Surveying equipment and tests

Mechanical hygrometers

This instrument offers you a non-destructive way of checking the moisture content within a solid floor. This is a key feature of the mechanical hygrometer. If an invasive test of a solid floor is contemplated, then proceed with caution.

These are used principally for testing floors. They are used most often by the flooring industry to assess whether a floor substrate has dried out sufficiently after laying for a moisture-sensitive floor covering to be installed on top. There is a real risk of damage being caused by laying flooring materials (often expensive) on a damp base. Therefore such testing can be crucial and many contract floorers have become experts in the test procedures.

In pre-purchase surveying you may not use a mechanical floor hygrometer often because of the time needed for a reading to be taken. However, you may find the instrument useful for a more intensive follow-up investigation. Whether or not you ever use one, you need to know how the instrument works and how it could help diagnose dampness.

Think twice before drilling into a solid floor...

There is generally less risk drilling walls than floors. Cable and pipe runs in walls are generally more predictable. Cables, for example, usually run vertically upwards or downwards from a socket or switch and would usually be found within the plaster thickness – where they are near enough to the wall surface for a standard metal detector to pick them up.

Cable and pipe runs in floors could be anywhere and in any direction – and could be too deeply embedded within the floor for a standard metal detector to flag them up. Damp-proof membranes can be directly under screeds often less than 50mm thick as built. Sometimes screeds are surprisingly thin in places – varying thicknesses may have been laid to compensate for an irregular sub-base or poor setting out of site levels. Drilling holes in floors into which electronic thermo-hygrometers can be inserted, or from which drillings can be carbide tested, can often be considered just too risky. Setting a drill depth guide in the hope that your drill bit will stop short of a hazard buried within a wall or floor is not a safe practice. The floor finish could also be part of a guaranteed tanking system – and should not be damaged by such invasive testing.

Mechanical hygrometers use either synthetic or natural hair. A natural hair hygrometer is reliable thanks to the predictable physical properties of blonde hair. Collections of blonde hair expand and contract within the instrument as humidity changes, and this causes a spring to move the needle on the dial. This type of hygrometer actually reacts quite quickly to humidity. If your hygrometer shows a steady 50% RH reading in room conditions, and is then placed in a 75% humidity chamber, you can watch the needle slowly moving clockwise towards the 75% mark. Some hygrometers also incorporate a temperature sensor – so both RH and temperature under the instrument can be read off the dial.

The instrument itself looks rather like a traditional ship's clock – usually 100mm diameter and with a plastic or glass face – but with just 1 hand. The dial is marked in 1% graduations from 0–100% relative humidity (RH). There will be vents either underneath or at the sides of the instrument, through which air may move. You will also find a small adjustment screw for calibration.

Figure 1: A natural hair hygrometer with screwdriver for adjusting. In practice, such a hygrometer would be housed in an insulating box and closely sealed with tape to the floor

Some suppliers of the instrument also sell an insulated box in which the hygrometer can be housed. This box helps create more stable temperature conditions for humidity measurement and makes it easier to seat the instrument on a floor. It is visible as it is usually bright yellow and helps to protect the hygrometer from accidental damage. Some hygrometer kits also include a test chamber for calibration and the hygrometer can be left in the chamber to equilibrate – the sealed plastic chamber contains a saturated salt solution. A saturated salt solution produces quite a reliable 75% RH environment at room temperature; if the hygrometer is adjusted from time to time during the calibration process to eventually read a steady 75%, it will then be suitably calibrated. Calibration is a little time consuming and you need to be patient; and rather like a guitar the instrument needs to be retuned every time it is used.

Testing the moisture content of a solid floor does involve a long wait. When the insulated box is sealed (ideally by using a butyl tape) to the floor, a pocket of room air is trapped under the box. This air eventually takes up and equilibrates with the moisture from within the floor. This could take at least 4 hours. You can speed up the process if you pre-prepare by placing a mat or taped plastic sheet over the floor area to be tested prior to the survey then take an initial reading after 2 hours and check again 2 hours later to see if the reading is still climbing markedly. The longer you wait, the more closely the reading reflects the true moisture condition deeper within the floor. When 2 readings are the same, possibly at 2-hour or 4-hour intervals, you will have achieved a fairly accurate indication of the floor's RH. An RH of more than 75% generally means the floor is significantly damp, irrespective of the materials used in the floor. Where the dpm is under a concrete slab, you may need to wait at least 24 hours for a reliable RH reading to be obtained.

Accuracy probably relies more than anything else on how carefully and recently the instrument has been calibrated – and annoyingly they must be calibrated for every survey visit.

Mechanical hygrometers are, according to manufacturers' specifications, probably a little less accurate than the electronic hygrometer probe, but have been used in the industry for many years and have been recognised by British Standards for some time, e.g. BS 8203: 2001+A1 2009. Electronic probes are certainly more easy to use – and can also be used in a drilled void – or plugged into the moisture meter to measure RH in a room or dropped into a floor void – so they are versatile. But, as mentioned already, drilling holes in solid floors can be risky.

Unless you have invested in several mechanical hygrometers you will only have knowledge of moisture condition in one place on the floor. You must remember that, in other tests for dampness described in this section, we stressed the need for you to take control readings. With this in mind, dampness investigators would need at least 2 and ideally 3 or 4 mechanical hygrometers.

This is where some of the more common surveyor's instruments score points, such as a surveyor's resistance or capacitance meter. Although there is an indication of the amount of dampness near the surface only, you will be able to map out changing moisture condition across a floor to show an epicentre of dampness or perhaps to monitor change in moisture condition from several visits. You could of course also drill a number of small holes (a slightly destructive and sometimes risky operation) and test the drillings with a carbide meter, or more conveniently use an inserted electronic thermo-hygrometer to find out whether the floor at each drill position appears to be significantly damp at depth (> 75% RH).

You only need to leave an inserted electronic probe in situ for 60 minutes for a reasonably reliable equilibrium relative humidity (ERH) to be recorded. The same hole can be tested on another day to assess whether there is any change in moisture condition – but make sure you keep track of changes in temperature as temperature influences RH readings.

As so many damp problems are caused by solid floor problems, it is surprising indeed that so few house surveyors actually test floors with anything other than the pin probes of a moisture meter, which only gives you an insight into surface dampness.

We hope that, armed with the right floor-measuring equipment, and knowing how to use it, more surveyors will use instruments that can measure moisture condition more reliably and accurately.