Which is more important Minnesota building code or manufactures specifications:
State code reads: roofing shall be secured and fastened to the supporting roof construction and shall provide weather protection for the building at the roof.  Roofing products shall be installed per the manufacture's installation instructions or Minnesota state code requirement whichever is more restrictive. (You can find manufactures instructions on the back of shingle products packages or from the material supplier.)

Underlayment & Ice and water shield protection:
Entire state of Minnesota requires special eave protection an ice barrier that consists of least two layers of Underlayment cemented together or of a self-adhering polymer modified bitumen sheet, shall be used in lieu of normal Underlayment and extend from the eave's edge to a point at least 24 inches (610 mm)inside the exterior wall line of the building. The code in Minnesota requires this special waterproof shingle Underlayment at the eaves or lower edges of the roof.
1:Ice and Water Shield In Valleys:
Valleys shall have a 36-inch-wide(914 mm) Underlayment of either one layer of Type I Underlayment running the full length of the valley, in addition to other required Underlayment, or a self-adhering polymer modified bitumen sheet complying with ASTM D 1970.
1a:Ice and Water Shield Manufacture Recommendations:
Install ice and water shield around all Penetrations, At the intersection of walls, And intersection of Pitch changes.
2:Roofing Felt - Tar Paper:
The code requires that Underlayment of one layer of non-perforated Type 15 felt lapped two inches horizontally and four inches vertically to shed water. If the pitch of the roof is under 4/12 the roofing felt must be overlapped 18" horizontally or the entire slope covered with ice and water shield.

Layers Allowed:
Re-covering versus replacement (reroofing asphalt shingles). Hail exposure map has been revised. Complete shingle and Underlayment tear-offs are required when reroofing in the following Minnesota Counties (from Figure R903.5 Hail Exposure Map):

Anoka MN - Benton MN - Blue Earth MN - Brown MN - Carver MN - Chisago MN - Chippewa MN - Clay MN - Cottonwood MN - Dakota MN - Dodge MN - Goodhue MN - Hennepin MN - Isanti MN - Jackson MN - Kandiyohi MN - Le Sueur MN - Lincoln MN - Lyon MN - Martin MN - McLeod MN - Meeker MN - Mille Lacs MN - Murray MN - Nicollet MN - Nobles MN - Norman MN - Olmsted MN - Pipestone MN - Ramsey MN - Redwood MN - Renville MN - Rice MN - Rock MN - Scott MN - Sherburne MN - Sibley MN - Stearns MN - Steele MN - Yellow Medicine MN - Wabasha MN - Waseca MN - Washington MN - Watonwan MN - Wright MN

Flashing Requirements:
Existing flashings may be re-used provided it has no holes and is serviceable. Under no circumstances may(tar) or other asphalt material be used in lieu of metal flashing. (WE REPLACE ALL ROOF FLASHINGS WITH NEW)
At walls: Flashing shall be installed at the juncture of the roof and vertical surfaces. Flashing and counter flashing shall be provided per roofing manufacture's instructions. Not less than no 26 gage corrosion resistant metal shall be used.
Chimney and step flashings: shall be installed into the masonry bed joints or adhered to the masonry. If the step flashing is adhered, Counter flashing must be installed to cover the top edge of the step flashing. This Counter flashing must be installed in either the masonry bed joints or via a reglet cut into the masonry. A cricket or saddle shall be installed on the ridge side of any chimney greater than 30 inches (762 mm) wide. Cricket or saddle coverings shall be sheet metal or the same material as the roof covering.
Kick-out flashing: shall be installed where the lower portion of a sloped roof stops within the plane of an intersecting wall cladding, in such a manner as to divert or kick out water away from the assembly.
At valleys: Valley linings shall be installed in accordance with manufacturer's installation instructions before applying shingles.
Valley linings of the following types shall be permitted.
1. For open valley: (valley lining exposed) lined with metal, the valley lining shall be at least 24 inches wide and of any of the corrosion resistant metals in Table R905.2.8.2.(no 26 gage corrosion resistant metal)
2. For closed valleys: (valley covered with shingles), use a valley lining of one ply of smooth-roll roofing complying with ASTM D6380 Class S Type III, Class M Type II or ASTM D3909 and at least 36 inches wide.
Other flashing: flashing against a vertical-front wall and the soil stack, vent pipe and chimney flashing, shall be applied according to asphalt shingle manufacturer's printed instructions.

Roof Decking:
Asphalt shingles shall be fastened to solidly sheathed decks or 1 inch thick nominal wood boards When old roofing materials are removed, the original roof boards are often found to be spaced; this was and still is permissible for wooden shakes . However , this creates a fastening problem, as the space between the boards does not allow nailing at all locations of the roof. Any roofs with spaces greater than ½ inch must be corrected(1/2" is Open to Interpretation-Code specifically states solidly sheathed)
Approved methods of decking / wood replacement:
1: Stripping: Install strips of at least 1 by 4 nominal boards in the spaces
2: Re-deck (Overlay): Overlay the Existing roof decking with either lumber sheathing or wood structural panel sheathing a minimum of 11/32 in thickness Panel Or 1 Nominal. No edge of this material can be left exposed to the weather; therefore, it must be metal edging or wood trim. (Open To Interpretation By Building Official Regarding Overlay - State Code Does Not Address This) The code requires that roof decks be solidly sheathed for asphalt shingles or mineral surfaced roll roofing. Solid sheathing may be plywood, OSB, or 1-inch nominal boards. When using 1-inch nominal boards, the shingle manufacturers instructions should be reviewed to determine if the manufacturer places any limitations on the use of their shingles on a board sheathed roof.
Wood shingles and shakes may be applied over solid or spaced sheathing. When using spaced sheathing, sheathing boards must be not less than 1X4’s and must be spaced on centers equal to the weather exposure to coincide with the placement of fasteners. If the center-to-center spacing of 1X4’s is 10 inches or more, additional 1X4’s must be installed between the sheathing boards. The portion of the roof receiving ice protection must be solidly sheathed. (R905)

Fasteners:
Fasteners for asphalt shingles shall be galvanized steel, stainless steel, and aluminum or copper roofing nails, minimum 12 gage (0.105 inch (2.67 mm)) shank with a minimum 3/8 inch (9.5 mm.) diameter head, ASTM F 1667, of a length to penetrate through the roofing materials and a minimum of 3/4 inch (19.1 mm) into the roof sheathing. Where the roof sheathing is less than 3/4 inch (19.1 mm) thick, the fasteners shall penetrate through the sheathing. Fasteners must comply with ASTM F 1667. Staples are not permitted for shingle application unless specifically noted in the manufacturer’s installation instructions on the shingle package.

Venting attic space:
Roof ventilation is required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of the roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilating openings shall be provided with corrosion-resistant wire mesh, with 1/8 inch (3.2 mm) minimum to 1/4 inch (6.35 mm) maximum openings.
Minimum area: The total net-free ventilating area shall not be less than 1:150 of the area of the space ventilated exempt that the total area is permitted to be reduced to 1:300, provided at least 50 percent and not more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents. As an alternative, the net free cross-ventilation area may be reduced to 1:300 when a vapor barrier having a transmission rate not exceeding one perm (57.4 mg/s-m2-Pa) is installed on the warm side of the ceiling. It might be necessary to add ventilation with your new roof to meet these standards. - If condensation problems exist in the attic, they too will be revealed when the roof is torn off. Properly designed attic ventilation can then be installed in order to help eliminate such problems.

Above is part of the 2007 Minnesota State Code as well as 2006 IRC and/or IBC . Certain cities and towns may have additional requirements that may have to be met. Check with your local city hall.