The majority of chimney flue location surveys we undertake are in Victorian houses. These have uniformly built, not overly thick, brick walls, which makes detection and location of flues fairly straightforward. Ground Penetrating Radar detects changes in the subsurface. There is a big difference in density between the flue voids and the masonry; therefore, they appear as a strong reflection in the GPR radargram.

Occasionally we are set a more challenging task. This was the case with a recent survey of an Elizabethan house in Plymouth. The walls were very thick and of stone construction. Wall thicknesses of 1 m were regularly recorded, but in places, they were thicker (we couldn’t detect the back face).

A fireplace in one of the thick stone walls we scanned, in an empty room gutted for renovation.
A fireplace in one of the thick stone walls we scanned

As well as flues, we were asked to detect and plot any other voids detected within the wall construction. The survey posed a number of challenges, most of which were solved satisfactorily by our survey team:

  • The thick walls meant we had to scan from both sides to ensure we penetrated the full thickness.
  • All walls were scanned using two antenna frequencies, 1.5 GHz and 900 MHz, to provide the best resolution and adequate depth penetration.
  • The walls were very rough and uneven, so the surface coupling of the antenna was poor.
  • The accuracy of the survey wheel’s tracking was variable due to the rough surface. Distance markers were manually added to the data to correct this, and corrections were applied in post-processing.
  • The walls were damp, resulting in signal attenuation. Scanning from both sides helped alleviate this problem.
  • The wall construction was very variable, possibly with a rubble infill. This caused GPR signal scatter, making data interpretation more difficult.

All GPR data was saved electronically and processed off-site. All detected chimney flues and voids were then plotted on CAD elevation drawings.

We successfully detected and tracked most of the flues; however, a few appeared to terminate unexpectedly and could not be fully traced due to poor data. It is possible that these were blocked.

Looking up inside of a chimney flue
Looking up inside of a chimney flue

See our Chimney flue location web page for further information about our flue location surveys.