A basement support beam is the main beam running down the middle of a basement, carrying the floor joists above and resting on steel posts and the foundation walls at each end. In most houses it is either a steel W beam or a built-up wood beam, and when the floor above sags or bounces, this beam and its posts are usually the first place to look.
This guide explains how the beam fits into the house’s load path, how steel and wood beams compare, what the warning signs look like, and what replacing or adding a beam involves. One rule runs through all of it: a licensed structural engineer has to size any beam or post you add or replace, because the right size depends on loads and spans that cannot be guessed from the basement.
What the Basement Beam Actually Does
Floor joists rarely span the full width of a house. Instead, they run from the outside foundation walls to a beam near the middle, which cuts their span roughly in half. That beam, sometimes called the main beam or girder, collects load from the joists on both sides and delivers it to a row of posts and to pockets in the foundation walls at its ends. Each post sits on a footing that spreads the load into the soil.
In a two-story house the beam may also carry a bearing wall from the floor above, which carries part of the second floor and sometimes the roof. That is why the beam often lines up with a central hallway wall upstairs. You will hear the same member called a girder, main beam or carrying beam.
Steel vs Wood Basement Beams
HUD’s Residential Rehabilitation Inspection Guide notes that metal structural parts in small residential buildings are usually limited to beams and pipe columns in basements, angles over small masonry openings, and beams over long spans elsewhere (HUD Residential Rehabilitation Inspection Guide, checked October 2026). In other words, the basement beam is one of the few places steel shows up in an ordinary house. Wood is just as common. Here is how the main options compare:
| Beam type | Depth for a given load | Handling | Typical issues |
|---|---|---|---|
| Steel W beam | Shallowest | Heavy one piece; usually needs several people or a lift | Rust where it sits on damp masonry; posts not fastened |
| Built-up dimensional lumber (2x10s or 2x12s nailed together) | Deepest | Assembled in place from light pieces | Sag over time, splits, plies not fastened, rot at wall pockets |
| LVL (laminated veneer lumber), built up in plies | Between the two | Plies carried in separately and fastened together | Must be fastened to the manufacturer’s pattern; keep dry |
| Flitch beam (steel plate between wood) | Shallower than wood alone | Heavy; bolted together | Bolt pattern is engineered; not a DIY assembly |
Why steel
Steel’s advantage is depth. For the same load and span, a steel beam is shallower than wood, which buys headroom in a basement where every inch counts. Steel does not shrink, creep under long-term load or rot. HUD’s guide puts it plainly: beams in small residential buildings rarely deflect, and if a steel beam is found deflected, the cause should be found and supports or plates added.
Why wood or LVL
Wood can be carried in one board at a time through a basement door, cut with ordinary tools and fastened to existing framing with nails and screws. Engineered lumber is more predictable than sawn lumber. Weyerhaeuser’s page on Microllam LVL beams says they resist warping, splitting and shrinking and can be built up on site to reduce heavy lifting (checked October 2026). A steel plate sandwiched between wood is a third option, explained in our guide to the flitch beam.
What Steel Basement Beam Sizes Look Like
Steel beams in houses are usually small wide flange shapes. The table shows three shallow W shapes with their real dimensions so you can recognize them and measure what you have. It is not a sizing guide: which shape a house needs depends on span, post spacing and the loads above, and only an engineer can work that out.
| Shape | Weight (lb/ft) | Depth d (in) | Flange width bf (in) | Ix (in4) | Weight of a 16 ft length (lb) |
|---|---|---|---|---|---|
| W8x18 | 18 | 8.14 | 5.25 | 61.9 | 288 |
| W8x24 | 24 | 7.93 | 6.50 | 82.7 | 384 |
| W10x26 | 26 | 10.3 | 5.77 | 144 | 416 |
Dimensions and Ix from the AISC Shapes Database v16.0. The 16 ft weight is simply the nominal weight per foot times 16. To identify a beam already in your basement, measure the depth and the flange width and compare with our guide to reading steel beam sizes. Older houses may have S beams (American Standard I-beams) with sloped inner flange faces, which are a different shape family.
