Roofing/Metal over asphalt shingles
Our house is ready for a new roof. Current roof is dimensional shingles - which I have enjoyed. Am leaning towards the add'l expense of metal due to perception of improved lifespan and insulation. Understood the insulation improvement came from installing metal over the existing shingles using wood strips between.
Roofer prefers synthetic underlayment only - disagrees that stripping provides insulation improvement or extends metal life. Home improvement store says need both underlayment and stripping. Store also states washers will need replacing in 10 yrs as they will dry rot and metal will need painting in 20 regardless of Valspar claims....if that's the case, why not just go with the less expensive shingles?
I am concerned about bore bees either way the metal in installed, but especially if we use the wood strips. They currently fly up V on horse shed and tunnel into wood slats. Home improvement sells diecut foam pieces, but says they fall out or insects eat through. Suggests spray foam filler.
Who is right? No strips, with strips, with underlayment and strips, don't worry about bees, use DC foam, use spray foam???
We are in East TN. Hot humid summers with cold rainy winters. Live on hill so plenty of wind. Have acres of woods around so lots of bees.
Your advice is greatly appreciated.
You have some great questions. First of all, many of your questions can be answered by the type of metal roof you choose. You may want to investigate my educational site on residential metal roofing at www.asktoddmiller.com for more information on product choices.
Putting the roof on strapping does not increase the life of the metal roof in my experience. Can it help with energy efficiency? Marginally, yes. However, to be real effective with energy efficiency you need to cross batten -- vertical battens followed by horizontal battens. This allows you to take advantage of convective airflow and actually vent the chamber beneath the new roof by bringing air in at the bottom and exhausting it out at the top.
I also like the synthetic underlayments and suggest them whether you use battens or not.
Adding battens does make the job more complex. It raises the roof level which means you need longer trims and may need to raise your gutters.
My ideal suggestion would be to make sure your attic is well vented (both intake and exhaust), add attic insulation if you need any,install underlayment and no battens, and install a metal roof with a reflective Energy Star coating if possible.
As far as exposed screws ... they are indeed a weak spot in these systems. The screws today are better than they used to be but there are still different quality levels. I always suggest if exposed fasteners will be used that they be stainless steel.
However, going back to my original statement, there are many metal roofs available which do not have exposed screws. Have you considered one of those?
If you do install a roof which creates gaps (not all do) then I do suggest foam or metal closures. I would not use spray foam -- too difficult to control. The foam closures have adhesive strips now oftentimes which work well or you can use a butyl tape to install them securely.
I hope I have helped with your questions. Feel free to email me direct at firstname.lastname@example.org