Structure: Approved Document A

O N L I N E V E R S I O N A1/2 THICKNESS OF WALLS IN CERTAIN SMALL BUILDINGS Diagram 7 Maximum height of buildings Find the orographic zone for the site from Obtain maximum Obtain value Calculate value Figure 2 Diagram 6 and allowable of Factor A of Factor S from: obtain Factor O from building height from Table b S = V × O × A Table a (or use Figure 3 from Table c Diagram 6) Table a Factor O Orographic category and average slope of whole Factor O hillside, ridge, cliff or escarpment Zone 1 Zone 2 Zone 3 0 1.00 Category 1: Nominally flat terrain, average slope < 1/20 1.0 1.0 1.0 50 1.05 Category 2: Shallow terrain, average slope < 1/10 1.12 1.07 1.05 100 1.10 Category 3: Moderately steep terrain, average slope < 1/5 1.24 1.13 1.10 150 1.15 Category 4: Steep terrain, average slope > 1/5 1.36 1.20 1.15 200 1.20 300 1.30 400 1.40 500 1.50 Table c Maximum allowable building height in metres S < 2km 2 to 20km > 50km < 2km 2 to 20km > 50km ≤ 25 15 15 15 15 15 15 26 11.5 13.5 15 15 15 15 27 8 11 14.5 15 15 15 28 5.5 8 11 15 15 15 29 4 6.5 8.5 12.5 15 15 30 3 5 6.5 10 12.5 15 31 4 5.5 8.5 11 13.5 32 3.5 4.5 7 9.5 11.5 33 3 3.5 6 8 10 34 3 5.5 7 87.5 35 4.5 6.5 7.5 36 4 5.5 6.5 37 3.5 5 6 38 3 4.5 5.5 39 4 5 40 3.5 4.5 41 3 4 42 3.5 43 3.5 44 3 Notes: Table a – Outside of the zones shown in Table a, Factor O = 1.0. Table b – For elevated sites where orography is significant a more accurate assessment of Factor A can be obtained by using the altitude at the base of the topographic feature instead of the altitude at the site, see Figure 2 Diagram 6 or, alternatively, Figure 3 Diagram 6. Table c – i) Sites in town less than 300m from the edge of the town should be assumed to be in country terrain. ii) Where a site is closer than 1km to an inland area of water which extends more than 1km in the wind direction, the distance to the coast should be taken as < 2km. Interpolation may be used in Tables b and c. Approved Document A Structure O N L I N E V E R S I O N 21