I know that "damp meters" are much derided here as they don't
accurately say if there is damp in a wall or not but, as they *are*
used by surveyors, what level of dampness do they normally say is
something that's a problem or are they just a bunch of rip-off artists
and will say that the walls need doing if the thing beeps at all?

Thanks.

Ed_Zep wrote:

I know that "damp meters" are much derided here as they don't
accurately say if there is damp in a wall or not but, as they *are*
used by surveyors, what level of dampness do they normally say is
something that's a problem or are they just a bunch of rip-off artists
and will say that the walls need doing if the thing beeps at all?

Thanks.

Would it not be better to ask at what damp levels a problem really does
occur, ie symptoms of a damp problem occur? Yes there is false positive
diagnosis.


NT

Ed_Zep wrote:
I know that "damp meters" are much derided here as they don't
accurately say if there is damp in a wall or not but, as they *are*
used by surveyors, what level of dampness do they normally say is
something that's a problem or are they just a bunch of rip-off artists
and will say that the walls need doing if the thing beeps at all?

Thanks.

Damp meters are very accurate, it is the interpretion of their output
which is usually dodgy. The output from the meter together with a
visual examination of the situation is route to an accurate diagnosis.
If you get say five different damp companies you will get five
different opinions.

Ed_Zep wrote:
I know that "damp meters" are much derided here as they don't
accurately say if there is damp in a wall or not but, as they *are*
used by surveyors, what level of dampness do they normally say is
something that's a problem or are they just a bunch of rip-off artists
and will say that the walls need doing if the thing beeps at all?

Thanks.

Damp meters are calibrated for timber, and can not (should not) be used
to determine moisture content of any other material. Generally timber
in a typical house will have moisture of around 5-10% but up to 20%.
But it varies widely - even through the day

They can be used to roughly check if a part of the room has more
moisture than other parts, but this involves a thorough check of
multiple parts of the whole room, in order to ascertain if areas have
more moisture than others. But this is a comparative test, not a true
measure.

If placed on the surface of, or slightly pressed into plaster, you can
expect an elevated reading, but this is more often due to surface
condensation. The surveyor will then angle the meter so that you can
see this. I've known of 'surveyors' breathing on a section of wall
first just to get a slightly higher reading to show the occupant.

Coincidently, we had a solicitors letter last week and a surveyors'
report alledging damp at a property. I surveyed it, and the occupant
said that their surveyor just placed the meter on the wall and showed
her the reading which was high. So "she must have damp".

When I explained how a dampness investigation should be done, she was
quite annoyed that the 'surveyor' could not substantiate his claims,
and had not done a proper survey. But the sorry part of it, was that
the surveyors fees and solicitors fees would be paid out of legal aid
funding, so they will not mind if they have not done a proper survey
and the claims are false.

dg

avocado wrote:
Ed_Zep wrote:

I know that "damp meters" are much derided here as they don't
accurately say if there is damp in a wall or not but, as they *are*
used by surveyors, what level of dampness do they normally say is
something that's a problem or are they just a bunch of rip-off artists
and will say that the walls need doing if the thing beeps at all?

Thanks.

Damp meters are very accurate, it is the interpretion of their output
which is usually dodgy. The output from the meter together with a
visual examination of the situation is route to an accurate diagnosis.
If you get say five different damp companies you will get five
different opinions.

how can they be accurate if the area of probe in contact with the wall
is uncontrolled, as it is with 2 pointy bits on a rough surface.
how can they be accurate if building materials have resistance of their
own which adds to the current the damp meter measures
how can they be accurate if salts present in materials change the
conductivity of the water present and hence the readings

In wood yes. theyre fine, but on masonry theyre just not measuring
damp, theyre measuring resistance which depends on several factors.


NT