W8x18 (W200x27) Beam: Dimensions, Spans and Uses

A W8x18, called W200x27 in metric, is a wide-flange steel beam about 207 mm (8.14 in) deep with a 133 mm (5.25 in) flange, weighing 26.6 kg/m (18 lb/ft). It is one of the most common beams in Ontario houses. In the 2024 Ontario Building Code floor table, a W200x27 carrying one storey spans 5.8 m to 7.3 m (about 19 to 24 ft), and carrying two storeys 4.4 m to 6.4 m (about 14 to 21 ft), depending on how much floor it supports. Real spans for your house come from your designer or engineer, but those numbers show where this beam fits.

This page puts the W8x18's full dimensions, code spans, family members and typical uses in one place.

W8x18 dimensions and section properties

Values are from the CISC section tables (metric) and the AISC Shapes Database (imperial). Engineers use the properties; the dimensions matter to framers, fabricators and anyone fitting the beam into a pocket.

A few practical notes on those numbers:

• The depth is 207 mm, not 200 mm or 8 in. The "8" and "200" are family names. If the beam has to sit flush with joists or fit a pocket in a foundation wall, work from 207 mm. For comparison, a 2x10 joist is 235 mm (9-1/4 in) deep and a 2x8 is 184 mm (7-1/4 in).
• The 133 mm flange is a little narrower than a 2x6 (140 mm). That affects how a wood nailer sits on top, so tell your framer and fabricator the nailer size early.
• Ix drives deflection, Sx and Zx drive bending strength. These are what the engineer checks. Why metric and imperial names don't convert exactly is explained in how to read steel beam sizes.

W200x27 spans in the Ontario Building Code

For houses within Part 9, the 2024 OBC (Article 9.23.4.3., Table 9.23.4.3.) gives maximum spans for steel beams supporting floors in dwelling units. The table works on supported joist length: half the sum of the joist spans on both sides of the beam. Here is the W200x27 row, with its neighbours for comparison.

One storey supported (maximum span, m):

Two storeys supported (maximum span, m):

In feet, the W200x27 one-storey row runs from about 24 ft (with roughly 8 ft of supported joist) down to about 19 ft (with roughly 20 ft of supported joist). The two-storey row runs from about 21 ft down to about 14 ft.

Reading it with an example: a basement beam under joists that span 4.2 m on each side has a supported joist length of 4.2 m (half of 4.2 + 4.2). Supporting one storey, a W200x27 is allowed up to 6.3 m between supports; supporting two storeys, 5.3 m.

The table explains its own limits in the code's explanatory note: simply supported spans, a laterally supported top flange (joists bearing on it at 600 mm or less, with wood strips nailed along both sides), 350 MPa steel, and a deflection limit of span/360. Sentence 9.23.4.3.(2) requires the beam to at least meet CSA G40.21 Grade 350W; our structural steel grades guide covers what that means when ordering.

Two more cautions:

• W200x21 is in the code table but does not appear in the current CISC section tables we checked. Confirm it can actually be supplied before designing around it.
• Roofs and brick change everything. Where a beam also carries a roof and a brick-veneered wall, the OBC uses separate tables (9.23.4.3.-A to 9.23.4.3.-J). In Table 9.23.4.3.-A, for example, the W200x27 is limited to between 3.33 m and 3.80 m depending on the supported roof and floor lengths, well under its floor-only spans.

The full tables, including all the W150 to W310 sections, are in our steel beam span tables guide. Beams outside Part 9's scope, or with point loads, cantilevers or openings, are designed by an engineer.

The W8 family: how the W8x18 compares

The W8 (W200) family covers a wide range. The W8x18 sits in the middle of the light sections with a 133 mm flange.

Metric masses and Ix are from the CISC tables; small differences from the AISC metric figures come from rounding.

What the comparison shows:

• Going from W8x18 to W8x21 adds about 17 percent more weight for about 22 percent more stiffness, with nearly the same depth and flange. It is the usual step up when a W8x18 just misses.
• Going deeper usually beats going heavier. A W10x22 (W250x33) weighs 32.9 kg/m, only a little more than a W8x21, but has far more stiffness and spans much further in the tables above. The cost is about 50 mm more depth. Our W-beam prices by size post looks at how that trade-off plays out on a quote.
• The W8x31 is an H-shape (203 by 203 mm), more often used as a column or where a wide bearing flange is needed.

Where a W8x18 gets used

The W200x27 shows up on drawings for:

• Basement main beams (girders) carrying the floor joists down the middle of a house, usually on steel posts. See basement steel beam size for how the girder is chosen.
• Load-bearing wall removal, as a dropped or flush beam carrying the floor above, often with new posts and footings at each end.
• Openings in exterior walls, such as wider patio doors, where it may be combined with a plate or angle to carry brick.
• Deck, porch and carport beams, painted or otherwise protected for exterior use.
• Light commercial framing: mezzanine and platform beams, roof framing for small buildings, and support framing for equipment, all engineer-designed.

It is popular because it is light enough to handle in a house (about 80 kg for a 3 m length, 162 kg for 20 ft), shallow enough to save headroom, and stiff enough for typical residential spans.

Ordering a W8x18

When you request a quote, have this ready:

1. The designation as drawn, W200x27 or W8x18, and the grade if the drawings give one.
2. Length, measured from the drawings or on site, plus the bearing at each end.
3. Connections: cap and base plates for posts, holes for bolting to posts or a nailer, end plates or angles.
4. Camber or special notes shown on the drawings.
5. Delivery details: site access, how the beam gets into the building, and whether you need installation.

Never substitute a different size without the designer's approval. Two beams with similar weight can have quite different stiffness.

W8x18 beams cut, drilled and delivered

We supply and fabricate W-beams like the W200x27 for basements, wall removals and additions, with plates, holes and connections ready to install, and we supply and install basement beams and posts. Send your drawings or measurements for a free take-off and quote, or see our steel products.