Metal trusses are the basis of a strong and durable roof for a garage, house, hangar or utility block. Their correct calculation ensures that the structure will withstand snow loads, gusts of wind and the weight of the roofing material without deformation. But design errors lead to deflections, corrosion in components, or even collapse. This article will help you figure out how calculate a metal truss independently - from determining loads to selecting profiles and creating working drawings.

Many people mistakenly believe that it is enough to take a β€œproven” scheme from the Internet and scale it to fit your size. However every farm is unique: it depends on the climate zone, the angle of the roof, the type of roof and even the material of the building walls. We will analyze real formulas, give examples of calculations for different regions of Russia and point out critical errors that lead to 80% of emergency situations.

In the article you will find:

  • πŸ“ Step-by-step calculation algorithm with formulas and coefficients
  • πŸ—οΈ Load tables for snow and wind by regions of the Russian Federation (SNiP 2026)
  • πŸ”§ Examples of selecting profile sections for trusses 6–12 meters long
  • ⚠️ Typical mistakes that self-builders make (with photos of the consequences)

1. Types of metal roof trusses: which one to choose?

Farms are classified according to belt configurations, type of grille and purpose. For private construction, 3 main types are relevant:

  • 🏠 Triangular - the simplest for single-pitched and gable roofs (angle 20–45Β°). Suitable for spans up to 8 m. Disadvantage: high metal consumption due to high altitude.
  • 🏭 Polygonal (polygonal) β€” optimal for spans of 9–15 m. They distribute the load evenly, but are difficult to manufacture.
  • 🚜 With parallel belts β€” used for flat roofs (slope up to 10Β°). They require a reinforced lattice, as they can withstand bending moments.

For garages and small houses they are often chosen triangular trusses with a lattice of braces (system Warren or Pratt). For hangars and industrial buildings - polygonal with cross grid.

Important: truss height should be 1/6–1/8 of the span length. For example, for a roof 9 m wide, the minimum height of the ridge truss is 1.1–1.3 m.

πŸ“Š What type of truss are you planning to use?
Triangular
Polygonal
With parallel belts
I haven't decided yet

2. Loads on the truss: how to calculate snow, wind and roof weight

The farm must withstand three types of loads:

  1. Permanent - the weight of the truss itself, roofing material, sheathing, insulation.
  2. Temporary (snow, wind) - depend on the region and the angle of the roof.
  3. Special β€” seismic activity (for zones 7+ points), ice.

Let's take a closer look snow and wind load β€” they are critical for Russia. We take the data from the updated SP 20.13330.2018 (analogous to SNiP β€œLoads and Impacts”).

2.1. Snow load (S)

Formula:

S = Sβ‚€ Γ— ΞΌ

where:

  • Sβ‚€ β€” standard weight of snow per 1 mΒ² (see table below).
  • ΞΌ β€” coefficient of transition from the weight of snow on the ground to the load on the roof (depending on the angle of inclination).
Region of the Russian Federation Sβ‚€ (kg/mΒ²) Examples of cities
I (southern) 50–80 Krasnodar, Sochi, Rostov-on-Don
III (central) 120–180 Moscow, St. Petersburg, Kazan
V (northern) 240–320 Murmansk, Norilsk, Yakutsk

Coefficient ΞΌ:

  • Tilt angle ≀ 25Β° β†’ ΞΌ = 1.
  • 25Β° < angle ≀ 60Β° β†’ ΞΌ = 0,7.
  • Angle > 60Β° β†’ snow is not taken into account (ΞΌ = 0).
πŸ’‘

For roofs with a slope of 30–45Β°, the snow load can be reduced by 30% due to natural snow melting. But this does not apply to regions with frequent thaws (for example, the Leningrad region).

2.2. Wind load (W)

Formula:

W = Wβ‚€ Γ— k Γ— c

where:

  • Wβ‚€ β€” standard wind pressure (see table).
  • k β€” coefficient of pressure change with height (for buildings up to 10 m = 0.75).
  • c β€” aerodynamic coefficient (depending on the shape of the roof).
Region of the Russian Federation Wβ‚€ (kg/mΒ²) Examples of cities
I (protected) 23–30 Kaliningrad, Pskov
IV (open) 35–42 Omsk, Novosibirsk
VII (coastal) 55–70 Vladivostok, Nakhodka

For gable roofs with an angle of 30–45Β° aerodynamic coefficient c:

  • Windward ramp β†’ c = -0,6 (flow separation).
  • Leeward slope β†’ c = +0,3 (pressure).
⚠️ Attention: Wind load can be negative (vacuum), which leads to β€œswelling” of the roof. This is critical for lightweight structures (corrugated sheets, metal tiles).

