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mansard_mayhem 12-05-2011 10:23 AM

mansard roof - venting the veritcal
We are in a house built in the early 70s which has a mansard roof. Two years ago we had the original shingles replaced with new ones because they were showing signs of age and curling. The curling was particularly bad on the front of the house which faces west.
Now that we have the new shingles on, they look much better but we are STILL having issues with curling. On sunny days, the shingles on the west side of the house (front) tend to "puff" out like they are incorrectly adhering. Since this appears to be temperature related I thought it might have something to do with the venting of the vertical portion of the mansard roof.
I went up into the attic, and it appears as though there is ~4" of pink fiberglass insulation on the ceiling, and then another ~4" of insulation (not fiberglass) has been blown in on top of that. From what I can tell, the "blown in" insulation goes right up to the edge of the roof, thereby blocking any vertical flow of air coming up from the soffit.
Q1: Does there need to be a space between the vertical and horizontal portions of the mansard roof?
Q2: If so, how frequent do the spaces need to be (between every truss, every other truss, two or three across the span)?
Q3: Does it seem plausible that this is causing the problems with our shingles? If not, what else could it be?

nealtw 12-05-2011 04:36 PM

Yup that will do it, I think you could do every third one for most of the house, but in the problem area you will need to look at all of them.

notmrjohn 10-16-2012 03:47 PM

Assuming you have enough soffit vents and large enough exit at top, you should pull away any insulation that is covering soffitt vents, install barriers to keep insulation off of them . And remove any loose material that prevents flow of air directly against underside of roof all the way to exit.

Three questions; 1. Where is exit for warm air? 2. You don't have any kind of "Radiant Barrier" directly applied to under side of roof or to rafters? painted on or foil type? 3. Is there a horizontal plate between more vertical rafters and flatter ones; or horizontal blocking there? 3a. Is the curling more prominent on vertical section of roof or flatter? (3A occurred to me several sentences down. Possibly more to come when/if.) you reply

Ideally you should have a properly sized entrance for every rafter bay with properly sized exits as high as they can be. Mansard roofs often have one or more cupolas, they're not there for decoration. They do provide roosting places for insect eating bats, swallows, and swifts but that's not the original purpose either.

I dunno where you are Mr or Ms mayhem ( How about adding location to profile? Would be helpful since roof ventilation also depends on winter temps and humidity.) but here I have front door in alcove facing west, Spring and Autumn not so bad, Summer and Winter, after leaves fall I cook roasts in there.

And what was your name in the States? I mean when you have non-roof related question? Though personally i like the name, images of French slaters and les enfants ripping roof off Bastille. Vive le fresh aire!

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