Replies

1. GBA Editor
   马丁·霍拉迪（Martin Holladay）|2014年2月17日下午2:54|#1

   Erik,
   First, I assume that you have checked with the manufacturer of the metal roofing that you intend to use, to be sure that the manufacturer approves the product's use for low-slope roofs.

   假设制造商批准了，我认为在屋顶下包括一个通风道没有太大的优势。

   In general, I'm not a fan of low-slope roofs in cold, snowy climates. But it's probably too late to convince you to design a roof with a steeper slope.

2. 
   埃里克·罗贝克（Erik Lobeck）|2014年2月17日下午03:21|#2

   马丁，
   谢谢您的意见。是的，制造商说的还可以，安装程序是保证的。因此，关于低螺距屋顶，这主要是局部分区的结果。恕我直言，在下雪的气候下，有时在雪和滴落的情况下，在坡度下的某些位置可能是不希望的（通常的替代方案是多个相交的山墙），有时可能会受益。在这种情况下，这是一个单脚奇，所有的雪滴都被引导到一个没有以下流量的区域（如果允许分区的话，我会在某种程度上增加球场）。我留下的是上面描述的屋顶设计。也就是说，继续问题：
   1_在护套上方进行搜索，如果我将金属直接放在底层上（没有排气空间），我会不会创建“蒸气三明治？”与下面的闭孔SF？
   2_Will venting not work in your opinion due to a lack of pressure / temp differential ? Or is there another reason?

3. GBA Editor
   马丁·霍拉迪（Martin Holladay）|Feb 17, 2014 03:37pm|#3

   Erik,
   Venting above the sheathing will work (assuming, of course, that the roofer specifies asphalt felt instead of one of the new, vapor-impermeable synthetic roofing underlayments). You are quite correct that the venting will help the roof sheathing dry out if it every gets wet. That is a real benefit.

   Your original question concerned ice damming. Because you have spray foam insulation with a high R-value, and because air leakage through your roof assembly is unlikely, I don't think that ice damming will be a serious concern. My answer was made in that light -- in an attempt to save you money.

   The entire question of whether we need to allow roof sheathing to be able to dry in one direction on the kind of roof you describe remains unsettled. Including a vent channel above the roof sheathing would be a conservative approach, with no downsides other than the increased cost.

4. 
   埃里克·罗贝克（Erik Lobeck）|Feb 17, 2014 03:46pm|#4

   马丁，
   Got it. That helps. Agreed. (Cost/benefit/conservative/etc) I suppose an additional benefit of not venting would be an increased assembly R value when we need it most-in the winter. (R-1 per inch for snow?)
   我可以更深入地研究一下，问为什么要说的问题，正如您所说的“未解决”？

   问候，
   Erik

5. GBA Editor
   马丁·霍拉迪（Martin Holladay）|2014年2月17日下午03:54|#5

   Erik,
   最终，所有屋顶都泄漏。如果建筑物的通风，无条件的阁楼，易滴屋顶泄漏更容易注意到，更容易定位，并且比屋顶有大教堂天花板更快地固定。

   如果您的屋顶泄漏，很可能有必要更换腐烂的屋顶护套。

   未解决的问题是：

   1. If you build a roof with closed-cell foam installed on the underside of the roof sheathing, will it take longer to detect a roof leak?

   2. Will more sheathing rot before roofing repairs begin?

   3.如果在安装闭孔泡沫的那一天干燥，是否有理由认为屋顶护套会通过屋顶泄漏以外的任何机制获得水分？

   Lots of people have opinions on these questions, but we don't really have a lot of data yet. So these types of roofs might last just as long as other types... or they might not.

6. 
   埃里克·罗贝克（Erik Lobeck）|Feb 18, 2014 12:07pm|#6

   Ok, if I wanted a reasonable level of safety without significant add cost in this scenario, what would your opinion be on utilizing pressure treated ply for the roof sheathing? Maybe in the hopefully unlikely event of a roof leak and membrane defect at least maybe there would be minimal chance of a structural roof rot condition?