Metal roof questions

[captain14]

I have a small lean to roof on the back of the garage. It is a 2/12 pitch and currently has roll roofing installed. The size of the roof is 6'3' x 7'.

It's time for a new roof.

There is a plywood and tarpaper underneath.

Can I remove the rolled roofing material and tarpaper and install a metal roof and purlins over the existing plywood deck? Or does the plywood deck need to be removed? There is a wood ceiling under the roof framing.

The gable end of the shed is vinyl siding with flashing installed between the rolled roof material and siding. The fascia is metal.

I have attached a picture of the current roof.

What is the expected life of a metal roof? The existing rolled roof will be ten years old this fall.

 

[Hooked]

If the plywood decking is solid I would simply install the metal on top of the existing roof. Although my garage roof has a higher pitch, I did that over 20 years ago and it's still as solid as when installed. Others may have a differing opinion.

 

[Kaizen]

Depends on what metal you are talking about. Mine is also a high pitch so i have not had any issues with roofing that uses screws with rubber washers that seal water out. However on a low roof like that I'd be looking at snap and lock roofing. Basically like old school standing seam but more of a DIY install. The fasterners are not exposed so less chance of leaks from standing snow. It is more expensive but usally a thicker product.
Metal in general is 20 years estimate.
Regadless i'd take off that last row of house siding and put a large flashing up the wall further so its not soaking the sheathing.
If that was mine, regardless of what metal used, I'd lay a whole layer of ice and water shield on the whole roof. Manufacturers like to see a slip layer under the metal. They have stuff that works kind of like tar paper under a shingle roof. Thats not a huge roof but the metal will move in the sun.

 

[reader2580]

I thought purlins are usually used where the existing shingle roof was not removed, or if you want ventilation under the metal.

I did a metal roof on my house. I did a full tear off and then put the metal roofing on top of the existing plywood. I placed a synthetic underlayment on the whole roof, and then did a peel and stick ice and water shield (not granulated) on the entire roof too. The two layer recommendation is mostly for future ease of removal as it could be nearly impossible to remove the peel and stick from the roof sheeting if necessary.