3. Calculation of forces in truss rods: method of cutting nodes

After determining the loads, you need to find forces in each truss rod. For this use:

  1. Knot cutting method β€” suitable for simple trusses (up to 6–8 knots).
  2. Moment point method (Ritter) - for complex structures.

Let's sort it out knot cutting method using the example of a triangular truss with a span of 6 m and a pitch of 1.5 m:

Example farm diagram

The truss consists of 5 nodes (A, B, C, D, E) and 7 rods. Upper belt: AB, BC, CD; lower belt: AD, DE; braces: BD, BE, CE. The load is applied in nodes B and C (300 kgf each).

Calculation algorithm:

  1. We determine the support reactions (RA and RE) from the moment equations.
  2. We cut out node A and compose the equilibrium equations (βˆ‘X=0, βˆ‘Y=0).
  3. We move successively to nodes B, C, D, E, finding the forces in the rods.

Formulas for node A:

βˆ‘Y = RA + S₁ Γ— sin(Ξ±) = 0

βˆ‘X = S₁ Γ— cos(Ξ±) + Sβ‚‚ = 0

where S₁ and Sβ‚‚ β€” forces in rods AB and AD, Ξ± β€” angle of inclination of the braces.

All nodes are in equilibrium (βˆ‘X=0, βˆ‘Y=0)|

The forces in symmetrical rods are equal in magnitude |

Compressive forces are marked with a β€œβ€“β€ sign, tensile forces with a β€œ+” sign |

Sum of projections of all forces onto the axis = 0-->

⚠️ Attention: If it turns out in the rod zero effort, this is not always an error. In triangular lattice trusses, some elements can be "unloaded" under symmetrical loading.

4. Selecting a profile section: how not to make a mistake with metal

After calculating the efforts, we select cross section of rods according to two criteria:

  1. Strength - check using the formula:
Οƒ = N / A ≀ R

where:

  • N β€” force in the rod (kgf),
  • A - cross-sectional area (cmΒ²),
  • R β€” design resistance of steel (for S245 = 2100 kgf/cmΒ²).

For compressed rods, we additionally check sustainability:

Οƒ = N / (Ο† Γ— A) ≀ R

where Ο† β€” longitudinal bending coefficient (depends on the flexibility of the rod Ξ» = l / r).

Recommended profiles for trusses:

  • πŸ”Ή Corner β€” 50Γ—50Γ—4 mm (for braces and racks).
  • πŸ”Ή Rectangular pipe β€” 40Γ—60Γ—2 mm (for belts).
  • πŸ”Ή Channel No. 8–10 (for support units).
Rod type Max. force (kgf) Recommended Profile Weight 1 m (kg)
Upper belt (compression) 1500 Pipe 60Γ—60Γ—3 mm 5,3
Lower belt (stretch) 2000 Corner 63Γ—63Γ—5 mm 4,8
Braces 800 Corner 40Γ—40Γ—4 mm 2,4

To save metal, use variable section profiles: in the middle part of the truss (where the forces are less) the wall thickness can be reduced by 20–30%.

πŸ’‘

For trusses with a span of more than 10 m, be sure to use prestressed lower chords (made of reinforcement or high-strength steel). This reduces deflections by 40%.

5. Truss nodes: how to connect elements correctly

Errors in node connections - the reason 70% of farm collapses. Basic fastening methods:

  • πŸ”© Bolted connections - easy to install, but require regular checking of tightness. Use bolts of strength class 8.8 or 10.9.
  • πŸ”₯ Welding β€” reliable, but creates residual stresses. For critical nodes, use fillet welds with leg β‰₯ 5 mm.
  • πŸ”— Rivets - used in factory farms (for example, LMK). Not suitable for self-building.

Requirements for nodes:

  • Minimum overlap of elements in a bolted connection - 1,5 Γ— d (where d - bolt diameter).
  • In welded assemblies, avoid β€œcross” intersections of seams - this reduces the strength by 30%.
  • For corners use gussets (oblique overlays) β‰₯ 6 mm thick.

Example of calculation of a bolted connection:

For a rod with force N = 2000 kgf and corner 63Γ—63Γ—5:

  1. Required bolt area: A = 2000 / (0.8 Γ— 2100) = 1.19 cmΒ².
  2. Select an M12 bolt (sectional area 1.13 cmΒ²) β€” doesn't fit (needs M14).
⚠️ Attention: At nodes with coaxial joint (for example, the connection of the upper chord) the bolts work for shear. Their number is increased by 50% compared to tensile calculations.