Signs Your Basement Beam or Posts Need Attention
HUD’s guide lists the usual causes of sagging or sloping floors as foundation settlement, excessive spans, cut or drilled structural members, overloading, and removal of supporting walls or columns, and says each case has to be diagnosed separately. What you can see from the basement:
- A visible dip in the beam. Sight along the bottom of the beam or stretch a string line from end to end.
- A floor that slopes toward the middle of the house. HUD suggests rolling a marble or using a level to find the direction and amount of slope.
- Posts that are tilted, rusted at the base, or not fastened. HUD’s guide says columns should be checked for connections at top and bottom and for corrosion at the base, and that off-center loading or tilting should be remedied.
- Crushing or loss of bearing. The beam end slipping in its wall pocket, wood crushed over a post, or shims that have worked loose.
- Splits, rot or insect damage in a wood beam, especially where it sits in a damp masonry pocket.
- Cuts and holes made for ducts, drains or wiring.
One point from HUD’s guide is worth knowing before anyone starts jacking: once a wood member is permanently deflected it cannot be straightened. Deflection can be stopped by adding supports or strengthening, but a sagged wood beam does not come back to straight just because it is pushed up. Where the floor is jacked to level, HUD advises doing it slowly and carefully.
The Posts Matter as Much as the Beam
Many basement beam problems are really post problems. A post that has settled because there is no real footing under it, a temporary screw jack left in place for decades, or a post rusted through at the floor will let any beam sag. Our guide to lally columns covers capacity, adjustable jack posts and when a post should be replaced. Adding a post in the middle of a long span is often the simplest fix, but the new post needs a footing sized for the load, and the beam has to be checked for the new support point.
Replacing or Adding a Basement Beam
The work usually follows the same order:
- Get it engineered. A licensed structural engineer works out the loads, sizes the beam, the posts and the footings, and details the connections and bearing at the walls.
- Pull the permit. Structural work on beams and posts normally needs a building permit and inspection.
- Shore first. Temporary posts and beams on either side carry the floor while the old beam is removed. Jack in small increments.
- Footings, then posts, then beam. New footings are poured and cured before they are loaded. The beam goes in, the posts are set and fastened top and bottom, and the load is transferred.
- Inspect and finish. Only close in after the inspector has seen the beam, posts and connections.
This is the same process as taking out a bearing wall upstairs, and our guide to removing a load-bearing wall covers the shoring and permit steps in more detail. To estimate how heavy a steel beam will be to carry in, use the steel beam weight calculator.
Basement Support Beam FAQ
What size steel beam do I need for my basement?
It depends on the span between posts, the width of floor the beam supports, any walls or floors above it, and the post and footing layout. A licensed structural engineer has to size it. Generic span tables found online do not account for your house’s actual loads.
Is a steel beam better than a wood beam in a basement?
Steel is shallower for the same load, does not shrink or creep and does not rot, which helps headroom and long spans. Wood and LVL are lighter to handle and easier to fasten to existing framing. Both work when they are engineered and installed correctly.
Can a sagging basement beam be jacked back up?
The floor can often be raised slowly, but HUD notes that a permanently deflected wood member cannot be straightened. The usual fix is to add support or strengthen the beam once the cause is found, not just to jack it.
Can I add another post under my basement beam?
Often, yes, and it is a common fix for a sagging span. The new post needs its own footing, needs to be fastened at top and bottom, and the beam needs to be checked for the new support. Have an engineer specify it.
Do I need a permit to replace a basement beam?
In most places, yes. Replacing or adding a structural beam, post or footing is structural work, and local building departments normally require a permit and inspection for it.
The Short Version
The basement support beam carries the middle of the floor above and passes the load through posts and footings to the ground. Steel W beams are shallow and stable, while wood and LVL are lighter to handle. Sags, tilted or rusted posts and lost bearing are the warning signs, and the posts are often the real problem. Any new or replacement beam, post or footing has to be sized by a licensed structural engineer and permitted.
Reference information only, not engineering advice. Disclaimer.