What Was a Bothy? The Scottish Highland Hut Whose Roof Lasted 300 Years

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[00:00:00]This is number 42 Arnold on the aisle of Lewis. A stone hut of the kind the Highlands still call a Bothorthy. Built in the 1870s, no chimney, no nails in its main timbers, no mortar between its stones, no electricity to this day. Walls of dry stack stone with a packed core of pete and earth. A roof of barley straw over heather turf. By every modern building code, it should have failed inside a generation.

[00:00:31]The Mloud family lived in it until 1965.

[00:00:36]Historic Environment Scotland has kept the Pete fire lit ever since. The building has operated for over 150 years on a system the Hebedes ran for at least three centuries. The same dry stonewalled chimneyless smokecured long house pattern archaeologists trace back to the Iron Age on Lewis.

[00:00:58]Same roof material year after year. Same hearth never allowed to die in the Hebdes. This building has a Gaelic name Thai doob. The black house the black is the soot. Today we're going to show you exactly how that soot worked. Starting with the double skin wall that broke 110 mph gales without a single bolt. The chimneyless pete fire that turned the inside of the roof into a chemical sealant and the closed loop that plowed last year's roof into next year's field. The word boi itself comes from the Scottish Gaelic bothan meaning hut. And in modern usage, it covers anything from a herders's stone sheel to the repurposed estate shelters mountain hikers sleep in today. But the building that produced the 300-year roof system is more specific, and historians call it by its Hebrdian name, the Black House.

[00:01:58]Arnold number 42 was not unique. It was a regional standard. The 1928 Royal Commission survey of the outer heedes cataloged hundreds of taidoo across Lewis Harris, North and South uist and Barra. Almost every croft on Lewis was one. As recently as 2016, the spab survey commissioned by historic environment Scotland walked the entire country and found 305 traditional thatched buildings still standing across Scotland. 236 of them now legally protected.

[00:02:37]At Geran on the western coast of Lewis, an entire village of nine black houses stayed continuously inhabited until 1974.

[00:02:48]Near Uig, an archaeological team led by Nesbbit Church Gilmore and Burgess excavated two structures inside an abandoned blackhouse village at Berro in 2013 down to their dry stone foundations.

[00:03:03]This was not one clever shelter in one valley. This was the dominant rural building type of an entire archipelago for at least three centuries of documented occupation. and it followed a pattern that goes back to the iron age.

[00:03:18]To understand why it lasted, you have to start with what the islanders had to fight. The climate of the outer heedes is by any modern metric hostile to buildings.

[00:03:30]The Met Office records mean wind speeds on Lewis between 16 and 18 mph year round, the highest sustained averages in the British Isles. The same Storway weather station logged its record gust at 113 mph in February 1962, a number that was tied again in January 2015.

[00:03:53]Annual rainfall sits between 50 and 60 in 1,200 and 1,500 mm across roughly 250 wet days a year. There is salt in the wind for 9 months out of 12. There is no protective tree line. Palinologists date the loss of native woodland on Lewis to roughly 4,000 years ago. The wind on Lewis is what the wind on the mainland would be without forests to slow it down. That climate is what every Hebbrdian building has to defeat. And the islanders had almost nothing to defeat it with. No standing timber, no clay for fired brick, no lime for mortar.

[00:04:36]Imported building material was prohibitively expensive on crafts that frequently operated below subsistence.

[00:04:43]The Napia Commission in 1883 and 1884 traveled to 61 places, held 71 meetings, and interviewed 775 individuals across the islands. And the picture that emerged was of an agricultural population working land that could not on its own feed them.

[00:05:04]What the islanders did have was nice and shist, the hardest rock in Britain, lying in slabs on the surface. And they had an effectively unlimited supply of pete. The black house is what you build when those are the materials in front of you, and that is the wind you have to break. The first layer of the system is the wall, and it is the most physically obvious thing about a black house. The second you walk up to one, it is roughly 3 ft thick, about 1 meter, and on older examples between four and 6 ft. It is not one wall. It is two. The outer is a dry stone skin of locally quarried laid coarse on course without a drop of mortar. The inner is a second dry stone skin running parallel about a foot away.

[00:05:53]The core rammed between the two skins like a sandwich is Pete blocks, turf, and top soil.

[00:06:00]Alexander Fenton, who spent his career documenting Scottish vernacular building for the Royal Museum of Scotland and Historic Scotland, called the flat top of the wall the Toba.

[00:06:12]This wall does three things at once. The two stone skins shed wind and stop rain.

