Frame type
A lean-to is a single-slope module framed off an existing sidewall. It has no ridge and no second column line of its own — the host building carries one side of it. That makes it the cheapest square footage available on any project, and the only frame type whose feasibility is decided by a building that already exists.
Practical span range
10–60 ft
Where this configuration is normally used
A single-slope module framed off an existing sidewall, with no independent ridge. The cheapest square footage on any project, provided the host frame can take the added load.
Skip it when: The host building was not designed for the reaction. This always requires a check of the existing frame.
Rafters bear on the existing sidewall columns at one end and on a new, lower column line at the other. You are buying roughly half a frame: one column line, one slope, no ridge, no second wall. That is why the cost per square foot is unlike anything else in this section.
What you are also doing is adding load to a structure designed without it. The new rafter delivers a vertical reaction and, because the lean-to roof also catches wind and snow, a lateral one, into columns that were sized for a different building. Snow is the sharper risk than dead load: the step in the roof creates a drift surface against the host wall that neither roof had before.
This is not a formality. Whether the host frame can take the reaction is the question that decides whether the lean-to is buildable at all, and it is answered by an engineer looking at the original design, not by a supplier looking at a floor plan.
Cheapest per square foot of anything here, by a wide margin, because half the structure already exists and is paid for. Add covered area to a building you already own and the marginal cost is a fraction of building the same area free-standing.
The exception swallows the rule when the host frame cannot take the load. Reinforcing existing columns, adding footings, or upgrading a foundation to accept the new reaction can cost more than the addition, and it is found out at the engineering stage, not the quoting stage. Get the check done before the budget is set.
Practical widths run to about 60 ft. Past that the lean-to rafter is long enough that it wants its own proper frame, and the reaction into the host wall becomes hard to justify.
Planning-stage estimate calibrated to published industry ranges. Not a substitute for a stamped design by a licensed engineer. How the figures behind those percentages were derived.
The widths this configuration reaches that have a page of their own, with what a clear span at that width costs and what a column line would save against it. Where the saving is meaningful, that number is the case for or against this configuration in one figure.
| Width | Clear span steel | Clear span package | Modular saves | What changes at this width |
|---|---|---|---|---|
| 40 ft | 2.04 – 2.97 lb/sf | $13.45 – $16.33/sf | — | Light frames. Span is rarely the cost driver at this width - eave height and snow load matter more. |
| 60 ft | 3.00 – 4.20 lb/sf | $15.00 – $18.00/sf | — | The practical floor for commercial work. Below this width, small-building suppliers price better. |
Each row is priced at the case on that width's own page — the eave and length that actually get built there — so the rows are comparable to those pages rather than to each other. Planning-stage estimate calibrated to published industry ranges. Not a substitute for a stamped design by a licensed engineer.
A single rigid frame carrying the full width with no interior columns. Maximum usable floor area and total layout flexibility, paid for in steel weight that rises faster than the span.
20–300 ft →One or more interior column lines split the width into modules, so each rafter spans only a fraction of the building. The savings grow with width and become decisive past roughly 150 ft.
60–480 ft →One continuous slope from a high eave to a low eave, with no ridge. Drains to one side, presents a taller street face, and fits sites where a gable would break a height limit.
20–200 ft →Two or more gable modules side by side sharing interior columns, covering great width with short, cheap spans. The tradeoff is a valley - an interior gutter line you must detail and maintain.
80–600 ft →Built-up members whose web depth follows the moment diagram - shallow at the base, deep at the haunch. The standard configuration for clear span, and the reason a rigid frame is the lightest way to cover a wide bay.
40–300 ft →Constant-depth columns. Heavier than a tapered equivalent, but they give a flat plane for racking, liner panel, masonry wainscot, and interior partitions, and they simplify sidewall framing.
20–150 ft →Not sure which one you need? The frame selector takes width, eave height, crane load and whether a column can stand inside, and tells you which configuration fits — and what the runner-up would have cost. Or go straight to quotes.