Shed roof or gable roof
The estimate prices the simpler of the two. A shed roof is one plane, sloping from a ledger on the house wall down to a beam carried on posts at the front. It has the fewest cuts, the least lumber and only one junction with the house, along its top edge.
Its limit is height. The high end has to fit under whatever is on the wall above, usually second-floor window sills or the eave of the main roof, and the low end has to leave headroom at the front. On a deep porch that squeeze can leave less slope than shingles need. IRC section R905.2.2 allows asphalt shingles at 2:12 and steeper, with doubled underlayment below 4:12; flatter than 2:12, the porch needs a membrane or metal roof instead. Work out the rise you actually have with the roof pitch calculator before settling on a design.
A gable porch roof runs its ridge out from the house and puts a triangular gable over the steps. It can match the pitch of the main roof and it marks the entrance, which is why it is the common choice on a front porch. It costs more than the shed priced here: a ridge, two sets of rafters with more angled cuts, and two sloped junctions with the wall that need step flashing instead of one straight run. The rafter calculator gives rafter lengths for either shape.
The ledger is where porch roofs leak
The ledger is the board fastened to the house that carries the back end of every rafter. It is lagged, screwed or through-bolted into the house framing, the rim joist or a header, never into siding or sheathing alone. On a brick veneer wall it cannot hang on the brick, so the porch gets its own beam and posts along the house side instead.
The flashing above the ledger is the most common source of porch roof leaks. Done properly, the siding along the ledger line comes off, the flashing runs up the wall behind the house wrap and down onto the new roof, and the siding goes back over it, held clear of the shingles. Done badly, a strip of metal is nailed over the face of the siding and caulked along its top, and it leaks once the caulk shrinks. Ask how the ledger will be flashed, and expect an answer that involves taking siding off.
The estimate carries a roll of aluminum flashing for this ($37 to $52); the sealant is for trim joints, not a substitute. One detail to check: aluminum corrodes against the copper-based preservatives in pressure-treated lumber, so it should be kept off a treated ledger with a strip of self-adhered membrane. The flashing guide shows how each type laps.
Permits, setbacks and what the posts stand on
A roofed porch is a structure attached to the house, so it needs a building permit and usually drawings. The inspector will look at the ledger fastening, the beam size, the post connections and the footings, typically before concrete is poured and again before the framing is covered.
Zoning is a separate question. Front porches often sit inside the front-yard setback, and an open deck or slab allowed there may not be allowed a roof. Lot coverage limits, historic districts and HOA rules can all apply, so find out before paying for drawings.
The posts need footings sized for the roof, including snow where it falls; the snow load calculator shows what a roof has to carry. An existing slab edge is not a footing, and deck footings were sized for a deck. The estimate prices post bases and rafter ties ($72 to $180), the hardware that holds the posts down and the rafters on in wind, which matters on a porch because wind gets underneath the roof as well as over it. New footings are not included.
Carpentry as much as roofing
More than half the hours on this job are framing, done before any roofing starts, so the right builder is often a carpenter or general contractor who shingles the roof in-house or brings in a roofer. The framing lumber ($784 to $1,792) is the largest material line. When comparing quotes, check that each names the post size and material, the beam and rafter sizes, rafter spacing, how the ledger is fastened and flashed, who pours the footings, and whether the permit and drawings are included. A much lower quote often leaves out footings or finish work.
A porch roof sits on the front of the house, so mismatches show. The estimate prices architectural shingles ($306 to $432); against a weathered main roof, or one in a discontinued color, even the closest current shingle will look new for a few years. If the main roof is due for replacement soon, doing both together is the only true match. Overhang depth, fascia, soffit and gutter profile should match the house too. Trim, gutters, a finished ceiling such as beadboard, paint, and wiring for lights or a fan are not in this estimate.