[00:06:18]The pet and earth core is hyroscopic, which is a long word for it absorbs moisture out of the air, slowly releases it back, and never lets a humidity differential drive water through the wall the way wind drives water through a modern masonry cavity. And the rounded gable corners, which every blackhouse in the Hebdes has, deflect wind shear instead of presenting it with a flat plane to push against. There are no straight 90deree corners on a blackhouse exterior by design because a 90deree corner is what a 113 mph gust uses as a crowbar.

[00:06:57]Path though is something else entirely.

[00:07:01]The second layer is the fire and this is the one a modern visitor cannot look at without wincing. There is no chimney.

[00:07:09]The blackhouse has a central hearth, a flat slab of stone laid into the packed earth floor with a pete fire on top of it that is lit on the day the family moves in and never deliberately allowed to die. The pete smolders rather than flames. Pete is partially decomposed plant material with low oxygen content and high moisture, so it burns between roughly 930 and 1,290° F inside the burning zone.

[00:07:39]But the smolder spread is only 1 mm per minute or about 1 in every 25 minutes.

[00:07:46]That is two orders of magnitude slower than an open log fire with less than half the heat and vastly more smoke.

[00:07:53]That is the trick. The smoke is the point. Slow oxygen starved combustion of Pete produces a smoke condensate that is chemically very different from a wood fire. much higher in phenolic compounds and carbonals per kilogram of fuel burned. The Polish food scientists Stoiwo and Sikorski in their work on smoking chemistry documented smoke condensates running 0.5 to 2% phenolic compounds molecules like guacol and siren gol the same molecules that make smoked salmon shelf stable. They also documented over 30% carbonils including formaldahhide.

[00:08:38]The blackhouse hearth was not built to keep people comfortable. It was a chemistry reactor running 24 hours a day for a 100red years. And it had to bond to something.

[00:08:50]The third layer of the system is the roof. And this is where the islanders solved a problem that should have stopped them building at all. They had no big timber. The Hebdes had no native forest. What rafters they had came from three sources. Atlantic driftwood, salvaged ship timber wrecked on the coast, and bogpine pulled out of pete cutings, often after a thousand years underground.

[00:09:17]None of that was the kind of straight, long, dry oak a mainland carpenter took for granted. So the islanders cheated the geometry.

[00:09:26]A traditional mainland cruc, the curved A-frame timber pair that holds up a thatched roof runs all the way from the ground to the apex, which means rafter length has to match wall height plus roof rise. The blackhouse sits the crux couples on top of the wall head instead.

[00:09:45]Footed on the inner stone skin with the thick wall acting as a buttress against lateral thrust. That trick alone shortens the rafter length you need by 30 to 40%.

[00:09:57]Driftwood you would normally throw away suddenly becomes structural. It does one more thing. By sitting the timbers inside the wall, the cruc couples now sit inside the smoke column rising off the hearth. Every cru timber in a black house is bathed in phenolrich pete smoke continuously year after year for as long as the fire burns. Modern timbers in a modern building get sprayed with chemical preservatives at the factory.

[00:10:26]The blackhouse impregnates its own structural timber in place. If that sounds like a guess about chemistry the islanders did not understand, the documentary record disagrees.

[00:10:39]In 1999, an archaeologist named John Lets working with English Heritage and the University of Reading published a study called smoke blackened thatch. Let spent more than a decade systematically sampling the underside of thatched roofs on surviving open hall medieval buildings across southern England. These were farmhouses that in the 14th and 15th centuries had no chimney, just a hearth in the middle of the room and a hole at the gable end where smoke worked its way out through the thatch. By 1999, he had compiled over 1,400 thatch samples. Roughly 250 of them turned out to be base coats from the original medieval roof. Straw bundles laid on the rafters when the building was first thatched, still up there centuries later, dated by dendrochronology of the crook timbers above them to between 1350 and 1600.

[00:11:40]The oldest of those base coats date to the early 14th century. They are now in the neighborhood of 700 years old, still functioning as structural thatch holding water out of medieval halls and blackened so completely by centuries of smoke that the original cereal varieties, including English bread wheat, rivet wheat, and rye, some of them no longer grown in Britain, are recoverable as preserved seeds embedded in the straw. Straw in any other context rots inside a single English winter.

[00:12:16]Smoke from an interior fire kept those straw bundles structurally sound for 700 years. The same mechanism is what makes the blackhouse thatch work. The thatch material on a Lewis blackhouse was almost never wheat. Wheat will not ripen reliably in the Hebradian climate. It was barley straw harvested with the roots still attached because the root pad sheds water better than cut straw.

[00:12:44]In leaner years it was oat straw, maram grass dragged up from the dunes, wild iris, rushes or heather over the heather turf base coat that sat directly on the rafters.

[00:12:58]None of those materials on their own has the rot resistance of cedar shingle or slate. None of them needs to. Pete Smoke deposits two distinct classes of preservative onto the underside of the thatch every minute the fire is burning.

