Roof Systems

Part of Permanent Shelter

The roof is the single most important element of any building. Walls can be patched, floors replaced, but a failed roof destroys everything underneath — stored food rots, tools rust, bedding becomes useless, and the structure itself begins to decay. Every civilization that built permanent settlements solved roofing first. Get the roof right and the rest of the building will stand for decades. Get it wrong and nothing else matters.

Why Roofs Fail

Understanding failure modes prevents them. Every roof that has ever collapsed or leaked failed for one of these reasons:

Failure Mode	Root Cause	Prevention
Leaking at the ridge	The peak where two roof slopes meet is inherently vulnerable — gravity pulls water directly into the joint	Ridge capping (see Ridge Capping)
Rot from trapped moisture	Roof materials stay wet because air cannot circulate beneath them	Ensure ventilation gap between covering and structure
Collapse under snow	Pitch too shallow or structure too weak for the accumulated snow load	Steeper pitch (45+ degrees) and adequate rafter sizing
Wind uplift	Wind gets under the roof edge and lifts the entire covering off	Secure eaves, weight or tie down roof covering, minimize overhang in high-wind areas
Pooling water	Flat or near-flat sections allow standing water that finds every tiny gap	Eliminate flat spots — maintain consistent pitch throughout
Structural failure	Undersized timbers, excessive rafter spacing, or poor joinery	Follow sizing guidelines, use proper joints, add bracing

The 24-Hour Rule

If a roof leak goes unrepaired for more than 24 hours in wet weather, damage cascades. Water soaks into structural timber, weakens joints, and promotes fungal growth. Fix leaks the same day you discover them — even a temporary patch of bark or hide buys you time.

Roof Pitch and Climate

Roof pitch — the steepness of the slope — is not a design preference. It is dictated by your climate and roofing material. The wrong pitch for your conditions will fail within the first season.

Climate	Recommended Pitch	Angle (degrees)	Reasoning
Heavy snow	Steep	45–60	Snow slides off before accumulating dangerous weight
Heavy rain (tropical)	Steep	45–55	Water sheds instantly, reducing leak potential
Moderate rain	Medium	30–45	Adequate shedding without excessive height
Arid / dry	Low	15–25	Minimal rain concern; lower profile resists wind better
High wind	Low to medium	20–35	Steep roofs catch wind like a sail; lower profiles reduce uplift

How to measure pitch without instruments: Cut a straight stick to a known length (say 100 cm). Hold it horizontally against the rafter with one end touching the rafter. The vertical distance from the free end of the stick down to the rafter gives you the rise. A 45-degree pitch has equal rise and run — the vertical drop equals the stick length.

Match Pitch to Material

Thatch requires a minimum 45-degree pitch to shed water before it soaks through. Clay tiles work well at 30–45 degrees. Bark shingles need at least 25 degrees. Always check your material’s minimum pitch before you cut rafters — changing pitch after construction means rebuilding the entire roof frame.

Basic Roof Shapes

Four roof shapes cover virtually every shelter need. Each has distinct advantages:

Gable Roof (Inverted V)

Two sloping surfaces meeting at a central ridge. The most common and practical shape for most climates.

• Pros: Simple to build, sheds rain and snow well, provides attic storage space, easy to ventilate at gable ends
• Cons: Gable ends are flat walls exposed to wind — can be pushed in by strong gusts
• Best for: General use, moderate to heavy precipitation

Hip Roof (Four Slopes)

All four sides slope upward to meet at a ridge or point. No vertical gable walls.

• Pros: Excellent wind resistance (no flat faces for wind to push against), stable in storms, sheds water in all directions
• Cons: More complex framing, less attic space, harder to ventilate
• Best for: Hurricane-prone or consistently windy locations

Shed Roof (Single Slope)

One flat plane angled from a high wall to a low wall. The simplest possible roof.

• Pros: Easiest to build, requires the fewest materials, no ridge joint to leak
• Cons: All water drains to one side (plan drainage accordingly), less interior headroom on the low side
• Best for: Lean-to structures, workshops, storage buildings, additions to existing structures

Flat Roof (Slight Slope)

Nearly horizontal with a slight pitch (2–5 degrees) for drainage. Common in arid regions.

• Pros: Creates usable rooftop space (for drying food, sleeping in hot weather), simple construction
• Cons: Leaks easily in wet climates, collects snow, requires frequent waterproofing maintenance
• Best for: Arid climates only — desert and semi-desert regions with less than 30 cm annual rainfall

Structural Components

Every pitched roof uses the same core components. Understanding their roles lets you size them correctly.

Ridge Beam

The horizontal timber running the full length of the roof at its highest point. All rafters lean against or attach to it.

