EN 1991-1-4: Wind Pressure on Flat Roofs (Section 7.2.3)

About this Flat Roof Wind Pressure Calculator

This calculator gives the net wind pressure on each zone of a flat roof to EN 1991-1-4 Section 7.2.3, using the recommended values with no National Annex. It works the peak velocity pressure from the basic wind velocity and terrain category, sets out zones F, G, H and I from the upwind edge (Figure 7.6), reads the external pressure coefficients from Table 7.2 and combines them with the internal pressure to give a net pressure per zone.

• Roof cladding designer checks the edge and corner suctions on a flat roof, and sees how a parapet reduces them.
• Structural engineer takes zone pressures for purlin, deck and holding-down design on low-rise sheds, warehouses and podium roofs.
• Checking engineer reviews a wind load submission zone by zone, with every factor from the terrain category to the net pressure on the page.

Each step is shown with its clause, so the result can be traced and reused on the next project. It is an engineering-grade calculation you can duplicate, adapt and keep in a CalcTree workspace.

More info on Flat Roof Wind Pressure

Peak velocity pressure at the parapet

The basic wind velocity is adjusted by the directional and season factors (Eq. 4.1). The terrain category sets the roughness length and minimum height from Table 4.1, and the roughness factor, mean wind velocity and turbulence intensity follow (Eq. 4.3 to 4.7). The reference height is taken at the top of the parapet, the building height plus the parapet height, and is never below the minimum height. The peak velocity pressure is then worked from Eq. 4.8.

Roof zones from Figure 7.6

The zones are set out from the upwind edge using the length e, the smaller of the crosswind dimension and twice the height. Zone F sits at each upwind corner, G runs between the corners, H lies behind them out to e/2, and I covers the rest of the roof. The page gives the depth and width of each zone and draws the layout, so the zones can be transferred straight to the roof plan.

Parapets and Table 7.2

Without a parapet the sharp eaves row of Table 7.2 applies. A parapet lowers the suction in zones F and G, and the page interpolates the coefficients linearly on the ratio of parapet height to building height between the tabulated rows, holding the last row above it. Zone I takes both an uplift and a downward coefficient, and each is reported as its own case.

Net pressure per zone

The net pressure is the peak velocity pressure times the difference between the external and internal coefficients (Section 5.2). Each zone takes the internal coefficient that makes its net pressure largest, so the suction zones pair with the positive internal value and the downward case on zone I pairs with the negative one. The internal coefficients are inputs, set by Section 7.2.9 where the openings cannot be quantified.

Common Calculation Errors to Avoid

• Taking the reference height at the roof, not the parapet Section 7.2.3 takes the reference height at the top of the parapet where there is one. Using the roof level understates the peak velocity pressure.
• Ignoring the parapet reduction, or applying it to every zone Table 7.2 reduces suction in zones F and G only. Zone H keeps its value, and a parapet ratio above the last tabulated row does not reduce suction further.
• Using c_pe,10 for small fixings The coefficients here are for loaded areas of 10 m² or more. Individual sheets, fixings and edge trims with smaller loaded areas need c_pe,1.
• Leaving out the zone I downward case Zone I can see pressure as well as suction. Checking only uplift misses the downward case the code also requires.
• Wrong sign on the internal pressure A positive internal coefficient adds to uplift on a suction zone, and a negative one adds to the downward case. Pairing the wrong internal value with a zone understates its net pressure.
• Treating a pitched or curved roof as flat The method covers roofs pitched up to 5° with sharp eaves or parapets. Curved or mansard eaves and steeper pitches need their own clauses.

FAQs

When is a roof flat under EN 1991-1-4?

This page treats a roof pitched up to 5° as flat. A steeper roof is a monopitch or duopitch roof under Sections 7.2.4 and 7.2.5, which have their own zones and coefficients.

Why does the reference height go to the top of the parapet?

The parapet raises the height at which the roof edge meets the wind, so Section 7.2.3 takes the reference height as the building height plus the parapet height. The page adds them and works the peak velocity pressure there.

How does a parapet change the edge suction?

A parapet lowers the suction at the corners and along the upwind edge, in zones F and G. Table 7.2 gives that reduction against the ratio of parapet height to building height, and the page interpolates between the rows.

Which internal pressure coefficients should I use?

Where the openings cannot be quantified and no face is dominant, Section 7.2.9 gives the two values the page starts from, and each zone takes the more onerous one. Where a dominant opening exists or the opening ratio is known, enter the internal coefficients for that case.

Does this cover National Annex values?

The page uses the recommended values with no National Annex. The basic wind velocity, directional, season and turbulence factors are inputs, so values from the National Annex for the country of use can be entered directly.

Can I use the results for cladding fixings?

Not directly. The coefficients are c_pe,10 for loaded areas of 10 m² or more, which suit purlins, deck spans and overall roof design. Fixings and small cladding areas need c_pe,1 from the code.