A homeowner from Massachusetts wrote:
My husband and I are going to put on a metal roof on our house. We have some experience after building a gamberel roof barn a few years ago and love the results. The barn was installed over purlins attached to the rafters. My questions are:
1. My house is a tri-level and I don’t have much room below the windows to install purlins, so I have to go directly over the existing roof. Would it be a good idea to install something like the “Z” fold insulation used under vinyl siding as an underlayment to separate the metal from the asphalt shingles? I see that you usually recommend 30# felt. Why is that better?
2. The manufacturer’s catalog suggests using closure strips: Is this still recommended if using over existing roofing or should I encourage air circulation to reduce condensation between the shingles and the metal roofing? Would the underlayment interfere with adequate ventilation? I have a ridge vented attic and eaves
3. How much overhang do you recommend if I’m not installing gutters, but just drip edge?
4. Do you recommend #14 screws for going into asphalt shingles with a 1/2″ plywood substrate?
5. Do you recommend snow guards? I don’t want snow crashing down onto my deck and blocking the sliding door access.
6. Our chimney is an exterior chimney at the gable end of the house but not at the peak. Any tricks to the flashing? I’ve flashed around the cupola on the barn so I’ve got some experience.
1) If the house is normal construction then I feel that insulation is best on top of the ceilings, not on the roof deck. I am a strong believer in underlayment though, either 30-pound felt or a synthetic underlayment.
2)Ventilating the attic is much more important than venting beneath the roofing. I would not be concerned about venting beneath the roofing if your attic is well vented.
3) Adhere to manufacturer instructions but generally I’d say 1.5 – 2″.
4) Use the screws supplied by your roofing manufacturer. You want adequate length to penetrate the sheathing in your case.
5) Snowguards will break up snowloads and prvent shrubs from being damaged, doorways from being blocked, etc. Talk with your installer or supplier about their experience with snow in your area.
6) Flashing depends a lot upon the exact type of metal roof. I always suggest cutting the flashing into the chimney if at all possible.