EN 1991-1-4: Wind Pressure on Duopitch Roofs (Section 7.2.5)

About this Duopitch Roof Wind Pressure Calculator

This calculator gives the net wind pressure on each zone of a duopitch roof to EN 1991-1-4 Section 7.2.5, for wind blowing at right angles to the ridge, using the recommended values with no National Annex. It sets out zones F, G and H on the upwind slope and I and J on the downwind slope (Figure 7.8), interpolates the external pressure coefficients of Table 7.4a on the roof pitch, and works all four load cases that pair suction and pressure on the two slopes.

• Portal frame designer gets the four roof load cases for rafter and frame design in one place, ready to combine with the wall pressures.
• Purlin and sheeting engineer reads the corner, eaves and ridge zone pressures to size purlins and roof sheeting across the slope.
• Graduate engineer follows Table 7.4a through interpolation and load case pairing step by step, with each value tied to its clause.

The four load cases are laid out side by side, so the governing case for each zone is easy to see. It is an engineering-grade calculation you can duplicate and adapt in a CalcTree workspace.

More info on Duopitch Roof Wind Pressure

Inputs and reference height

The inputs are the fundamental basic wind velocity, the directional, season and orography factors, the terrain category, the height to the ridge, the roof pitch, the plan dimensions along and across the ridge, and the two internal pressure coefficients. The reference height is the height of the building to the ridge, and the peak velocity pressure is worked from Section 4 at that height.

Zones on each slope

The zones are set out from the upwind eaves using the length e, the smaller of the crosswind dimension and twice the height. On the upwind slope zone F sits at each corner and G between them, both e/10 deep, with H over the rest of the slope to the ridge. On the downwind slope zone J is a strip e/10 wide behind the ridge and I covers the rest. The page gives each zone's extent and draws the layout.

Table 7.4a and pitch interpolation

Table 7.4a gives a suction and a pressure coefficient for most zones and pitches. The page interpolates linearly between the tabulated pitches, suction with suction and pressure with pressure, and never across zero. Where the table gives a single value it is used for both cases, and above 45° the suction case for zones F, G and H is held at zero because the code gives a pressure value only. A check confirms the pitch lies within the table.

The four load cases

The upwind and downwind slopes take, in turn, pressure and pressure, suction and suction, pressure and suction, and suction and pressure. A slope in suction takes the positive internal coefficient and a slope in pressure the negative one, and the net pressure on each zone follows from Section 5.2.

Common Calculation Errors to Avoid

• Checking only the suction case Above a shallow pitch the upwind slope can be in pressure while the downwind slope is in suction. Leaving out the mixed load cases can miss the governing frame or bracing action.
• Interpolating across zero Where a zone has both a suction and a pressure value, interpolating between a negative value at one pitch and a positive value at the next gives a meaningless coefficient. Interpolate each sign separately.
• Using wind parallel to the ridge zones This page covers wind at right angles to the ridge. Wind along the ridge uses Table 7.4b and a different zone layout, and has to be checked separately.
• Setting out zones from the ridge Zones F, G and H are measured from the upwind eaves and J from the ridge downwind. Getting the origin wrong moves the high suction strips to the wrong part of the roof.
• Treating a low pitch as duopitch Below 5° the roof is treated as flat under Section 7.2.3, which has its own zones and coefficients.

FAQs

Why does a duopitch roof need four load cases?

For most pitches Table 7.4a gives both a suction and a pressure coefficient on each slope, because the flow can either separate or reattach. Pairing the two values on the upwind slope with the two on the downwind slope gives four combinations, and each can govern a different member.

What pitch range does the page cover?

The page covers pitches from 5° to 75°, the positive range of Table 7.4a. A pitch outside that range fails the check on the page. Troughed roofs with negative pitches are not covered.

How is the reference height chosen?

The reference height is the height of the building to the ridge. The peak velocity pressure is worked at that height, or at the minimum height for the terrain category if that is higher.

Can I use these pressures for local cladding design?

The coefficients are c_pe,10 for loaded areas of 10 m² or more, which suit purlins, rafters and frames. Cladding and fixings with smaller loaded areas need c_pe,1.

Does the page handle wind along the ridge?

No. Wind parallel to the ridge uses Table 7.4b with a different zone layout. Run that case separately and take the worse of the two for each member.