Monitoring moisture condition case studies
(Level 2) - Slate roof timbers: Using pin probes of the conductable meter
A roof in East Dulwich had quite serious roof leaks through the slating that had caused noticeable wetting to structural timbers, but rot had not yet set in.
Chinks of light gave away the position of quite a large hole near the ridge (see figure 1), not to mention the puddles of rainwater collecting on an old camping groundsheet spread out over ceiling joists underneath. Rainwater had been trickling down rafters, to soak battens and purlins. It took just a few minutes to mark several monitoring positions on batten, rafter and purlin (as shown in figure 3). Electronic moisture meter (EMM) readings using the pin probes were taken at A, B, C, D and E. After the roof slating had been overhauled, further visits to check the roof timbers confirmed satisfactory drying out. Visually, the puddle on the groundsheet had dried up. The table of readings below confirmed drying out. It had even rained quite persistently after the repair work.
Figure 1: Damaged roof - before overhaul
Figure 2: The client shows how water from the roof leaks collects in a camping groundsheet draped over the ceiling rafters
Figure 3: Monitoring position A
| Date | A Batten | B Batten | C Purlin | D Rafter | E Batten |
|---|---|---|---|---|---|
| The very high % readings for D and E on 7/3/03 reflect very wet or saturated timber rather than offering an accurate moisture content. | |||||
| Pre-repair 7/3/03 | 17.3% | 19.1% | 16.1% | 38.5% | 57.2% |
| Post-repair 12/3/03 | 16% | 15.3% | 14.4% | 17.2% | 34.2% |
| 18/3/03 | 11.4% | 13.4% | 11.3% | 12.7% | 10.1% |
The client was pleased with the extra service provided. Yet again, the usefulness of electronic moisture meters was proven; it was not possible to be sure the roof timbers were drying out from visual observation alone.
Monitoring by drying contractors
Many drying contractors undertake monitoring of drying out using level 3 techniques – often invasive techniques, using inserted electronic thermo-hygrometers. This method of checking progress of drying down has proved to be extremely useful and most often reliable.
Once the contractor has addressed health and safety issues, preliminary clearing out of the property, removal of standing water, sterilisation, etc. an assessment is made of how much moisture needs to be removed from the walls, floors, etc. and the extent of the water damage. It is at this stage that moisture meters have a leading role. The contractor takes readings over surfaces within the flood-affected zone of the property and records them (and should also take a few readings away from the wet zone for comparison). The moisture meter readings may be used to create a ‘moisture map’, showing on plan, by sketches of elevations of rooms or by isometric sketch, the patterns of moisture meter readings as found. Some contractors note down exactly where some specific moisture meter readings are taken, and some even attach stickers or labels to walls to identify positions of readings. Once drying equipment is installed, the contractor will often set up fixed monitoring stations (typically half a dozen drilled sleeve positions for a typically sized flood zone), and on each visit to the property humidity and temperature readings for each monitoring position are recorded and tabulated.
As well as monitoring moisture in materials, the contractor will also log inside and outside air humidity and temperature on every visit. It is important that the contractor knows that a suitable drying chamber is maintained during the drying, and it is critical that air humidity is reduced early on in the drying contract to prevent secondary mould development (likely when internal RH is 65% or above).
Monitoring is a way of life for drying contractors.