Are soil radon measurements valid for avoiding CTE compliance?
Why we submitted this consultation
In the Canary Islands, almost every municipality in Tenerife and Gran Canaria is classified as Zone II under Appendix B of Spain's Technical Building Code (CTE). This requires radon protection measures in new buildings, a significant cost that can run into hundreds of thousands of euros on multi-building projects.
In our daily work as radon measurement and protection specialists, we kept encountering the same question from developers and builders: if you measure radon directly in the soil of a plot and the results are low, can you justify not installing the full protections required by the regulations? It is an understandable question, because no one wants to take on a significant cost if they believe their land does not have a real radon problem. But it needed a clear official answer.
That is why, from Radón Canarias, we decided to submit the consultation directly to the CTE consultation platform, to obtain a formal statement from the Ministry that would settle the matter once and for all. The specific questions we submitted were:
- Could low radon concentrations in the soil justify not installing a protection barrier?
- Are prior radon measurements required on a vacant plot where construction is planned?
The full consultation along with the official Ministry response is available for download:
Download CTE consultation on soil radon measurements (PDF)
The Ministry's response
The Subdirectorate General for Architecture and Building replied clearly on 6 October 2021. The statement contains two fundamental points that are worth separating.
Soil measurements are not contemplated in the CTE
The response states it explicitly:
"Section HS 6 Protection against radon does not currently contemplate measurements in the ground."
This means the CTE does not recognise soil radon measurements as a valid method for determining a building's protection obligations. The regulatory system is based exclusively on the municipal classification in Appendix B, not on site-specific soil measurements.
Protection cannot be waived in risk zones
This is the most important point. The Ministry's response states unambiguously:
"For land located in a risk zone listed in the aforementioned appendix, the established protection measures could not cease to be implemented based on ground measurements or other data supposedly indicating low radon concentration risk, since the CTE does not contemplate this possibility."
The conclusion is direct: if a municipality is classified as Zone I or Zone II, protection measures are mandatory regardless of what soil measurements on the plot might show.
What about land outside risk zones?
The Ministry's response does address the reverse scenario. If land lies outside Zones I and II of Appendix B, but measurements or other information suggest an elevated radon risk:
"It is clear that protection measures against radon can be voluntarily incorporated, since we would be on the side of safety."
In other words, soil measurements can serve to add voluntary protections where regulations do not require them, but never to remove protections where they are required.
The alternative solutions route
The response also mentions, in general terms, the possibility of adopting alternative solutions under Article 5.1 of Part I of the CTE:
"The designer or the construction manager may, under their responsibility and with the prior agreement of the developer, adopt alternative solutions, provided they document that the designed building meets the basic CTE requirements because its performance is at least equivalent to that which would be obtained by applying the Basic Documents."
This means it is possible to depart from the specific solutions in Basic Document HS 6, but under two conditions: the documentary justification must demonstrate equivalent performance, and liability falls on the designer or construction manager. It is not a blank cheque to eliminate protections, but a route for proposing technically justified alternatives.
Why soil measurements are unreliable for this purpose
Beyond the regulatory question, there are technical reasons why measuring radon on an open plot cannot reliably predict the concentration inside a future building:
-
The building changes ground conditions. The foundations, floor slab and the structure itself create confinement effects and pressure differentials that alter the dynamics of radon emission from the soil.
-
Soil concentrations are highly variable. They depend on moisture, soil permeability, atmospheric pressure, temperature and the time of year. A one-off measurement does not represent annual behaviour.
-
No direct correlation has been established. There is no validated methodology to convert an outdoor soil radon measurement into a prediction of indoor concentration for a building not yet built.
What this means for the Canary Islands
For building projects in Tenerife and Gran Canaria, where almost every municipality is Zone II, the situation is clear: CTE protection measures must be implemented without exception. There is no shortcut through prior soil measurements.
This does not prevent soil studies being carried out for other purposes, such as geological characterisation of the ground or assessing radon permeability to optimise the design of protection solutions. What they cannot be used for is to justify removing the required protections.
What does make sense is measuring radon once the building has been constructed, using passive detectors placed inside the dwellings or premises. This is the correct way to verify that radon concentrations remain below the reference levels. At Radón Canarias we supply these passive detectors and advise on their correct placement and analysis of results.
Regulatory references:
Technical Building Code, Section HS 6: Protection against radon
CTE Consultation Platform: Subdirectorate General for Architecture and Building, Ministry of Transport, Mobility and Urban Agenda
