United StatesASCE 7-16
## Building properties
Define roof geometry, dimensions, and enclosure classification as usual. For the Directional Procedure, **Roof Type** supports **Flat**, **Gable**, **Hip**, and **Monoslope** (the Envelope Procedure is limited to flat and gable roofs).
**Enclosure Type** supports **Enclosed**, **Partially Enclosed**, **Partially Open**, and **Open**. Select **Open** for structures with no walls (carports, canopies, agricultural sheds, and covered pavilions) where wind flows freely over and under the roof. For Open enclosures the Directional Procedure computes design wind pressure on the free roof per **Cl. 27.3.2** (Figs. 27.3-4, 27.3-5, 27.3-7) using the free-roof net pressure coefficients ($C_N$), and enforces the **Cl. 27.1.5** minimum design wind force of **16 psf** times the projected frontal area. Open enclosure support is currently limited to **monoslope** and **gable** free-roof shapes.
Directional-specific inputs and computed values include:
* **Wind Direction**: choose **N-S Direction (Normal to Ridge)** or **E-W Direction (Parallel to Ridge)**. This sets which building dimension is treated as $L$ (parallel to wind) and $B$ (normal to wind).
* **Horizontal Dimension Parallel to Wind Direction** ($L$) and **Horizontal Dimension Normal to Wind Direction** ($B$): derived from **Width (Perpendicular to Ridge)** and **Length (Parallel to Ridge)** based on the selected wind direction.
* **L/B Ratio (for Directional Procedure)**: used for the leeward wall $C_p$.
* **h/L Ratio (for Directional Procedure)**: mean roof height divided by $L$; used for roof $C_p$ interpolation.
* **Roof Angle in Wind Direction** ($\theta_{dir}$): equals the roof slope for most cases, or 0° when wind is parallel to the ridge (E-W on a gable roof).
A diagram under **Wind Direction** in the summary shows windward and leeward surfaces for the selected direction.
Optional **Include Roof Overhangs?** = **Yes** adds **Roof Overhang External Pressure Coefficient** and **Roof Overhangs Bottom Surface Pressure** for the directional procedure (Cl. 27.3.3).
## Intermediate calculations
The calculator follows the ASCE 7 Chapter 27 workflow:
1. **Velocity Pressure** ($q_h$) at mean roof height, including exposure, topographic, and ground elevation factors where applicable.
2. **Wind Directionality Factor** ($K_d$) per Table 26.6-1 (computed on the sheet, but not multiplied into the directional summary pressures).
3. **Internal Pressure Coefficient** ($GC_{pi}$) from enclosure type, optionally reduced by the Internal Pressure Reduction Factor ($R_i$) for large-volume partially enclosed buildings (see [Internal pressure reduction (Ri)](#internal-pressure-reduction-ri)).
4. **Roof External Pressure Coefficients** table: $C_p$ for windward wall, leeward wall, sidewall, and roof surfaces from Fig. 27.3-1, with linear interpolation for $L/B$, $h/L$, and $\theta_{dir}$ where permitted by the figure notes. For flat/low-slope roofs with $h/L \geq 1$, the "Roof (0 ft to h/2) Min" coefficient can be reduced with tributary area per Fig. 27.3-1, footnote b (see [Cp roof reduction factor (RF\_b)](#cp-roof-reduction-factor-rf_b)).
For sloped roofs ($\theta_{dir} \geq 10°$), the roof table reports separate windward minimum/maximum and leeward coefficients. For low-slope roofs ($\theta_{dir} < 10°$), roof zones are split by distance from the windward edge (0 to $h/2$, $h/2$ to $h$, $h$ to $2h$, and beyond $2h$), with separate minimum and maximum $C_p$ values where Fig. 27.3-1 requires both.
### Internal pressure reduction (Ri)
For **Enclosure Type = Partially Enclosed**, a **Large-Volume, Unpartitioned Building?** input appears. Selecting **Yes** exposes:
* **Unpartitioned Internal Volume** ($V_i$)
* **Total Area of Envelope Openings** ($A_{og}$), summing wall and roof openings
The Internal Pressure Reduction Factor is computed per Eq. 26.13-1:
$R_i = \min\left(1.0,\ 0.5 \left(1 + \frac{1}{\sqrt{1 + V_i / (22{,}800\, A_{og})}}\right)\right)$
$R_i$ multiplies $GC_{pi}$ and feeds every net design pressure table on the sheet (MWFRS Envelope, MWFRS Directional, and Components and Cladding). It defaults to 1.0 (no reduction) unless the input is set to **Yes**.
### Cp roof reduction factor (RF\_b)
For the MWFRS Directional Procedure on flat/low-slope roofs ($\theta_{dir} < 10°$) with $h/L \geq 1.0$, a **Tributary Area for Cp Roof Reduction** input appears. The reduction factor $RF_b$ is linearly interpolated from that area per Fig. 27.3-1, footnote b:
| Tributary area | $RF_b$ |
| ------------------ | ------ |
| $\leq 100$ ft² | 1.0 |
| 250 ft² | 0.9 |
| $\geq 1{,}000$ ft² | 0.8 |
$RF_b$ multiplies the $-1.3$ roof pressure coefficient in the **Roof (0 ft to h/2) Min** zone only. It defaults to 1.0 (no reduction) unless the input is engaged.
## Results
The summary includes:
* **Wind Pressures**: design pressures for each MWFRS surface, reported as **Wind Pressure (Positive Internal)** and **Wind Pressure (Negative Internal)** to cover both internal pressure cases.
* **Wind Pressures for Windward Wall**: height-varying pressures on the windward wall when velocity pressure varies with height (including topographic effects).
* **Note: Minimum design wind pressures (Directional)**: ASCE 7 requires minimum ultimate pressures of 16 psf on windward walls and 8 psf on leeward walls and roofs (Cl. 27.1.5). Compare calculated values against these minimums in your design.
Design pressure form:
$p = q_h (G C_p \mp GC_{pi})$
The calculator applies $K_d = 0.85$ in other modes but omits it from the directional pressure equations shown in the summary.
When **Include Parapets?** is enabled, see [Calculate Parapet Wind Loads (ASCE 7-16)](/docs/calculators/us/wind-loads/wind-loads-parapets-asce-7-16) for parapet pressure outputs.
## Frequently asked questions
