Water and Ice on part or whole roof?

The roof "breathes" from underneath, with proper ventilation (e.g., vents at eaves and ridges, good airflow in cold attic). The top surface can be airtight.

Our 1920s cottage roof (in snowy mountains) was redone before we bought the house and only had water-and-ice shield for the first 4 feet. We had ice dams and ceiling damage the first winter. We then had the whole thing stripped down to the wood, next summer, and water-and-ice shield added from the bottom to the top.  We still get ice dams, due to  faulty insulation, but no more leaks.  Our roof surface is asphaltic shingles with a 12/12 pitch at the peak, 6/12 roof over 12-foot porch awnings, ending in a 3-ft  overhang having only 1/12 pitch.  The roof also has cold attic dormers (with their own eaves) and several valleys that attract and hold snow and ice.  We have rakes for the eaves and sometimes we get up and shovel the bad spots on the shallow roof areas when it's over 20 inches deep.

Many of our neighbors have metal roofs. We put standing-seam metal on the 12/12 roof on the unheated carriage barn and it has never had a problem.

The first 2 runs (the underlay comes in a 4 foot wide roll so about 8 feet from the gutter up need this underlay.  As it is a fairly new procedure, the jury is still out on it.  But going by the old way of roofing it seems the first 6-8 feet are adequate.

The roof does need to breath so whole or bottem edges comes down to your particular icing issue.

My back roof should have at least the bottem half done, the front roof only need the bottem couple of feet done. 

if its a low pitch 4/12 or less I would do the whole roof

building codes in my neck of the woods requires it to be 4’ in over the living space plus the eave

you would have got better answers if you had given more info ..your description is extremely vague ..so ..is it a pitched roof?  flat?  material ?..

A complete metal roof is the only way to go.

It will virtually eliminate solar heating of the attic space, ice dams, leaks, and maintenance.

And a WHITE metal roof is best

Since this is a reroof, my question is..... have you had problems in the past. 

If not then I would go with the the same amount (length) of Ice and water shield. 

If there is none on the roof when they do the tear off,  then I would go 6-10 feet up from overhang (or whatever code determines)... also do all valleys, ridges and around chimney ...anyplace that is prone to leaks. 

If you have a shallow pitch (2/12 or less) typically on Fla rooms ... you may consider doing the entire roof...**Otherwise ...l do not recommend doing an entire roof  do to moisture build up .........roof does need to breath.    

if you have ice buildup on the roof, it means your insulation in the attic is inadequate -- allowing heat from the house to melt snow up toward the peak of the roof, which water then runs down and freezes lower down.  That is how an 'ice dam' forms -- after which water will end up sneaking under the tar paper or membrane and getting into your attic and from there onto your ceilings [where it eventually destroys the wallboard as well as leaves discolored marks on the ceiling].  -- grampa [from experience in Vermont]

Not sure what your asking.If you live in a region where you get lots of snow a slanted roof is the best way to go.The more angle the better.You want the snow to melt as soon as possible so you don't have to get up there and shovel it off by hand.A darker color shingle will help it melt quicker in the winter time and retain more heat but have the opposite affect in the summer and get at least a 25- 30 year shingle.If they are stripping the roof make sure they put tar paper on or use that new membrane material.