Replies

1. Expert Member
   MALCOLM TAYLOR | Aug 07, 2014 10:19pm | #1

   Two other people you should ask as well as the posters here are:
   Your building inspector (Our seismic code no longer allows it, your code may not either).
   The metal roofing manufacturer.

2. GBA Editor
   Martin Holladay | Aug 08, 2014 04:18am | #2

   David,
   Building a roof without solid sheathing (these days, solid sheathing usually means plywood or OSB) can work for a barn or a wood shed, but it's not a good idea for a house.

   There are two main reasons why you don't want to do what you propose.

   The first reason is that it's important to have a good air barrier above your fibrous (air-permeable) insulation.

   The second reason is that it's important to have some way of handling the inevitable condensation on the underside of the metal roofing. (If you don't anticipate the condensation, the moisture can saturate your insulation.)

   The best installation has a plywood or OSB roof deck. If you want a cathedral ceiling (that is, if you want to install your insulation along the sloping roof), and you intend to use fibrous (air-permeable) insulation, then the building code requires you to have a ventilation gap between the top of your insulation and the underside of your roof sheathing. The best way to establish this gap is with a ventilation baffle (either a commercial baffle or a site-built baffle) installed with attention to airtightness. For more information on ventilation baffles, see How to Build an Insulated Cathedral Ceiling.

   Above your plywood or OSB roof sheathing, you will need to install a layer of roofing underlayment (for example, asphalt felt). When condensation forms on the underside of the metal roofing, the roofing underlayment holds the condensation long enough to allow the moisture to evaporate before it soaks the roof sheathing.

   I like to install 1x4 or 2x4 purlins or furring strips (rake to rake) above the roofing underlayment when I install metal roofing. The air gap between the asphalt felt and the metal roofing encourages fast evaporation of the condensation.

3. 
   dparker | Aug 08, 2014 07:46am | #3

   Thanks Martin, Point taken about condensation. I have seen it. Re: air barrier. Should I run baffles from soffit to ridge,keeping air out of attic? It is a flat ceiling ,not cathedral.

4. GBA Editor
   Martin Holladay | Aug 08, 2014 08:04am | #4

   David,
   When you described your ventilation baffles, I assumed that you were talking about a cathedral ceiling. If it is a vented unconditioned attic, you don't necessarily need plywood. The condensation is likely to dry quickly because of the volume of air in the attic.

   So your plan might work, as long as your local building inspector and your engineer (if any) are happy.

5. 
   dparker | Aug 08, 2014 08:09am | #5

   Thanks for the input and quick response.