Flat roof trusses are parallel chord trusses, meaning the top and bottom chords run parallel instead of meeting at a peak, with steel connector plates or steel webs holding a web of diagonals in between. They frame flat and low-slope roofs on garages, additions, and light-commercial buildings, spanning farther than solid joists while a small built-in slope carries water off the roof. This guide covers what they are, how far they span by depth, wood versus steel, and 2026 cost. It sits within our learn about roofing library.
What is a flat roof truss?
A flat roof truss is a prefabricated structural component with parallel top and bottom chords joined by a triangulated web. The parallel chords are why engineers call it a parallel chord truss. The triangles convert roof loads (dead, live, snow, wind) into tension and compression along the members, so the truss spans farther than a solid beam of the same weight.
The same parallel-chord design is used for floors, where it is called a floor truss. The roof version differs mainly in the loads it carries and in the slight slope built into the top chord. Trusses are engineered to a signed and sealed design for a specific span, spacing, and load, then delivered to the site ready to set. For how trusses compare with rafters across all roof shapes, see our overview of roof trusses.
Are flat roof trusses actually flat?
No. A flat roof truss is never dead level. The top chord is pitched a small amount, typically between 1/4 inch and 1/2 inch of fall per foot, so rain drains toward the eaves or interior drains instead of ponding. A truly level roof holds standing water, which overloads the structure and fails the membrane early.
Builders create the slope in one of two ways. The top chord can be manufactured at a slight angle (a sloping flat truss or mono truss), or the whole truss can be set level and tapered insulation added on top to build the fall. The International Building Code treats anything below 2/12 (about 9.5 degrees) as low-slope, and most flat trusses land well under that.
Camber matters here too. Longer wood and steel trusses are built with an upward camber, often 1 to 2.5 inches, so that once the roof loads deflect them, they settle closer to the intended plane rather than sagging into a low spot that collects water.
Flat roof truss types
Flat roof trusses are named for how the web members are arranged between the parallel chords. The pattern controls how the truss handles load and how far it spans. The table below compares the common configurations.
| Truss type | Web pattern | Best for |
|---|---|---|
| Warren | Diagonals forming a row of triangles, few or no verticals | Long, efficient spans with minimal material |
| Pratt | Vertical webs plus diagonals sloping toward the center | Heavier loads where verticals carry compression |
| Howe | Diagonals sloping toward the ends | Shorter spans and specific load paths |
| Sloping flat (mono) | Top chord built at a slight pitch | Roofs needing drainage slope framed in |
| Open-web / girder | Deeper webs, often carrying other trusses | Long clear spans and load-gathering points |
Warren and Pratt patterns dominate residential and light-commercial flat roofs because they balance span, depth, and cost. A girder truss shows up where one member has to carry the ends of several others, such as over an open living space.
How far can a flat roof truss span?
A flat roof truss span depends on its depth. The working rule for economical wood roof trusses is a span-to-depth ratio near 1:12, so a truss needs roughly one inch of depth for every foot of span. Deeper trusses span farther. Steel and open-web trusses beat wood at long spans because steel carries more load per inch of depth.
| Clear span | Approx. wood truss depth (1:12) | Typical material |
|---|---|---|
| 12 ft | 12 in | Wood parallel chord |
| 16 ft | 16 in | Wood parallel chord |
| 20 ft | 20 in | Wood parallel chord |
| 24 ft | 24 in | Wood parallel chord |
| 30 ft | 30 in | Deep wood or open-web steel |
| 40+ ft | 40 in or more | Steel or open-web steel joist |
Wood parallel chord trusses are practical to roughly 30 to 35 feet before the depth grows awkward. A 12-inch wood truss might reach about 20 feet under a 30 psf live load, while a similar steel truss can push past 30 feet at the same depth. Deflection limits, not just strength, often set the final depth: the IBC requires at least L/240 under live load, and designers often tighten that to L/360 on flat roofs to limit ponding.
These figures are planning estimates. Actual span, depth, and web layout come from the truss manufacturer’s sealed engineering for your snow load, spacing, and building code.