• Sizing: diameter should be at least 15–20 cm for spans up to 4 meters. For longer spans, increase diameter or add intermediate support posts.
• Must be the straightest, strongest timber you have — it carries the combined weight of both roof slopes.

Rafters

The sloping timbers running from the ridge beam down to the wall top plate. They are the primary load-bearing members.

• Spacing: 60–90 cm apart (closer spacing for heavier roof coverings like sod or tile)
• Diameter: 10–15 cm for spans up to 3 meters from ridge to wall
• Cut the bottom end (the “birdsmouth”) to sit flat on the wall plate — a V-notch that hooks over the beam

Purlins

Horizontal poles running across the rafters, perpendicular to them. They provide the surface that the roof covering attaches to.

• Spacing depends on roof covering: 20–30 cm for thatch, 25–35 cm for tiles, 30–50 cm for bark sheets
• Diameter: 5–8 cm — they carry less load than rafters
• Lash or peg to each rafter where they cross

Collar Ties

Horizontal beams connecting opposing rafters about one-third to one-half of the way up from the wall plate. They prevent rafters from spreading apart under load.

• Install on every second or third rafter pair minimum
• Use lap joints or lashing at each rafter connection
• Essential for roofs over 4 meters wide

Wall Plate (Top Plate)

The horizontal beam along the top of each wall where rafters rest. Distributes the roof’s weight evenly along the wall.

• Must be continuous or well-joined — a broken top plate creates a point load that can crack walls

Materials Overview

Material	Minimum Pitch	Lifespan	Weight	Insulation	Skill Level
Thatch (reed/straw)	45°	15–30 years	Light	Excellent	Intermediate
Clay tiles	30°	50–100+ years	Heavy	Poor	Advanced
Bark sheets	25°	5–15 years	Light	Moderate	Beginner
Sod/turf	20–35°	20–50 years	Very heavy	Excellent	Intermediate
Wood shingles	25°	20–40 years	Moderate	Moderate	Intermediate

Start with What You Have

Choose your roofing material based on what is abundant within 500 meters of your building site. Hauling heavy materials over distance is a massive labor cost. If you are surrounded by grassland, thatch. If you are near clay deposits, make tiles. If you are in dense forest, use bark or split shingles. The best roof is the one you can build and maintain with local resources.

Waterproofing Principles

No single layer of natural material is truly waterproof. Roofing works by slowing water down and directing it away before it penetrates. Three principles govern this:

1. Overlap from bottom to top: Every course of material must overlap the one below it. Water hits the top layer, runs down onto the next layer, and continues downward. It never encounters an upward-facing joint. Minimum overlap: two-thirds of the material length for thatch, one-third for tiles and shingles.

2. Thickness absorbs rain duration: A 5 cm thatch layer stops a brief shower. A 30 cm thatch layer stops a daylong downpour. Match your roof thickness to the longest continuous rainfall your region experiences.

3. Smooth unbroken slope: Any interruption in the slope — a valley, a change in pitch, a chimney penetration — is a potential leak point. Minimize penetrations. Where unavoidable (chimneys, vent holes), build flashing: an extra layer of material (bark, clay, or flat stone) that bridges the gap between the roof covering and the penetrating element.

Overhang: Your Walls’ Best Defense

The roof overhang — the portion extending beyond the wall face — is not decorative. It is the primary protection for your walls against rain.

Climate	Recommended Overhang
Arid	15–30 cm
Moderate rainfall	30–60 cm
Heavy rainfall	60–90 cm
Adobe or cob walls	60–90 cm minimum (walls dissolve in rain)

Rain does not fall straight down — wind drives it sideways. A 60 cm overhang with a 45-degree wind-driven rain still keeps the upper two-thirds of a 2.5 m wall dry. Without any overhang, every rainstorm soaks the wall from top to bottom.

Overhang vs. Wind

Longer overhangs catch more wind. In high-wind areas, limit overhang to 45 cm and protect walls with exterior plaster instead. An overhang that gets ripped off by wind does more damage than no overhang at all.

Key Takeaways

• Roofs fail from ridge leaks, trapped moisture, snow overload, wind uplift, or undersized structure — prevent all five.
• Match roof pitch to climate: 45–60 degrees for snow/heavy rain, 30–45 for moderate rain, 15–25 for arid zones.
• Four shapes cover all needs: gable (general purpose), hip (wind resistance), shed (simplest), flat (arid only).
• Core structure is ridge beam, rafters (60–90 cm apart), purlins, collar ties, and wall plate — size each to your span and load.
• Choose roofing material based on local availability: thatch, tile, bark, sod, or shingles each have specific pitch and maintenance requirements.
• Waterproofing depends on bottom-to-top overlap, adequate thickness, and unbroken slope with minimal penetrations.
• Roof overhang protects walls: 30–60 cm minimum in moderate climates, 60–90 cm for earth walls or heavy rainfall.