[00:13:14]Together they make a black sticky residue. The inhabitants called it soot, but chemically it is closer to a thin layer of pine tar. The phenolic compounds, molecules in the guacol and seringal family, disrupt fungal cell membranes and stop the rot fungi that would otherwise digest the cellulose in the straw. The carbonal compounds formaldahhide and acrolin and over a 100 related aldahhides and ketones cross-link the proteins in plant fibers and the proteins in any insect that tries to chew through them.

[00:13:52]A 2023 study in the journal Animals by Delifan Phillips and colleagues at Kansas State confirmed in the laboratory that the carbonal heavy fraction of smoke condensate directly repels tyrrophus storage mites, the same family of mites that eats untreated straw. The blackhouse roof is at the molecular level a slowrelease insecticide and fungicide that the inhabitants did not have to buy or apply.

[00:14:21]and the thatch returns the favor every June. The fifth layer of the system is the bit that turns the blackhouse from a clever building into a closed ecological loop. Every year, usually in June, after the worst of the spring gales had passed, and before the summer cattle work began, the Croft family or a thatching crew stripped the outermost layer of the previous year's blackened thatch off the roof. They did not burn it. They did not dump it. They loaded it onto a sledge and dragged it to the barley field. That stripped thatch was now soots saturated.

[00:14:59]It was loaded with carbon particulate from a year of Pete smoke with ammonia and nitrogen condensed out of the same smoke with phosphorus and potassium from the Pete ash and with a heavy load of phenolic residue that suppressed weeds and discouraged crop pests in the top soil. The family plowed it under. The next year's barley grew up out of last year's roof. Then they thatched the building again using that year's barley straw. The straw on the roof in 1880 was the straw the field had grown in 1879.

[00:15:34]The straw the field grew in 1880 went onto the roof in 1881.

[00:15:41]There was no waste. There was no fuel input from outside the craft except the pete. The hearth, the roof, and the field were one organism. The cattle made it a body, and that was the sixth layer.

[00:15:56]A working blackhouse was a long house.

[00:15:59]The family lived at one end, the cows lived at the other end, and the two were divided only by a low partition or a hungkin curtain.

[00:16:08]To a modern public health inspector, this is appalling. To a 19th century coffter on Lewis, it was applied thermodynamics.

[00:16:17]A Hebrdian cow, smaller and less productive than a modern dairy breed, gave off something in the range of 400 to 600 W of body heat continuously day and night. Two cows in the buyer, typical for a smaller craft, were a steady kowatt of supplemental heating, dry and constant.

[00:16:38]Four cows on a larger coft were two. The cattle also produced manure that was shoveled out twice a week and combined with the stripped thatch in the field cycle.

[00:16:49]The same single Pete hearth that smoked the roof now also benefited from animal heat radiating up the length of the building from the buyer end.

[00:16:59]It was the buyer end of the long house that the late Victorian public health movement could not tolerate.

[00:17:06]New regulations introduced around 1900 required buyer and dwelling to be separated by a loadbearing wall. Once that wall went in, the heat sharing was gone and the case for keeping a chimneyless smokecured roof collapsed with it. The Hebrdian black house was in every measurable way the most efficient closed loop building system that ever stood on a treeless North Atlantic coast. A thick double skin wall packed with Pete and earth broke 113 mph wind without a single bolt of metal. A chimneyless pete fire turned the interior into a chemistry reactor running 24 hours a day. A cruc roof footed on top of the wall salvaged usable rafters out of driftwood. A barley straw thatch impregnated with phenolic petem smoke resisted rot, fungi, and insects without a single industrial chemical. An annual stripping cycle moved last year's roof into next year's field. And a shared single roof with the cattle buyer cut the heating bill by a steady kowatt.

[00:18:15]A modern building science laboratory with hydrothermal modeling, antimicrobial coatings, and forced air ventilation still struggles to match the integrated performance of a Lewis blackhouse running on pete and stone.

[00:18:30]The blackhouse was not a hut. It was a small engine, a wall, a fire, and a roof tuned to one another so precisely that every byproduct of one mechanism became the raw material of the next.

[00:18:45]Hebrd and cfters understood something that modern green building consultants now pay tens of thousands of dollars to put back into a house. The best wall is the one that breathes pete and earth between two skins of stone. The best roof is the one that cures itself in the smoke of its own fire. And the best fire is the one that fertilizes next year's barley while it heats the room it sits in.

[00:19:11]Subscribe to Medieval Way for more buried engineering. Drop a comment and tell me which of the six layers, wall, hearth, crook, smoke, field, or buyer, surprised you most. Until next time, build to