QUESTION: Hello Bruce
I know that the conventional wisdom on attic ventilation is that gable vents should not be combined with a ridge/soffit vent system.
But, is it OK to combine gable vents with metal dormer vents that are placed near the ridge of the roof?
And, is it OK to add soffit vents to the mix of gable and dormer vents?
The structure in question is about 900 sq feet. It is a gabled structure, with gable vents, and also dormer vents, two dormers on one side near the top of the roof, and one dormer on the other. Currently, there are no soffit vents. The roof is sloped, and does not have dormer structures, i.e., no valleys.
Thank you for your help.
Other venting methods combined with opposing gable vents have been noted to result in a 'short circuit' of air flow which can reduce the overall effectiveness of the gable vents. If the case is that other types of venting are present and have been in place for several years and there is no indication of poor performance; then it is reasonable to assume that it is okay as is. Gable vents or ridge vents combined with soffit vents are systems that are best not intermixed, but don't predictably result in performance concerns when they are. Consider the rationale for the desire to add venting as it is likely that the two gable vents provide sufficient ventilation required by Building Code.
Bruce Ryan II, RRO
Professional Roof Consultants, Inc.
---------- FOLLOW-UP ----------
Thank you so much for your answer to my question.
You remarked that if the current ventilation is working properly, there was not any reason to to think it would not continue to do so, despite the fact that the current ventilation is not consistent with conventional wisdom.
I do no know wheter or not this roof has mold and if the ventilation has been working correctly. I do not own the building. It is owned by a union motion picture local in Los Angeles of which I am on the Board of Directors.
I want to make sure that I understand your answer to my quesion:
1)Are you saying that the metal dormer vents (the ones that are commonly seen on hip roofs) that are currently near the ridge of the roof operate the same way as a ridge line roof, and therefore should not be combined with gable vents?
2)Currently, there are no soffit vents. The roofer proposes to install 4 soffit vents.(I have no idea of what 4 soffit vents mean. I am know about continuos soffit vents) Do you think the addition of soffit vents could bring an adverse effect to the pre-existing combination of gable vents mixed with metal dormer vents that are near the ridge?
3) Also, as a general question, not pertaining to this specific roof, I frequently see soffit vents combined with gable vents. Do you think gable vents work more efficently without soffit vents?
Bye the way, I've written to you about this matter because I wrote to you in 2009 about an attic ventilation problem I was having with a house I own in NY. It was raining inside my roof - and it wasn't raining outside. The procedure you recommended to me to get to the root of the trouble was very helpful. It turned out that the builder misinstalled the ridge line vent. He cut the opeing for the roof way more narrow than what was stated in the ridge line manufacturers instructions. I called the builder. He installed another ridge line vent. The condensation stopped. But, the builder would not take responsibility for the mold, so I sued him. I won a settlement. I should have written you about this long ago. Please accept this belated update and thank you.
I am please to hear that the advice we offered previously had a positive result!
To your follow up questions:
1) The dormer vents will have a tendency to affect the air flow of two opposite gable vents. Should they not be combined? It is generally not good practice but if there are no performance concerns then it is fine. You stated that you have no knowledge of the condition, so you will have to rely on your contractor to assess the condition and make the case to you as to why they want to add the vents.
2) My bet on the four soffit vents to be installed are perhaps more dormer vents positioned low on the roof to promote passive air movement within the attic propelled by the gravity of hot air rising out of the ridge dormer vents.
3) Gable vents are fine by themselves but they generally do not perform well without air movement outside the structure. They can usually successfully accomplish the task of attic venting, which is to moderate the propensity for moisture build up, but with lower efficiency comes increased weather protection as they do not require roof penetrations, which also adds to the exterior appearance of the structure. Soffit and ridge vents perform best when combined, but soffit vents can work well with gable vents, but can also result in the same short circuit of air flow as ridge venting. This again is only a problem when there is a problem that is specific to the structure or use.
I hope that this resolves your questions and did not result in more confusion. In summary, I would tell the contractor to leave the venting as is unless they have performed a calculation and can show that the ratio does not meet code requirements or they are changing the color or type of roof system.
Please also recognize I do not have specific information as to the configuration of the roof and structure and my recommendations are generic.