By The Roofing Brief Team. Last reviewed: October 2026.
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. MiTek’s engineering design guidelines cap a parallel chord roof truss at a span of 24 times its depth (a 24 in deep truss spans no more than 48 ft), and IRC R802.10.1 requires the truss design drawings to be approved by the building official before the trusses are set. For more background, browse 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, and the MSR Lumber Producers Council lists it as a 2×4 floor or roof truss that can be designed with a chase. Under IRC R802.10.2, metal-plate-connected wood trusses must be designed and manufactured to ANSI/TPI 1. 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 in the loads it carries, in the slight slope built into the top chord, and in its depth limit: MiTek caps floor trusses at a span-to-depth ratio of 20 (a 14 in deep floor truss spans no more than 23 ft 4 in) and parallel chord roof trusses at 24. 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 defines a low-slope roof as one under 2 units in 12 (17 percent, about 9.5 degrees), and IRC R905.12.1 requires single-ply membranes to have a design slope of at least 1/4 unit in 12.
Deflection matters here too. An SBCA analysis of the truss design note on ponding walks through a 30 ft truss designed at 1/4 inch per foot: once dead load deflects it, part of the span drops below 1/4 inch per foot, which makes it a susceptible bay that the IBC requires to be evaluated for ponding instability. Camber, a steeper design slope or tapered insulation keeps the finished surface above that threshold.
Flat roof truss decision table
Use the span, load and ceiling below the truss to pick the framing type and the limits the design has to meet.
| If | Then | Source |
|---|---|---|
| Span fits the sawn joist tables (up to 21 ft 8 in for a DF-L No. 2 2×12 at 16 in, 30 psf ground snow) | Sawn joists work; a truss is optional | IRC Table R802.4.1(3) |
| Span past the sawn joist limit, residential loads | Wood parallel chord roof truss at least span/24 deep (15 in for 30 ft) | MiTek engineering guideline |
| Ducts or plumbing must cross the framing | Parallel chord truss designed with a chase | MSR Lumber Producers Council |
| Long commercial span or noncombustible framing required | Open-web steel joists, span no more than 24 x depth | SJI 100-2020, 5.2 |
| Design slope 1/4 in per ft or less | Ponding instability evaluation (susceptible bay) | IBC; SBCA ponding analysis |
| Plaster or stucco ceiling hung from the truss | Live load deflection limit L/360 | IBC Table 1604.3 |
| No ceiling attached | L/180 code limit; MiTek designs roof trusses to at least L/240 live load | IBC Table 1604.3; MiTek |
| A truss member needs cutting or notching in the field | Stop: approval from a registered design professional first | IRC R802.10.4 |
Flat roof truss types
Flat roof trusses are named for how the web members are arranged between the parallel chords. The MSR Lumber Producers Council notes that chord size and web configuration are set by span, load and spacing, which is why the truss designer, not the buyer, picks the final pattern.
| 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 |
In a Pratt pattern under gravity load the verticals work in compression and the diagonals in tension; in a Warren pattern the diagonals alternate between the two. A girder truss carries the ends of several other trusses, and IRC R802.10.1 requires the truss-to-girder-truss and ply-to-ply connections to appear on the design drawings.
How far can a flat roof truss span?
A flat roof truss span depends on its depth. MiTek’s guideline sets a maximum span-to-depth ratio of 24 for parallel chord roof trusses, and the Steel Joist Institute specification (SJI 100-2020, section 5.2) sets the same 24-times-depth cap for open-web steel joists. Dividing the span in inches by 24 gives the shallowest depth either rule allows; deflection limits and snow load usually push the engineered depth deeper.
| Clear span | Shallowest depth allowed (span divided by 24) | Floor truss at the same span (span divided by 20) | Source |
|---|---|---|---|
| 16 ft | 8 in | 9.6 in | MiTek guideline; SJI 100-2020 5.2 |
| 20 ft | 10 in | 12 in | MiTek guideline; SJI 100-2020 5.2 |
| 24 ft | 12 in | 14.4 in | MiTek guideline; SJI 100-2020 5.2 |
| 30 ft | 15 in | 18 in | MiTek guideline; SJI 100-2020 5.2 |
| 40 ft | 20 in | 24 in | MiTek guideline; SJI 100-2020 5.2 |
| 48 ft | 24 in | 28.8 in | MiTek guideline; SJI 100-2020 5.2 |
The exact span for a given depth, load and spacing comes only from the manufacturer’s span tables or sealed engineering. Deflection limits, not just strength, often set the final depth: IBC Table 1604.3 limits roof member deflection under live or snow load to L/180 where the member supports no ceiling, L/240 under a nonplaster ceiling and L/360 under a plaster or stucco ceiling, and designers often tighten the limit on flat roofs to limit ponding.
