Thatching

Part of Permanent Shelter

Thatching is the oldest roofing method still in use. It requires no specialized tools, no kilns, no metal fasteners — just dried plant material bundled and layered on a roof frame. A well-thatched roof is waterproof, an excellent insulator (keeping warmth in during winter and heat out during summer), and nearly silent in rain. Thatched roofs have covered buildings from Stone Age Europe to modern-day Africa, Japan, and Indonesia. If you have access to grass, reeds, or straw, you can roof a building within days.

Selecting Thatching Materials

The material you use determines how long your roof lasts and how thick it needs to be. Not all plant material works — the key qualities are hollow or tubular stems that shed water, stiffness to resist compression, and length for proper overlap.

Material	Availability	Lifespan	Water Resistance	Notes
Water reed (Phragmites)	Wetlands, riverbanks	25–40 years	Excellent	The gold standard — hollow stems shed water superbly
Long straw (wheat, rye)	Farmland	15–25 years	Good	Must be uncombed — machine-threshed straw is too short
Grass (tall meadow species)	Grassland, savanna	8–15 years	Moderate	Widely available but shorter-lived
Palm leaves	Tropical regions	5–10 years	Good	Excellent in tropical climates; requires different laying technique
Sedge / cattail	Marshes	10–20 years	Good	Similar to reed but softer; wears faster

The Length Rule

Thatching material must be at least 60 cm long — ideally 90–120 cm. Shorter material cannot achieve the overlap needed for waterproofing. If your only available grass is short (under 60 cm), double the thickness to compensate, or consider a different roofing material.

What Makes Bad Thatch

• Leaves without stems: Broadleaves mat flat, trap water, and rot quickly. Use stems, not foliage.
• Soft, pithy stems: If the stem crushes easily between your fingers, it will compress under rain and lose its water-shedding ability.
• Already rotting material: Any discoloration, mushiness, or mold means the material is already decomposing. Use only clean, dry, undamaged stems.

Harvesting and Bundling

When to Harvest

Timing matters for longevity:

• Reeds: Late autumn or winter, after the plant has died back and dried naturally on the stem. Green reeds contain moisture that promotes rot.
• Straw: At grain harvest — the stems are dry and the grain has been removed.
• Grass: Late summer or early autumn, when fully mature and beginning to dry. Cut before seed heads shatter.

How to Harvest

1. Cut stems as close to the base as possible — you want maximum length.
2. Cut cleanly with a blade (sickle, knife, or sharp stone tool). Tearing damages the stems and creates entry points for water.
3. Lay cut material in one direction with all stem bases (butt ends) aligned.

Bundling

1. Gather a handful of stems — a bundle should be about 15–20 cm in diameter when loosely held.
2. Tap the butt ends against a hard surface to align them evenly.
3. Tie each bundle with two ties: one near the butt end (10 cm from the base) and one near the middle. Use flexible twine, strips of bark, or twisted grass.
4. Stand bundles upright in a dry, ventilated area for 2–4 weeks to fully cure. Bundles must be dry before going on the roof — wet thatch on a roof will rot from the inside.

Quantity Needed

A thatched roof uses enormous quantities of material. For a 3 x 4 meter building with 45-degree pitch, you need approximately 150–200 bundles of reed (15–20 cm diameter each). Harvest and bundle material well in advance of building — running short mid-roof means an exposed, leaking structure.

Preparing the Roof Deck

The roof frame must be ready before you begin thatching. You need:

1. Rafters in place: Spaced 60–90 cm apart (see Timber Framing).
2. Purlins (battens) attached: Horizontal poles running across the rafters, spaced 20–30 cm apart. These are what you tie the thatch bundles to.
3. A solid, even surface: Walk the purlins and check that they are firmly attached and evenly spaced. Loose purlins mean loose thatch.

Purlin Spacing Determines Bundle Exposure

The spacing between purlins controls how much of each thatch course is exposed to weather. Tighter purlin spacing (20 cm) means each course overlaps more, creating a thicker, more waterproof roof — but uses more material. For reed thatch at 45 degrees, 25–30 cm purlin spacing is the sweet spot.

Laying Courses: Bottom to Top

This is the core technique. Every thatched roof in history follows this principle: start at the eaves (bottom edge) and work upward toward the ridge. Each course overlaps the one below it so water running down the roof surface never encounters an upward-facing joint.

Step-by-Step

1. First course (eave course): Position bundles along the bottom edge of the roof with butt ends pointing down and slightly overhanging the eave by 5–10 cm. The butt ends face the weather — this is the exposed surface.

2. Secure each bundle: Tie each bundle to the lowest purlin using twisted hazel rods (called “sways” or “liggers”), twine, or split bamboo. Alternatively, pin each bundle with a long wooden peg or “spar” (a pointed, twisted hazel stick) driven through the bundle and hooked over the purlin.

3. Pack tightly: Push bundles together until there are no visible gaps. Each bundle should press against its neighbor firmly.

4. Second course: Move up one purlin. Position the next row of bundles so their butt ends overlap the butt ends of the first course by two-thirds. Only one-third of the first course’s length remains exposed — the rest is covered by the second course.

5. Repeat: Continue course by course up the roof. Maintain the two-thirds overlap rule consistently. The overlap is what makes the roof waterproof — water runs down a butt end, drops onto the course below, and continues downward. At no point does it reach the inner surface of the thatch.

