pressure of the blown cellulose, and you have to create openings, either in the skylight headers or in the adjacent rafters. Notching or drilling the top edges of the rafters should not be a problem. Just make sure there is a continuous vent path in each rafter bay.

Answered by Riversong
Posted Aug 9, 2010 4:52 PM ET


Depends on the skylight and location. I'm certainly no authority on the subject; just providing the benefit (however dubious) of my Texas experience. Contact the skylight manufacturer for the best advice.
I've never had an issue with quality glass skylight installations when I've followed directions, but that might be more good luck than good management. For most, there should be no need for any additional venting other than that included in the manufacture.
The acrylic domed skylights generally have a gutter in the extruded aluminum frame with drain holes leading to the outside. Condensation is designed to travel down the dome into the gutter and drain to the outside. Two main leaking issues that I have experienced are broken or cracked domes (easy to find) and aged or insufficient sealing on the base housing/sheathing interface. (Water pools on the upper side, seeps under the shingles and creeps in under the base housing.) However, I have sometimes seen "leaking" as a result of an overzealous corner weld in the factory that fried the internal gutter edge combined with a low pitched roof. Water that should flow down the skylight gutter and exit to the outside, instead gets to the defective corner aluminum miter joint and flows inside rather than out..
On one home with two skylights, the homeowner with good intentions to stop two leaking skylights, slathered mastic over the entire skylight frames, covered the drain holes and effectively pushed all the condensation into the roof.
I hope this helps.

Answered by Tony
Posted Aug 9, 2010 6:13 PM ET


The skylight manufacturers will have nothing to say about roof venting, which is what Will is asking about.

I'm a stickler on adequate, complete and continuous roof venting. I will never build a "hot roof". I drill or notch any headers (or adjoining rafters) that would block the ventilation flow in each rafter bay, such as around skylights (which I tend to avoid) and chimneys (which I tend to build) as well as valleys and hips (which I also tend to avoid because they make ventilation very challenging).

Answered by Riversong
Posted Aug 9, 2010 9:19 PM ET


When I've installed the typical 22-1/2" wide curb mounted skylights I've done what Robert is describing and drilled holes or cut notches thru the tops of the rafters, above and below the skylights, to allow air communication into the adjacent rafter bays. Not perfect, but hopefully adequate. It helps if you have pre-planned this and allowed plenty of rafter depth, so that removing some wood is not a structural problem. The inspector here knows this and will actually redline it on the plans.

Answered by David Meiland
Posted Aug 10, 2010 1:15 AM ET


The question concerns ventilation between the roof sheathing and the insulation below; it has nothing to do with condensation on skylights. Skylights interrupt venting channels, so some method is usually developed to allow air to flow around skylights.

Obviously, there are many locations on most roofs where it's very difficult to get a straight shot from the soffit to the ridge. In addition to skylights, problem areas include valleys, hips, chimneys, and dormers.

Considering the fact that most new homes have chopped-up roofs with many such features, builders have several options:
1. Argue with the architect who designed the roof. (Good luck.)
2. Build a hot (unventilated) roof with closed-cell spray polyurethane foam or rigid foam on top of the roof sheathing.
3. Install 2x4 purlins (cross hatching) between the top of the rafters and the roof sheathing.

Of course, none of these options apply to the original question, which concerns a skylight being retrofit in an existing roof.

Answered by Martin Holladay
Posted Aug 10, 2010 6:39 AM ET

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