The depths above are rule minimums, not designs. IRC R802.10.2.1’s prescriptive truss provisions cover only roof slopes from 3:12 to 12:12, so every flat truss is engineered to ANSI/TPI 1 for the actual snow load, spacing and wind; MiTek’s guideline also sets L/240 live load and L/180 total load as its minimum roof truss deflection limits.
Wood vs steel flat roof trusses
Wood parallel chord trusses cost $60 to $500 each and steel trusses $150 to $700 each in HomeGuide’s 2026 price guide, so wood is the usual pick on homes, garages and small additions. Open-web steel joists take over on long clear spans, heavier commercial loads and buildings that need noncombustible framing. Span, load and fire rating decide; both materials share the same 24-times-depth span cap. For pitched steel options on barns and garages, see our guide to metal roof trusses.
| Factor | Wood parallel chord | Steel / open-web |
|---|---|---|
| Span cap | 24 x depth (MiTek guideline) | 24 x depth (SJI 100-2020 5.2) |
| Cost per truss (HomeGuide, 2026) | $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 |
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. Sawn joists cover spans up to the IRC table limits: 26 ft 0 in for a Douglas fir-larch No. 2 2×12 at 16 in on center under a 20 psf roof live load, and 21 ft 8 in under a 30 psf ground snow load (IRC Tables R802.4.1(1) and (3)). Past those spans, trusses or engineered joists take over.
- 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; below 3:12, IRC R802.4.4 requires the ridges, hips and valleys that support rafters to be designed as beams.
For the framing detail behind solid-joist flat roofs, including the four ways to build 1/4 inch per foot of slope into a level structure, see our guide on flat roof framing.
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, per HomeGuide’s 2026 roof truss price guide. Installed, HomeGuide puts roof trusses at $5 to $14 per square foot, or $7,500 to $35,000 on average, in line with our roof truss cost breakdown. Flat trusses sit within that range and price up with longer spans.
Spacing is the easiest driver to quantify. The worked example below prices a 24 ft by 40 ft garage roof (960 sq ft) with HomeGuide’s 2026 ranges; a manufacturer’s quote against the sealed design replaces these numbers for a real roof.
| Line | 24 in on center | 16 in on center | Source |
|---|---|---|---|
| Trusses along a 40 ft wall | 21 (20 spaces) | 31 (30 spaces) | Layout arithmetic |
| Wood truss material at $60 to $500 each | $1,260 to $10,500 | $1,860 to $15,500 | HomeGuide 2026 per-truss range |
| Installed at $5 to $14 per sq ft | $4,800 to $13,440 | $4,800 to $13,440 | HomeGuide 2026 installed range |
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. Which route you pick drives the insulation package, and our breakdown of flat roof insulation covers the assembly options and where the tapering actually goes. The sequence below is the standard order for a small residential or garage roof.
- Confirm the truss design drawings delivered with the shipment (IRC R802.10.1) match 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 design drawings; where they are silent, IRC R802.10.3 points to the SBCA BCSI guide. Never cut, notch or drill a truss member without a registered design professional’s approval (IRC R802.10.4).
- 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. Asphalt shingles need at least 2:12 (IRC R905.2.2), so a flat truss roof at 1/4 to 1/2 inch per foot takes a single-ply, modified bitumen or built-up membrane; 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. Either way, IRC R802.10.2 requires the wood truss to be designed and manufactured to ANSI/TPI 1.
How far can a flat roof truss span?
A flat roof truss can span up to 24 times its depth: MiTek’s guideline sets that cap for wood parallel chord roof trusses (a 24 inch deep truss spans no more than 48 feet), and SJI 100-2020 section 5.2 sets the same cap for open-web steel joists. 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?
Within the IRC span tables, sawn joists usually cost less than engineered trusses, which HomeGuide prices at $60 to $500 each in wood for 2026. Once the span passes what a sawn 2×12 can reach (21 ft 8 in for Douglas fir-larch No. 2 at 16 inches on center under a 30 psf ground snow load), trusses or engineered joists are the code-compliant option, and their open web carries ducts and wiring.
What is the depth of a flat roof truss?
The shallowest flat roof truss allowed is the span divided by 24, per MiTek’s guideline for parallel chord roof trusses: 10 inches for a 20-foot span and 12 inches for a 24-foot span. The engineered depth is often deeper, because the truss also has to meet deflection limits of L/240 live load and L/180 total load and the local snow load.