Two choices decide whether a roof frame is strong enough: how far apart the rafters sit, and how deep each rafter is. The spacing and the size trade against each other: wider spacing needs deeper lumber, closer spacing lets you use shallower lumber, and the snow load in Lake County sets the floor for both. Get the pair right and the roof stays flat for decades. Get it wrong and the roof sags between the rafters, the sheathing dips, and the shingles telegraph every low spot. This guide compares 16 in and 24 in spacing and 2×6, 2×8, 2×10, and 2×12 lumber with rule-of-thumb maximums, so you can see which combination fits your span. The lengths themselves come from the rafter calculator, which works out the rafter length, the count, and the board feet for any spacing and size you choose.
16 in vs 24 in On Center
On center means measured from the center of one rafter to the center of the next. At 16 in on center, the rafters sit 16 in apart, and a 4 ft by 8 ft sheet of sheathing lands on a rafter at every edge and in the middle. At 24 in on center, the rafters sit 24 in apart, and the same sheet lands on a rafter at every edge and once in between. The count difference is large: a 40 ft building needs 31 rafters per slope at 16 in on center, or 62 in total, and 21 per slope at 24 in on center, or 42 in total. That is 20 fewer boards, which is a real saving in lumber and in cutting time. But each rafter at 24 in on center carries half as much load again as one at 16 in, because it supports a 24 in wide strip of roof instead of a 16 in strip, so the lumber must be deeper to carry it.
The roofing decides the spacing more than the framing does. A shingle roof needs 16 in spacing in most cases, because the sheathing spans between the rafters and shingles need a flat, solid deck: at 24 in spacing the sheathing must be thicker or rated for the wider span, or it will dip between the rafters and show through the shingles. A metal roof can use 24 in spacing, because the metal panels are carried by purlins that run across the rafters, and the purlins spread the load between them. That is why a metal roof shed is often framed at 24 in on center with purlins at 24 in on center, while a shingled house roof is framed at 16 in. If you are re-roofing and changing the roofing type, check the spacing before you order the roofing: a metal roof on 16 in rafters is fine, but a shingle roof on 24 in rafters needs the sheathing checked first.
2×6 vs 2×8 vs 2×10 vs 2×12
Lumber size is the depth of the board: a 2×6 is 5.5 in deep, a 2×8 is 7.25 in deep, a 2×10 is 9.25 in deep, and a 2×12 is 11.25 in deep. Depth is what carries the load, and it matters far more than width: a 2×10 carries nearly three times the load of a 2×6 on the same span, because the strength grows with the square of the depth. The table below gives rule-of-thumb maximum runs for #2 lumber under the Chicago area snow load, which is 25 psf ground snow load per ASCE 7. The run is half the span on a gable roof, so a 28 ft span is a 14 ft run and a 2×8 at 16 in on center fits it, while a 2×6 does not.
| Lumber size | Depth | Max run at 16 in on center | Max run at 24 in on center |
|---|---|---|---|
| 2×6 | 5.5 in | 12 ft | 10 ft |
| 2×8 | 7.25 in | 15 ft | 13 ft |
| 2×10 | 9.25 in | 19 ft | 16 ft |
| 2×12 | 11.25 in | 23 ft | 19 ft |
Read the table as a starting point, not as a code figure. The real maximum depends on the lumber species and grade, the snow load, the pitch, and whether the rafter is tied by a ceiling joist or carried by a ridge beam. A low-slope roof needs deeper rafters than a steep roof on the same span, because the load sits flatter and bends the rafter more. A 2×8 that works at 16 in on center may not work at 24 in on center on the same span, and a 2×6 that works on a 12 ft span at 6/12 pitch may not work on the same span at 4/12 pitch. If your span is near the top of a row in the table, go one size up: the cost of the deeper board is small against the cost of a sagging roof, and the deeper board also leaves room for insulation between the rafters if the attic is finished later.
What the Combinations Cost
The lumber cost follows the board feet, and the board feet follow the size and the count together. Take the 28 ft span, 6/12 pitch, 40 ft long roof from the pillar examples: at 16 in on center with 2×8 rafters it needs 62 boards of 16 ft 8 7/16 in each, which is 1,380.6 board feet and $1,518 to $2,624 in material. At 24 in on center with 2×10 rafters it needs 42 boards of the same length, which is 42 times 27.83 board feet per rafter, or about 1,169.0 board feet, and $1,285 to $2,222 in material. The wider spacing saves boards but the deeper lumber costs more per board foot, so the totals are closer than the board counts suggest. Run your own combination through the rafter calculator and compare the material range before you decide: the calculator shows the count, the board feet, and the cost for the spacing and size you pick.