6. Thickness check: At any point on the roof, measure the total thatch thickness perpendicular to the slope. For reed, you need at least 25–30 cm. For straw, at least 30–35 cm. For grass, 35–40 cm. Thinner than this and heavy rain will penetrate.

Side Edges

At the gable ends, trim the thatch flush with the wall face or extend it 15–20 cm as a small overhang. Keep the edge neat by trimming with a sharp blade after each course is secured.

Working Both Slopes

Thatch both slopes simultaneously, alternating courses left and right to keep the weight balanced on the roof frame. An unbalanced roof can shift during construction.

Securing Bundles

The method of attachment determines how long the thatch stays in place during wind and storms.

Method	Tools Needed	Wind Resistance	Notes
Sway and tie	Hazel/willow rods, twine	Excellent	Rod is laid along the course and tied to purlins with twine at each rafter
Spar pinning	Pointed twisted hazel sticks	Very good	Spars are driven through the thatch and hooked over purlins — fast method
Direct lashing	Cordage	Good	Each bundle lashed to the purlin individually — simplest but most tedious
Wire or bamboo strips	Wire or bamboo	Excellent	If available, run horizontal strips over each course and tie to purlins

The Sway Method in Detail

A sway is a long, flexible rod (hazel, willow, or any pliable wood) laid horizontally across the face of each course of thatch. The sway presses the bundles flat against the roof. You tie the sway to the purlin beneath at every rafter position using a length of twine passed through the thatch with a thatching needle (a large, long needle — even a sharpened stick works). This is the most wind-resistant attachment method and the standard technique used by professional thatchers.

Ridge Finishing

The ridge — the peak of the roof where two slopes meet — is the most vulnerable point. Water, wind, and snow all concentrate here. See Ridge Capping for detailed ridge methods. In brief:

1. Bring the top courses from both slopes up and over the ridge beam.
2. Fold the tips of the top course bundles over the ridge and interleave them with the opposite slope’s top course.
3. Secure with a heavy ridge sway and additional ties.
4. Cap with a thick layer of thatch bundles laid along the ridge line, straddling both slopes, and lashed firmly.

The ridge cap is the first part of a thatched roof to wear out and the first place leaks develop. Plan to replace it every 5–8 years even if the rest of the roof is sound.

Pitch Requirements

Thatched roofs demand steep pitch. The steeper the pitch, the faster water runs off, and the less time it has to soak through the material.

Pitch	Suitability	Expected Lifespan
60°	Ideal — water barely touches the surface	Maximum for material type
50°	Excellent — standard for professional thatch	Near-maximum
45°	Good — minimum recommended	Moderate reduction
35°	Marginal — water begins to soak through	50–60% of maximum
Below 30°	Unacceptable	Rapid rot, leaks within 1–2 years

45 Degrees is the Floor

Below 45 degrees, rainwater slows enough on the thatch surface to begin penetrating between stems. Every degree below 45 costs you years of roof life. If your frame is already built at a shallow pitch, add thickness — a 35-degree pitch can work with 40+ cm of thatch, but it is much better to build the frame correctly from the start.

Lifespan and Maintenance

Expected Lifespan by Material

Material	Average Lifespan	Maximum with Maintenance
Water reed	25–40 years	50+ years
Long straw	15–25 years	30 years
Meadow grass	8–15 years	20 years
Palm leaf	5–10 years	15 years

Annual Maintenance

1. Inspect the ridge: Look for thinning, bird damage, or exposed purlins. The ridge wears fastest.
2. Check for moss: Moss holds moisture against the thatch, accelerating rot. Scrape it off with a flat stick or rake.
3. Patch thin spots: Push additional loose material into any area where the thatch has compressed or worn thin. Pack from the butt end upward.
4. Remove debris: Clear fallen leaves, branches, or bird nests that trap moisture.
5. Check fastenings: Re-tie any loose sways or replace broken spars.

When to Re-Thatch

You do not re-thatch the entire roof at once unless it has been neglected. Instead, re-thatch in sections:

• Ridge: Every 8–12 years (reed) or 5–8 years (straw/grass)
• South-facing slope: Wears faster due to sun exposure. Re-thatch 5–10 years before the north-facing slope.
• Full re-thatch: Only when the base layer is rotted through — typically 25–40 years for reed.

Key Takeaways

• Water reed is the best thatching material (25–40 year lifespan), followed by long straw (15–25 years) and grass (8–15 years). All material must be at least 60 cm long.
• Harvest when dry (autumn/winter for reed), align butt ends, bundle at 15–20 cm diameter, and cure for 2–4 weeks before use.
• Lay courses from the eaves upward with two-thirds overlap. Each course’s butt ends face down and outward. Total thickness: 25–40 cm depending on material.
• Secure bundles with sways (horizontal rods tied through to purlins) for maximum wind resistance. Pack bundles tightly with no visible gaps.
• Minimum roof pitch is 45 degrees — steeper is better. Below 45 degrees, water soaks through faster than it drains.
• The ridge is the weakest point. Cap it heavily and plan to replace the ridge cap every 5–12 years.
• Annual maintenance (moss removal, patching, ridge inspection) extends roof life by 30–50%.