# A roof made of geometry, wearing straw

> Straw isn't waterproof. A thatch roof sheds rain because of its angle and how much of each layer is left showing — a course diagram is the geometry, drawn plainly.

August 19, 2026 · https://justog.club/stories/thatcher-course-diagram

A cutaway through a roof: courses of straw stacked over a rafter, each one overlapping the one
below it. Two labels beside the layers — COURSE, EXPOSURE — and a third, off to the side: PITCH 50.

Straw doesn't repel water in any material sense. This drawing is the reason the roof works anyway.

## Why does the pitch matter more than the straw?

Because it decides whether water has time to soak in before it runs off.

At a shallow angle, rain sits on the surface long enough to work its way between the stems. At 50
degrees — steep, close to the angle of a ladder against a wall — water is moving fast enough that
it's gone before it gets the chance. The pitch isn't a stylistic feature of a thatched roof. It's
the load-bearing fact, and everything else on the diagram exists to support it.

## What is "exposure" actually measuring?

How much of each course is left showing to the weather.

A course of straw is laid, and then the next course is laid over most of it, leaving only a
narrow band exposed at the bottom — that band is the exposure. A smaller exposure means water has
to travel down through more layers stacked on top of each other before it reaches the rafter, so a
roof with a tight exposure is one built with more redundancy, not more straw.

## Why draw it as a cutaway instead of a photograph?

Because the whole point is a relationship nobody sees from outside.

From the ground, a finished thatch roof is one continuous golden surface. What the cutaway shows —
the way each course tucks under the one above it, the depth of the overlap — is invisible on a
finished roof and is the entire reason a finished roof works. It's the same instinct behind
[a bookbinder's sewing diagram](/stories/bookbinder-sewing-diagram): the structural part is
the part that gets covered up.

## Is thatching a dying trade?

It's a small one, and it has current work.

There are working thatchers with order books, and a roof laid correctly today is expected to last
decades before it needs rethatching. The trade is small in numbers, not in relevance to the
buildings that depend on it.

## Does this diagram belong to anybody?

The geometry doesn't. A named building's actual roof might.

Course spacing, exposure ratios and pitch requirements are physics and convention, not property —
nobody owns the fact that water runs off a steep angle faster than a shallow one. A specific
listed building's roof, documented and attributed, is a different matter, and using a real one by
name is a matter of asking whoever holds the record. The invented roof on this hero sidesteps that
question and gives the wearer a diagram of the principle rather than someone else's building.

## Does it work for other roofing trades?

The logic transfers; the specific numbers don't.

Slate, shingle, tile and standing-seam metal roofing all use their own version of overlap and
pitch to do the same job water does for thatch. What varies between them is how forgiving each
material is of a shallower angle — a plasterer works through the same problem from the other
direction, deciding [how thick a coat needs to be where nobody will ever run a hand over it](/stories/plasterer-mix-ratio).

## Does it print well?

Yes — line drawing on a mid-tone ground is close to ideal for this kind of detail.

The overlapping courses want to stay as clean parallel bands rather than individually textured
straw, which would collapse into noise at print size. Sepia line on ochre reads warm and legible;
keep the labels — COURSE, EXPOSURE, PITCH 50 — set larger than a technical drawing would, since fine
type is the first thing to disappear into the weave.

## How do I get one made?

Describe your roof at [JustOG](/) — the pitch, how tight the exposure should read, and whether the
labels stay on. Pick a direction, drag the crop frame, see it composited on the real garment, and
it's made to order and shipped.

Designs other people have published are in the [shop](/shop).

Fifty degrees, and the water is gone before it asks the straw a question.