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House design · Choose the structure and foundations · design-13

Understand structural loads and when an engineer is required

Every building must carry its own weight (dead load), people and furniture (live load), wind, snow and earthquake forces. Codes set these per location, so local values come from your code or engineer, not from rules of thumb.

How

  1. Dead load: the weight of walls, roof, PV, tanks on stands, green roofs and stored food or water. Heavy items like roof-mounted tanks need specific design.
  2. Live load: occupants, furniture, snow access and maintenance foot traffic on roofs. Example (US, IRC Table R301.5 as adopted in Minnesota): 30 psf for sleeping rooms and 40 psf for other rooms. Heavy stores (water, food, books, batteries) may exceed these and need specific design.
  3. Wind: higher on exposed hilltops and coasts; roofs and PV racking need tie-downs from rafter to foundation.
  4. Snow: in cold climates the roof must carry the local design snow load plus drifting near parapets and PV. Values come from your national or state map; for example Minnesota sets roof snow load at 0.7 times the state ground snow load.
  5. Seismic: codes group sites into seismic categories. For example, the US cob appendix allows prescriptive building only in Seismic Design Categories A to C; others need engineered design (ICC). New Zealand requires specific engineering on liquefaction-prone ground.
  6. Find the values for your address. US: the free ASCE Hazard Tool gives ASCE 7 wind, tornado, seismic, ice, rain, flood, snow and tsunami values by address.
  7. Europe: snow loads come from EN 1991-1-3, wind from EN 1991-1-4 and earthquake design from EN 1998 (Eurocode 8). Each country's national annex, from its standards body, sets the local values.
  8. Europe and the Mediterranean: the 2020 European Seismic Hazard Model (ESHM20) publishes hazard maps at hazard.efehr.org. Use it for a first look, then take design values from your national annex.
  9. Australia example: house wind classes come from AS 4055. The timber framing code AS 1684.3 gives solutions only up to class C3; higher classes need engineering design.
  10. Hire a structural engineer when: your system is outside a prescriptive code table, you have two storeys of natural walls, long spans, heavy roof loads, high wind, snow or seismic zones, retaining walls, or a sloping or poor soil site.

Safety: Undersized structure can collapse. Do not guess member sizes.

Do these first

Sources

  1. 2015 IRC Appendix S: Strawbale Construction. International Code Council (copy hosted by Ecological Building Network), accessed 2026-11
  2. Cob Code Appendix Approved for the 2021 IRC. International Code Council, Building Safety Journal, accessed 2026-11
  3. Construction systems (Smarter Homes guides). Building Performance, New Zealand Government (MBIE), accessed 2026-11
  4. Straw bale. Your Home, Australian Government, accessed 2026-11
  5. Minnesota Rules 1309.0301: IRC Section R301 amendments (design criteria). Minnesota Office of the Revisor of Statutes, accessed 2026-11
  6. ASCE Hazard Tool (Building America Solution Center library entry). Pacific Northwest National Laboratory for US DOE, accessed 2026-11
  7. Get Eurocodes and National Annexes. Calcs.com documentation, accessed 2026-11
  8. Eurocode 8: Design of structures for earthquake resistance. European Commission Joint Research Centre (Eurocodes: Building the future), accessed 2026-11
  9. ESHM20 Overview: 2020 European Seismic Hazard Model. European Facilities for Earthquake Hazard and Risk (EFEHR), accessed 2026-11
  10. Technical Design Guide 40: Building Timber-framed Houses to Resist Wind. WoodSolutions, Forest and Wood Products Australia, accessed 2026-11

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