[rbry]

I am installing a new composition shingle roof on a 2.5/12 shed roof which is attached to the back of my home over the kitchen and dining area. The cathedral ceiling in made of 2x6's with 2x2 furring strips attached to their bottom- I suppose the person who installed the system figured that made it OK to use R30 insullation!. I have removed all rock and insulation and will replace water damaged decking once the existing shingles are removed tomorrow. The rafters are solid blocked with 2x6 to accommodate nailing between sheet edges. There are three blocks per rafter bay including the one at the wall plate. Since I have the roof completely opened up i am considering adding a ventilation path to the system. I have read a lot of different posts regarding how to vent a roof such as mine and after digesting it all have concluded that the best way will be to cut and place vents in the blocks at the wall plate and then drill several holes in each of the other two blocks. Where i am getting a little stuck is what to do with all that hot air at the top of rafter where the roof meets the wall. I am somewhat resistant to products like cor-a-vent just because of the slight pitch and very light but predictable snowfall that i get at my elevation. I am considering drilling holes between the rafters toward the top of their run and then installing two or three equally spaced roof vents. Another option would be to avoid additional penetrations and place vents in the soffit at rake. If anyone out there has had success with a project like this I would greatly appreciate any insights. Also with the existing rafter depth of 7in I am considering an r19(6.5 inch thickness but easily expands to 7in) batt insulation- if my holes are the top of the rafter blocks will the insulation prevent air flow?

This is my first post so if I have made any mistakes in protocal i apologize in advance.

thanks

Robert

 

[Perry525]

Ventilation rain snow wind.

Its difficult to frame an answer, not knowing where you are.

With a shallow pitched roof you must always allow for the wind to blow the rain and snow up the roof.

With a shallow pitch, it is often best to go for a one piece rubber roof that is definitely going to be waterproof.

If there is a chance of wind blown rain entering the roof, you must make provision for ventilation to dry the inside.

However, ventilation equals loss of heat from the home, as the passing wind not only pulls the air through your roof, it pulls it from your home as well.

From a comfort and cost point of view, moving towards the most modern ideas, of a sealed, air tight home makes a lot of sense.

Lining the roof with sheets of polystyrene, carefully cut to a tight fit, but positioned at least one inch below the roof (if the inside of the roof is smooth) or two inches below if rough \will provide and escape route for any rain and will give the roof a chance to dry out. As long as the roof can dry during the summer months you should not have a problem. (You do not need constant ventilation)

 

[rbry]

Thanks for your advice. I guess i need to add some crucial info to my profile!. The house I am working on is in northern California and was built in the 1880's so there are lots of things - old school framing, various remodels, unconventional carpentry etc. - that i am having to factor in as I undertake the project. The roofer removed two layers of shingles yesterday and I was then able to start addressing the damage from a leak which was the result of improper flashing. The sheeting and rafters in the immediate vicinity of the leak was all damaged and unrepairable. What struck me about the rest of the plywood roof sheeting was how dry and somewhat brittle parts of it was. I made the decision to replace it all with 1/2in. Cdx. About 90% of the nails were slightly loose in the sheeting which made me think that there was considerable expanding and contracting over the years. with all things considered and still not sure where I sit on the sealed roof or vented roof discussions I have decided with this roof to go vented. In order to provide flow through the rafter bays i will install a product called Durovent and at the roof to wall i will have the roofer install ridge vent which he will cut in half to allow for this application.

Hope it works!

thanks

Robert

 

[Perry525]

Roof repair/replacement.

Installing a vented system will do no harm (except for loss of heat from the home and of course in summer it merely replaces the hot air in the roof with hot air from outside) the system works by the passing wind causing a low pressure area to the lee of the building which sucks the air from the roof)
To work it needs a wind and matching vents on all sides and the top of the roof.
You may care to consider laying SIP's panels over the rafters. These panels, a sandwich of polystyrene and plywood or oriented strand board are very good at keeping the heat out. They separate the roof covering from the rafters and stop the conduction of heat through the frame of the home.
To work they need a water vapor proof plastic sheet underneath to stop any water vapor rising from the home in winter and entering the joins in the panels.