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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).

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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

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. Thanks,

  • Seems like my terminology might have been bad - when I say "finished" I mean it needs to be replaced immediately - 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. – Jonathan Apr 26 '15 at 21:15
  • Can you please advise which option you went with and how that is working out for you? – user61904 Oct 28 '16 at 1:22
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I own a mansard and we are battling this issue. We had our roof replaced last year and the roofer that did it didn't even install 1/10th of the recommended ventilation. Last winter was brutal in the north and we sustained substantial water damage from condensation inside of our home. We are working with another roofer to get the proper edge venting installed. Think of a cold glass on a hot summer day, that is what the inside of my roof decking looked like last winter.

Do not skimp or you will be sorry and sustain a substantially decreased lifespan of your roof, not to mention you could incur damage to the interior of your home if you live in the north (hot inside of the roof + extreme cold outside = large amounts of condensation).

Make sure you have a proper vapor barrier as well. Loose fill insulation = more required ventilation. I would also consider a power vent with both thermostat and a humidistat. I am not a professional, but I am a Mechanical Engineer by education and since our issue, I have researched proper ventilation of mansards extensively. Go with a very reputable roofer that will stand behind their work if you have condensation issues.

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If there's enough space over the windows your are better off with option #1. If money, looks or the if you don't have enough space over your windows on the ledge, go with (I am assuming from the picture) 3x vmax-at 1(square) or vmax-at 2 (triangle) on each side of the house... Add 1 maximum 301 vent per 1000sf at the top of the roof and you shall have no more problems

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One strategy would be to incorporate the attic into the conditioned space and then use a sandwich of barriers and insulation under a furred/vented over-roof to control ice damming. This works everywhere, no matter what kind of roof you have, what kind of attic you have, or where you live. It also has the advantage that you have an attic that's now more than suitable for being turned into a semi-finished or finished (instead of unfinished) space!

More details about this approach can be found in this piece from the Building Science Corporation and in particular, Figure 3:

Figure 3 from BSI-083: Mea Culpa Roofs

As to that pesky flat-as-a-pancake middle? Well, what you're dealing with is no different challenge-wise than a commercial flat roof, and I'd suggest reading up on flat roofs and parapets to deal with it. In short:

  • Make sure the control layers (water, air, vapor, heat) are continuous through all sections of the roof, including the inner parapet.
  • Let things move if they want to (in particular, flat membranes are notorious for moving around on unfortunate building residents).
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I think if you have space over your roof and also have budget then you should go with option1

  • You haven't offered any reasoning behind your suggestions. – isherwood Oct 2 '18 at 12:52

protected by Community Apr 29 at 17:57

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