How to properly ventilate a Mansard Roof?


Old 04-26-15, 02:16 PM
Thread Starter
Join Date: Feb 2015
Location: Canada
Posts: 8
Received 0 Votes on 0 Posts
Question How to properly ventilate a Mansard Roof?

I recently purchased a home with a Mansard roof, I had some roofers come to give quotes to replace the top (it is finished) and they all recommend adding ventilation (known issue with Mansard's).

Name:  Home_Front Exterior.jpg
Views: 2135
Size:  38.4 KB

The problem is that they are offering two different solutions and I don't know which is best... hoping for some professional recommendations from the community.

1- Maximum Mansard Air-Intake drip edge
Product information for Air Intake Soffits
This is what 95% of the mansard homes on my street have and is the more expensive option

2- Ventilation Maximum (VMAXAT1 or VMAXAT2) uniformly positioned at the eaves
There is 1 house on my street that has it and it is less expensive

Any advice, first-hand experience, or help for me to decide which would be the right solution would be very much appreciated.
Sponsored Links
Old 04-26-15, 02:45 PM
Join Date: Oct 2008
Location: New England
Posts: 10,483
Received 33 Votes on 30 Posts
Hi Shaitan,
Canada so I'm assuming a cold climate.
I have no first hand experience with the Mansard style homes, so forgive me if I ask some stupid questions.
Ventilation works best with both upper and lower openings. I didn't see any mention of high vents, unless they are proposing to use the second version near the top. What's up there? Is there a flat roof, you said "finished" but not sure what you mean, the work was finished or the flat area is outdoor living space. As I said, dumb questions.

One of the first steps in sizing ventilation is determining the interior surface area that is subject to air leaks and moisture penetration. On a gable style roof with a flat attic floor they simply use the floor area. When you go to a sloping roof then you have to think about the inside surface area, which is usually more. So, are there knee walls inside that sloped roof we see in the picture with a small floor area. then what is above that?

Old 04-26-15, 03:00 PM
Join Date: Apr 2014
Location: Nova Scotia
Posts: 750
Received 0 Votes on 0 Posts
Adding soffit vents are not going to do any good unless there is a clear path for the air to flow under the roof. If none were installed when the house was built, you can be almost certain there is nowhere for the air to go on the virtical part of the mansard. So unless you want to tear out the outside walls on the second floor, I'm not sure I would bother with soffit vents.... unless it can be verified there is a clear path for air to the upper part of the roof.

As for the flat part on top, is it all cathedral inside, or is there a small attic space? do you know what the pitch of the roof is abd what type of roofing it has on it now ( or what is being put on it)
Old 04-26-15, 03:13 PM
Thread Starter
Join Date: Feb 2015
Location: Canada
Posts: 8
Received 0 Votes on 0 Posts
Seems like my terminology might have been bad - when I say "finished" I mean it needs to be replaced immediately - the previous owner block all the soffits (thought they were mistakes) - so now I need to tear it down - while I am doing that I am going to improve the ventilation and thus my questions.

The roof is NOT flat (small pitch, not sure how many degrees), and there is an attic that I can get to and almost stand-up in (so pretty large).

Right now the roof top is the same as the sides, shingles (falling apart), and I plan to put new IKO 50 year fiberglass shingles on - would have loved metal but can't afford it.
Old 04-26-15, 03:27 PM
Join Date: Apr 2014
Location: Nova Scotia
Posts: 750
Received 0 Votes on 0 Posts
If your roof is less than a 3:12 pitch, make sure the roofers install an ice and water membrane under the entire roof. From the pic, it does look less than that. 3 tab shingles are not supposed to be installed on a slope less than that without a secondary membrane under it. ( read the directions on the package).

If there were vents in the soffit before, then you are right, they need to be restored.

For the upper roof I would use a turbine vent, but that is my personal preferance. They will draw alot more air though the attic and are much less likely to leak due to snow build up or driving rain.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: