---
title: In Wildfire Testing, a House Is Most Vulnerable at Its Deck and Its First Five Feet
description: "Full-scale fire testing keeps landing on the same weak points in a house: the first five feet of ground around the walls, and the wooden frame under the deck."
author: Darie Nani (Editor-in-Chief)
updated: 2026-07-31T21:23:56.204Z
canonical: https://richdadmagazine.com/article/wildfire-testing-deck-joists-zone-0-house-survival
image: https://cdn.nanimediahouse.com/ibhs-zone-0-wildfire-test-78237.webp
categories: Real Estate
content_type: News
region: United States
publication: Rich Dad Magazine
schema_type: Article
---

Crews are working 87 large fires in the United States this week, and 4.8 million acres have burned since the start of January. That is close to half again the 3.25 million acres burned by the same date last year, and about a third above the 3.6 million acre average for the date over the past decade. The number of fires is running high too, 44,004 so far against a ten year average of 34,340.

For anyone whose house sits in or near that country, the useful question is not how big the season gets. It is which parts of a house actually fail, and in what order.

The Insurance Institute for Business & Home Safety has spent years answering that by burning full scale test structures rather than modeling them. The institute is a scientific research organization funded by the property insurance industry, with its research center in Richburg, South Carolina, and a second office in Tampa. Its findings keep landing on the same small and unglamorous details: the first five feet of ground around the walls, and the wooden frame underneath the deck.

## Watering the Shrubs Does Not Save the Wall

The first five feet around a house has a name in fire research, Zone 0. Researchers at the institute [ran 17 controlled experiments](https://ibhs.org/wildfire/vegetationinzone0/) on what happens when plants sit inside it, and found that vegetation there amplifies the damage, raising both the chance the structure ignites and how badly it burns.

The part worth knowing is the mechanism, because it runs against what most people assume they are doing when they keep the beds green. Well hydrated plants still ignite and burn in the conditions of a suburban wildfire. What the water changes is timing: a plant with more moisture in it takes longer to catch. During that delay the wall behind it is preheating. By the time the vegetation does ignite, the structure is hotter and more susceptible than it would have been. The institute's own summary is that Zone 0 vegetation poses a significant ignition risk regardless of species or moisture.

Local wind speed and direction, and the work of firefighters on the day, explain much of why investigators walking a burned neighborhood afterwards find some plantings consumed and others untouched.

## A Deck Burns From Its Joists Upward

An attached deck is one of the weakest parts of a house in fire country. Once it is alight it puts flame and radiant heat against the siding, the windows and doors, and the area under the eaves, and that is the path by which a building is badly damaged or lost.

A deck takes two different attacks at once, embers landing on the top surface and flames underneath. On the top, ignition typically began in the gaps between deck boards where they cross a joist, which is exactly where wind blown plant debris collects. That finer fuel is what lets an ember take hold. Redwood surfaces proved particularly vulnerable to it.

Underneath, [the testing found the supporting timber matters more](https://ibhs.org/wildfire/vulnerability-of-decks-to-embers-and-flames/) than the boards a homeowner actually looks at. Once a joist ignites it can burn for a long stretch and hold flame against the underside of the deck. The observation that follows is the clearest thing in the research: where a joist was not engulfed, the boards above it did not burn. Most decks in North America are a wood or plastic composite surface laid over a wood substructure, which is the combination the testing found most vulnerable.

A metal substructure cut that vulnerability substantially against both embers and flames, though the institute notes it may not be an affordable way to retrofit a deck that already exists. That is why it also tested walking surfaces, and found a continuous noncombustible surface helps. The institute has separately examined whether the deck standards now in use, the State Fire Marshal standard 12-7A-4A and ASTM E2632, predict how an assembly behaves in a real fire.

## Sheds and Detached Garages Need the Same Treatment as the House

None of this works as a single purchase. The institute's position is that no one hardening measure fully protects a building, and that homes ignite through a combination of ember attack and direct flame, which have to be answered separately. It sets out two layers: a foundation that deals with embers, of which a noncombustible Zone 0 is the main element, and a second layer against flame, taking in the roof, vents, walls, fences, the deck and the planting close by.

That extends past the house itself. Accessory structures, a shed or a detached garage, have to be hardened to the same standard as the main building.

## FAQ

**Q: What is Zone 0?**
Zone 0 is the first five feet of ground and planting immediately around a house. Testing by the Insurance Institute for Business & Home Safety found that vegetation inside it amplifies damage, increasing both the chance a structure ignites and the severity of the burn, regardless of the species or how wet the plants are.

**Q: Does keeping plants watered protect a house?**
No. Well hydrated plants still ignite and burn in suburban wildfire conditions. Moisture increases the time a plant takes to catch, and during that delay the wall behind it preheats, so the structure is more susceptible to ignition by the time the plant does burn.

**Q: What is the most vulnerable part of a house in a wildfire?**
An attached deck is among the most vulnerable. The supporting joists are the weak point rather than the boards on top: once a joist ignites it can burn for a long period and hold flame against the deck's underside, while boards over a joist that was not engulfed did not burn.

**Q: How do you make a house wildfire resistant?**
The institute's research points to a system rather than a single measure, because no one fix fully protects a building. It describes a foundational layer against embers, centered on a noncombustible Zone 0, and a second layer against flames covering the roof, vents, walls, fences and deck. Sheds and detached garages need hardening to the same standard as the house.