There is one more cost that does not show in the board feet: the birdsmouth. A deeper rafter leaves more wood above the seat cut, which is why the seat cut must never take more than a third of the rafter depth. On a 2×6, which is 5.5 in deep, the seat cut must leave at least 3.67 in of wood above it. On a 2×8, which is 7.25 in deep, it must leave at least 4.83 in. A shallow rafter with a deep birdsmouth is the most common way a rafter fails, and it is the reason a 2×6 is a poor pick for a long span even when the table says it fits: the birdsmouth eats too much of the depth. The mistakes post covers the birdsmouth depth rule in detail, including how to measure the seat cut on the pattern rafter.
Spacing for a Metal Roof Shed
A metal roof shed is the one place 24 in spacing is the default rather than the exception. The rafters sit at 24 in on center, the purlins run across them at 24 in on center, and the metal panels screw to the purlins. The rafter size still follows the span: a 10 ft shed span at 3/12 pitch takes 2×6 rafters at 24 in on center, and a 12 ft span takes 2×6 at 16 in or 2×8 at 24 in. The purlins are usually 2×4 lumber laid flat, and their spacing matches the panel manufacturer’s requirement, which is commonly 24 in on center. Do not skip the purlins and screw the panels straight to rafters at 24 in spacing unless the panel is rated for it: the panels will oil-can between the rafters and the screws will work loose. The shed roof post works through the shed lengths and the overhang, and this page covers the size and spacing that go with them.
How to Decide
Start with the span and the pitch, then read the table for the smallest lumber size that covers your run at your spacing. Then ask three questions. First, is the pitch low? If it is 4/12 or less, go one size up from the table, because low-slope rafters bend more under snow. Second, is the spacing 24 in? If it is, read the 24 in column, not the 16 in column, and do not use the 16 in size at 24 in spacing. Third, is the attic going to be finished? If it is, the deeper rafter leaves room for insulation and for the birdsmouth, and the extra cost now is far less than reframing later. Then confirm the size with the Lake County building department or a structural engineer, because the table is a rule of thumb and the code is the rule. The rafter calculator will give you the length, the count, and the material range for the size you settle on.
Related reading: rafter calculator, plus our guides on shed rafter length calculator and common rafter vs jack rafter, and the roofing calculators hub.
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Frequently Asked Questions
What size rafters do I need for a 12 ft span?
For a 12 ft span at 16 in on center, 2×6 rafters are the usual starting point, because the run is only 6 ft. At 24 in on center, 2×6 still works on a 12 ft span at a mid-range pitch, but 2×8 is the safer pick if the pitch is low or the roof carries a heavy snow load. Chicago ground snow load is 25 psf per ASCE 7, so confirm the final size with the local building department before you cut.
Can I use 2×6 rafters for a 16 ft span?
A 16 ft span has an 8 ft run, and 2×6 rafters at 16 in on center are a starting point for that run at a mid-range pitch. At 24 in on center, or at a low pitch such as 4/12, 2×8 is the safer pick for a 16 ft span. The rule-of-thumb table on this page puts 2×6 at 16 in on center at runs up to about 12 ft, so a 16 ft span is well inside it, but the pitch and the spacing still decide the final size.
Is 16 in or 24 in rafter spacing better?
Neither is better on its own: 16 in spacing uses more rafters and less lumber per rafter, and it gives the sheathing closer support, which is what shingle roofs need. 24 in spacing uses fewer rafters and needs deeper lumber, but it saves boards and suits metal roofs, where purlins carry the panels across the wider spacing. A 40 ft building needs 62 rafters at 16 in on center and 42 at 24 in on center, so the saving is real, but each rafter at 24 in carries half as much load again as one at 16 in.
How far can a 2×10 rafter span?
As a rule of thumb, a 2×10 rafter at 16 in on center covers runs up to about 19 ft, which is a 38 ft span on a gable roof, and at 24 in on center it covers runs up to about 16 ft. A 2×12 at 16 in on center covers runs up to about 23 ft. These are starting points for #2 lumber under the Chicago area snow load, not code figures: the species, the grade, the pitch, and the local code all move the real number, so confirm it with the building department or a structural engineer.
What size rafters for a lean-to roof?
A lean-to roof is a shed roof, so the run is the full span and the rafter needs to be deeper than a gable rafter on the same span. A 10 ft lean-to at 3/12 pitch should use 2×6 rafters at 16 in on center, and a 12 ft lean-to should use 2×6 at 16 in on center at a minimum, or 2×8 if the pitch is low or the spacing is 24 in. The lean-to post works through the lengths, and the shed roof post covers the overhang and the spacing for a metal roof shed.
Zayn Robert is a content specialist focused on roofing, construction, and home improvement topics. He creates easy-to-understand guides that help homeowners learn about roof measurements, materials, maintenance, and repair decisions. His work combines detailed research with practical insights from roofing professionals at Zuniga Roofing Inc.