air barriers

THE CHURCH I ATTEND RECENTALY HAD A METAL ROOF INSTALLED OVER TWO LAYERS OF ASPHALT SHINGLES. I'M CONCERNED THAT THE CONTRACTOR DID NOT USE ANY FELT OR PROVIDE FOR ANY TYPE OF AIR CIRCULATION TO KEEP MOISTURE FROM ACCUMULATING UNDER THE METAL. COULD YOU EXPLAIN WHAT THE CORRECT INSTALLATION PROCEDURES ARE FOR INSTALLING METAL ROOFING OVER EXISTING TWO LAYERS OF ASPHALT SHINGLES.

I have a commercial building in Birmingham, AL. and I am placing a metal roof over a small section of the building. This section is 60' x 12'. The area has 3 parapet(sp. ?) walls on the 2 long sides and one on the short. A gutter borders the fourth short side. I will be adding the structural frame on top of the parapet walls and in between the walls on the flat roof. There will be a gap between the metal roof and the flat roof from 2-5'. My questions are: 1. What pitch should I use if I shed the water along the 60' length 2. Do I need the mechanically ventilate the dead space? Or can I install some sort of louver for ventilation etc. 3. What is the recommended center length (16 oc, 24 oc)? 4. What gage metal should I use? Thanks for your response. Scott G.

Obviously, I do not know the exact details of your installation. However, in most cases, the foam closures are installed to prevent infiltration by insects, etc. They also help keep out wind-driven rain. Generally, I suggest they be installed. They will not have any negative effect at all. If you have further questions regarding these, I would suggest contacting the manufacturer of the roofing.

You have the concept absolutely correct. The most imortant part of a roof assembly starting from the inside is the "air barrier" which must be on the warm in winter side of the assembly which in Wahington is the inside. This barrier needs to be monolithic with seled joints and wrapped around the electrical boxes etc. It prevents the warm moist air from moving through to the outside and condensating usualy on the underside of your roof material or decking. Insulation works better with a good air barrier as it keeps dry and traps air. Dead air is a good insulator similar to the air space in thermal windows as it resists the direct transfer of heat and cold.Ensure that the insultion is tight to the air barrier with few gaps. On the outside of the insulation and directly under the roofing material you need a moisture barrier. 30# felt is the minimum required however when you put on a long tern roof you need to upgrade the underlayment so that ir will perform as long as the roof material. I am a little confused on your terminology as TRI FLEX 30 is a brand of spun reinfoced mat that is an excellant underlayment which weighs around 30# for a 9 square roll. Please ensure that your contractor seals the side laps but not the horizontal laps as the TRI FLEX has a low water permiable rating and if sealed will act as an air barrier and needs to allow air to psss on the laps. Also seal any roof penetrations etc like jacks with a house wrap tape. The one thing you are missing is an air space that is ventilated. The US codes now require a minimum of 1 1/2" of vented air space on cathedral assemblies like yours. I would suggest using 6" of insulation which will give you an R20 rating, then 2x2 vertical strapping with the plywood over. You can vent the eaves either with soffit vents of a eaves vent mesh available from GAF called Cobra. Most roofing manufacturers have a vented ridge detail. This assembly will provide you with good energy savings and good home comfort. Unfortunately the various US building codes are very weak in this regard. I would recommend trying to get a copy of the Canadian National building code which is excellant in this regard (probably from all the experimenting over the years on trying to keep warm). Good luck.