Thatch is kept watertight by holding its depth, at least 30 cm and 35-40 cm for grass, scraping off moss, patching thin spots from the butt end upward, and renewing the ridge cap every 5-12 years because the ridge wears out 2-3 times faster than the main coat. A patch needs about 9-12 bundles of reed per square metre of roof, so gather material before the repair, and the roof must keep a pitch of at least 45 degrees.
Part of Building Maintenance Schedules
Thatch repair is the patching of thin or damaged areas of a plant-stem roof, and re-ridging is the renewal of its ridge cap, the part that wears out first.
Thatch Fails By Thinning, Not By Sudden Collapse
The maintenance guide gives the thatch jobs: replace missing sections, add thatch to thin areas, minimum depth 30 cm for adequate water shedding; daylight visible from inside means the thatch is too thin. The thatching guide gives the depth by material, at least 25-30 cm for reed, 30-35 cm for straw and 35-40 cm for grass (Thatching), so the maintenance guide’s single 30 cm is enough for reed and straw at the low end and too little for grass. It also gives lifespans (reed 25-40 years, long straw 15-25, meadow grass 8-15), and the annual routine: inspect the ridge, scrape moss, patch thin spots by packing loose material from the butt end upward, clear debris and re-tie loose sways.
Worked Example: Material For A Patch
Inputs from the thatching guide: a 3 m by 4 m building at 45 degrees takes 150-200 bundles of reed, each 15-20 cm across.
- Roof area: two slopes, each 1.5 m of run at 45 degrees, so 1.5 ÷ cos 45° = 1.5 ÷ 0.707 = 2.12 m long by 4 m wide: 2 × 2.12 × 4 = 17 m².
- Bundles per square metre: 150 ÷ 17 = 8.8 to 200 ÷ 17 = 11.8, so about 9-12.
- A patch of 2 m²: 2 × 8.8 = 18 to 2 × 11.8 = 24 bundles, plus a margin for waste.
- Ridge: the capping guide’s woven thatch cap takes 20-30 bundles of the best material (Ridge Capping).
The ridge wears 2 to 3 times faster than the main covering, which is why it is renewed on its own cycle: every 8-12 years for reed, 5-8 for straw or grass (thatching guide), or 5-12 years overall (ridge guide). The south-facing slope wears faster and is re-thatched 5-10 years before the north.
What You Need
- Thatching material at least 60 cm long, dry and in bundles, harvested in advance
- Twine or cord, sways (rods) and a thatching needle
- A ladder and a crawl board
- A flat stick or rake for moss
- A spark-free site: no flame, no hot work, no burning
Method: Patch And Re-Ridge
Step 1: Mark the thin and damaged areas from inside, by daylight, and outside by sight.
Step 2: Scrape moss and clear debris so you can see the sound thatch.
Step 3: Cut out rotted or bird-damaged material back to sound thatch.
Step 4: Lay new bundles from the bottom of the patch upward, butts down, each course overlapping the one below by two-thirds, and tie them to the sways or battens.
Step 5: Check the depth, measured perpendicular to the slope, against the table above.
Step 6: Renew the ridge cap in dry weather, bringing the top courses over the ridge, interleaving them from both slopes, securing with ties and a ridge sway, and capping with a thick layer of bundles.
Common Mistakes
- Patching on top of rotten thatch. Rot spreads from the base layer.
- Short material. Under 60 cm cannot overlap enough to waterproof.
- Wet bundles. Wet thatch rots from the inside; cure bundles dry for 2-4 weeks.
- Leaving moss. It holds moisture against the thatch.
Safety
Thatch is slippery, fragile and flammable
A thatched roof at 45 degrees or more is hard to stand on, and the frame beneath may be rotted. Use a crawl board and a second person. Thatch ignites from a spark and burns fast; allow no flame, pipe, torch or cut-off wheel near it, and keep chimneys capped with an arrestor (Ember Protection). Repair work generates dry dust and chaff, so keep fires out of the area while it lies about.
What’s Next
- Learn next: Metal Roof Inspection — the alternative covering
- Combine with: Leak Detection & Emergency Patching — finding where thatch is letting water in