6. Drawings and 3D models: how to avoid design errors

Even correct calculations are useless without precise drawings. Recommendations:

  • πŸ“ Use programs: AutoCAD, Compass-3D or free FreeCAD.
  • πŸ” Indicate on the drawings:
    • Coordinates of all nodes (accurate to mm).
    • Lengths and sections of rods.
    • Types of connections (M12 bolts, K5 welding, etc.).
  • πŸ–₯️ For complex trusses, create a 3D model in SketchUp or SolidWorks - this will help identify collisions.

Example of drawing structure:

[Top view] β†’ Span dimensions, truss spacing (1–1.5 m).


[Side view] β†’ Angle of inclination, height at ridge.


[Nodes A, B, C] β†’ Detailing of connections with gusset sizes.


[Specification] β†’ List of profiles with quantity and weight.

Typical errors in drawings:

  • Not specified tolerances to the length of the rods (Β±2 mm).
  • Missing marking rods (for example, B1, P3).
  • Not drawn mounting holes for fastening to the Mauerlat.
πŸ’‘

Before production, print out the drawings at a scale of 1:1 and assemble a mock-up of the truss from cardboard. This will help check the geometry and avoid mistakes when cutting metal.

7. Common mistakes and how to avoid them

Analysis of emergency cases shows that 90% of problems arise due to five key mistakes:

  1. Unaccounted loads:

    Example: in the Moscow region they did not take into account the snow load of 240 kg/mΒ² (instead of 180 kg/mΒ² according to β€œaverage” data). The result is a truss deflection of 15 cm in winter.

  2. The metal is too thin:

    Using a 35Γ—35Γ—3 mm angle instead of 50Γ—50Γ—4 mm leads to loss of stability compressed rods.

  3. Incorrect nodes:

    Welding without edges or M10 bolts instead of M12 are the cause of 60% of collapses.

  4. No stiffening links:

    Trusses without horizontal connections β€œwalk” along the lower belt under wind loads.

  5. Corrosion in nodes:

    Lack of anti-corrosion treatment (for example, Tsinkol) reduces the service life of the farm by 2–3 times.

How to check a finished truss before installation:

All rods correspond to the drawings in terms of cross-section and length|

There are no cracks in the welds (check with magnetic particle method)|

Bolts are tightened to 70–90 Nm (use a torque wrench)|

The nodes are treated with anti-corrosion (especially at the joints) -->

FAQ: Answers to frequently asked questions

Is it possible to use wooden trusses instead of metal ones?

Wooden trusses are cheaper, but inferior to metal ones in:

  • πŸ”Ή Strength: maximum span - 8 m (for metal - up to 30 m).
  • πŸ”Ή Durability: require treatment with antiseptics every 3-5 years.
  • πŸ”Ή Fire hazards: metal does not burn, wood - flammability group G2-G3.

Wood is justified for light roofs (gazebos, verandas) in dry climates.

How to calculate a truss for a roof with an attic?

For mansard roofs (broken structure), consider:

  1. Additional load from attic floor (150–200 kg/mΒ²).
  2. Changed center of gravity β€” moves down, which increases the load on the walls.
  3. Insulation: The weight of the mineral wool (20–50 kg/mΒ³) is added to the permanent loads.

Recommended scheme: polygonal truss with a broken top chord and reinforced racks.

Do I need to coordinate the truss with the architect?

Yes, if:

  • πŸ“„ The building belongs to capital buildings (house, garage with foundation).
  • πŸ—οΈ Truss span > 12 m or building height > 7 m.
  • 🏒 Construction is underway in seismic zones (Crimea, Kamchatka, Baikal).

For outbuildings (shed, shed) approval is not required, but the drawings must comply SP 20.13330.2018.

How to save on metal without losing strength?

5 proven methods:

  1. Use pipes instead of corners - they are 15% lighter with the same strength.
  2. Apply perforated profiles (for example, punch corner) - metal savings up to 20%.
  3. Enlarge truss step up to 1.5–2 m (but not more than 1/20 of the span).
  4. Replace welding with bolted connections - it’s 10–15% cheaper.
  5. Buy metal in bulk from a manufacturer (for example, MMK or Severstal) β€” discount up to 30%.
What paint should I use to protect my truss?

Optimal options:

  • πŸ”΄ Primer-enamel Hammerite β€” applied without preliminary primer, service life 10+ years.
  • 🟠 Zinc-rich paints Zinga β€” create cathodic protection, like galvanizing.
  • 🟒 Polyurethane enamels Polyton β€” for aggressive environments (coastal zones).

Application technology:

  1. Cleaning metal by sandblasting or brushing.
  2. Degreasing White spirit.
  3. Primer GF-021 (1 layer).
  4. Painting in 2 layers with intermediate drying for 24 hours.