Wood vs steel flat roof trusses
Wood parallel chord trusses are cheaper and standard on homes, garages, and small additions. Steel or steel open-web joists take over on long clear spans and heavier commercial loads because they hold depth without adding weight. Span is the deciding factor: choose wood under about 30 feet, look at steel beyond it.
| Factor | Wood parallel chord | Steel / open-web |
|---|---|---|
| Practical span | Up to ~30-35 ft | 30 ft to 60+ ft |
| Cost per truss | $60 to $500 | $150 to $700 |
| Weight | Lighter, hand-set on small jobs | Heavier, often craned |
| Fire / pests | Combustible, treatable | Noncombustible, no rot or termites |
| Typical use | Residential, garages, additions | Commercial, warehouse, long spans |
Parallel chord trusses in either material often use less framing than a solid beam for the same span, but the precise cuts and connector plates raise the per-piece manufacturing cost. That trade, less material but more fabrication, is why trusses win on long spans and lose to a simple joist on short ones.
Flat roof truss vs joists vs rafters
Flat roof trusses span farther and leave the interior open, but cost more per piece than dimensional lumber. Solid joists (2x lumber or engineered I-joists) are cheaper for short spans under about 16 feet. Rafters suit pitched roofs, not flat ones. The choice comes down to span, clear-space needs, and whether ducts and wiring need to run through the framing.
- Flat roof trusses: longest spans, open web lets HVAC and plumbing pass through, engineered and delivered ready to set.
- I-joists and dimensional joists: lowest cost for short spans, solid web means chases must be cut or added, simple to source.
- Rafters: for sloped roofs only; a flat or low-slope roof uses trusses or joists, not conventional rafters.
For the framing detail behind solid-joist flat roofs, including how slope is built into a level structure, see our guide on flat roof framing. This truss guide and that framing guide cover two different ways to build the same low-slope deck.
How much do flat roof trusses cost?
Wood flat roof trusses run about $60 to $500 each for the material, and steel trusses run $150 to $700 each, driven mostly by span and depth. Installed, roof trusses average $5 to $14 per square foot, or roughly $7,500 to $35,000 for a full roof, per 2026 contractor pricing, in line with our roof truss cost breakdown. Flat trusses sit within that range and price up with longer spans.
Cost drivers include span (longer needs deeper, heavier trusses), spacing (16-inch on center uses more trusses than 24-inch), load (higher snow or commercial loads add material), and delivery or crane fees for long or heavy units. Getting the manufacturer’s quote against a sealed design is the only way to price a specific roof.
How flat roof trusses are installed
Flat roof trusses are set on the bearing walls at the engineered spacing, usually 24 inches on center, braced, then sheathed. The slope is either built into the truss or added with tapered insulation before the membrane goes on. The sequence below is the standard order for a small residential or garage roof.
- Confirm the sealed truss design matches the span, spacing, and load on the plans.
- Mark truss layout on both bearing walls, typically at 24 inches on center.
- Set each truss upright on its bearings, keeping the sloped chord oriented for drainage.
- Install permanent lateral and diagonal bracing per the truss engineer’s bracing plan.
- Sheath the top chords with the specified deck, then add tapered insulation if slope was not framed in.
- Apply the flat or low-slope membrane and flashing to finish the drainage path.
What sits on top is a separate decision. Most flat truss roofs get a single-ply or low-slope membrane rather than shingles; our low-slope roofing guide covers the material options below 4/12 pitch.
Frequently asked questions
What is a flat roof truss called? A flat roof truss is called a parallel chord truss, because its top and bottom chords run parallel rather than meeting at a ridge. The same design used horizontally for floors is called a floor truss. When the top chord is pitched slightly for drainage, it may be called a sloping flat truss or mono truss.
How far can a flat roof truss span? Wood parallel chord flat trusses span practically up to about 30 to 35 feet, using roughly one inch of depth per foot of span (a 1:12 ratio). Steel and open-web steel trusses span from 30 feet to 60 feet or more. Deflection limits and snow load, set in the manufacturer’s engineering, fix the exact span and depth.
Are flat roof trusses truly flat? No. Flat roof trusses carry a small built-in slope, usually 1/4 to 1/2 inch of fall per foot, so water drains instead of ponding. The slope is either manufactured into the top chord or created with tapered insulation on a level truss. A dead-level roof holds standing water and fails early.
Are flat roof trusses cheaper than joists? For short spans under about 16 feet, solid joists are usually cheaper because the lumber costs less than an engineered truss. Flat trusses become the better value on longer spans, where they use less total material and leave an open web for ducts and wiring. Span length is the main cost pivot.
What is the depth of a flat roof truss? Flat roof truss depth follows roughly the 1:12 span-to-depth rule, so a 20-foot span needs about a 20-inch deep truss and a 24-foot span about 24 inches. Deeper trusses span farther and deflect less. Steel trusses hold more span per inch of depth than wood, which is why long spans switch to steel.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.