Environmental Monitoring: The Rotunda, Rotunda Trust

The building now

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King post °C

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King post humidity %

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Floor °C

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Floor humidity %

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Outside °C

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Moisture inside g/m³

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Moisture outside g/m³

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Closest measured surface to condensation °C

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The king post figures are the air at the king post, 16 metres up and the highest position in the array, where the warmest and driest air gathers; the floor figures describe the space people stand in. The difference between them is one of the more useful things the record shows. Every figure above is either measured directly or derived from measurements by standard psychrometric relations. The moisture content of the timber itself is not measured.

Live readings at each position. Dark markers are air, red markers the surface beside them.

Four ways to read the record. Click a box to change every chart below.

Temperature

The air inside the building moves far less than the air outside (grey), and the fabric itself (deep red) moves less again. Thick brickwork and a lead roof average the weather rather than following it. Every solid line is a measurement; broken lines are not measurements, whether projected or forecast.

Sunlight

Measured illuminance at the weather station, scaled against a clear-sky calculation for this latitude and date, so the line reads about one on a cloudless day and falls with cloud. Ahead of now it is the same calculation attenuated by forecast cloud. The building lags it by several hours: sun on the lead in the early afternoon reaches the timber towards evening.

Humidity

Humidity at the same heights, with the zone above 80 per cent shaded, where mould can germinate. No target range is drawn. A building’s range is a seasonal cycle rather than a fixed pair of numbers, and this one’s will come from a full year of measurement under EN 15757. Warmer air reads drier, so the high positions run low in summer, which carries its own risk of over-drying.

Moisture in the air

The actual water content of the air, in grams per cubic metre, averaged across the sensors within the drum. Humidity percentages depend on temperature; this does not. Green marks the times when the air outside holds more water than the air inside, when letting the building breathe would add moisture, and russet the times when it holds less, and breathing would dry it.

Condensation margin

Most sensors read a surface as well as the air beside it. The heavy line is the surface closest to condensation at any moment, the faint line the one furthest from it, and neither is a fixed sensor. While the line stays clear of zero, nothing measured is wet. The margin has narrowed through the season. Heating needed to protect the primary timbers does not improve the sarking board risk. That can only be improved by upgrading the roof itself, with some limited insulation.

Wind

Two anemometers, deliberately different. The stronger line is London City Airport, 2.5 km north on open ground, taken as the free-stream wind. The fainter line is the mast beside the building, which stands in the lee of the drum and the trees. The gap between them is the building’s wind shadow. Arrows along the top fly with the wind, so an arrow pointing right is air moving west to east.

Rainfall

Measured at the weather station beside the old cannon dock, as depth fallen in the hour to each reading, taken from the gauge’s cumulative total rather than its reported rate. The gauge tips in steps of 0.254 mm, so light rain resolves coarsely. Readings during the station move on 10 August 2026 are excluded. Rain events show up inside as surface temperature, and later as moisture the fabric gives back.

Why it matters

An iron strap and timber pegs at a rafter joint. Connections of this kind are where the difference in how iron and timber move is felt. FennecMedia, CC BY 4.0.

Timber takes its moisture from the air around it and changes size as that moisture changes. Held damp, it decays. Held very dry, it shrinks, checks and works loose at the joints and iron straps that hold this roof together. Between those extremes the level matters less than the movement: every swing is a cycle of strain at the connections, and two hundred years of them is what the condition survey is describing.

So the object of the exercise is a building that moves as little as possible, and the first step is to find out what it does now. The record below is that measurement, running since 30 July 2026.

Looking up from below the Doric capital, rafters fanning out from the ring beam — Looking up from below the Doric capital: all 24 rafters fan out from the ring beam. The shape that makes the building remarkable also makes it stratified, warm air gathering at the apex, which is why sensors sit at three heights rather than one. FennecMedia, CC BY 4.0.

What is measured, and what is not

Eight sensors report air temperature and humidity, each paired with a probe reading the temperature of a solid surface beside it, because condensation depends on how cold a surface is rather than on the humidity of the room. They sit at floor level, on a ring five metres up, at walkway level eleven metres up and at the king post 16 metres up, with four aspects on the ring so a building-wide condition can be told from a local one. A weather station outside supplies the comparison, and half-hourly observations from London City Airport supply the wind and cloud the site cannot measure honestly. The king post position has been measured since 22 September 2026, and the charts show it from that date.

No target humidity has been adopted. Conservation practice is to measure a building for a full year and derive any target from the climate it has made for itself, so the charts mark only the zone above 80 per cent, where mould can germinate, and otherwise show what is there. On present evidence the building's problem is dryness rather than damp.

The inner colonnade with water damage visible — The inner colonnade, with water damage from the failing lead roof visible throughout. Monitoring records the conditions in which that deterioration has progressed, and the conditions any repair would have to work in. FennecMedia, CC BY 4.0.

Every opening in the building is fixed: mesh insets in eight windows, panels in the two doors, holes at each of the twenty-four eaves, and the meshed opening at the apex. Air passes between low and high as the temperature difference dictates, and none of it can be adjusted. Whether anything follows would be a matter for the Ministry of Defence as owner, for listed building consent and for consultation with Historic England and the local planning authority. Nothing is proposed here. The record exists so that any option could be weighed on evidence rather than assumption.

The full technical account is set out in Environmental Monitoring at the Rotunda: Technical Note, material behaviour and the physics of moisture in timber, the standards the method follows, the instruments and how they are placed, the reasoning behind each sensor position, the damage regimes the record is read against, why it is shown at these four views, how the outlook is forecast and how well it verifies, and the calibration of the array (PDF, seven pages). The instrument-by-instrument commissioning and calibration record is held by the Trust and available on request.

References

On the building: E. Cole, S. Newsome and V. McCaig, The Woolwich Rotunda: From Waltzes to Wargames (Liverpool University Press / Historic England, 2025); J. Clarke, “Cones, Not Domes”, Construction History 21 (2005). Standards and sources for the monitoring method are listed in the technical note. Charts show live measured data; brief gaps are radio weather, not building weather.