Damp Beam

[Shawld2]

Hi

We live in a C16 property one of the rooms has a oak beam vertically from ceiling to floor which is sat upon a stone plinth which is approx 6" high. I recently purchased a moisture meter and when I tested the base of the beam it is reading 38% damp at the base to 7% 1 foot higher... is there anything we can do to treat the beam? can they be dried out? any advice appreciated

Thanks
Lee

 

[Andrew Bodenham]

Shawld2,

First thing to ask is "is there a problem?". If the column is showing signs of rot at the base then maybe you'll need to take action. Otherwise there's no need to do anything. If you can keep good air circulation around the stone plinth and beam that'll help excess moisture to evaporate away.
Damp-meter readings are notoriously unreliable, and are only really useful for comaprative work or plotting a trend over time. Even then surface irregularities can produce spurious readings. Unless you enjoy being worried about 'damp' I'd put the meter in the back of a drawer and trust your nose, eyes and fingers instead.

 

[Shawld2]

Hi

Thanks for your reply, there is a small amount of decay at one corner of the beam directly at the point where it meets the stone plinth. We have only noticed it as we removed the skirting board so no clue as to how long it has been like this. Being new to the property and a novice of old buildings, we were a little concerned especially if this is a supporting beam (it joins the main ceiling beam running through the house but its inset into a wall) should we be concerned? we had a historical survey done 6 months ago before purchase and nothing to do with this beam was highlighted.

 

[Shawld2]

A little background information on why we removed the skirting board may help: I posted on this forum about a week ago as our neighbour was undergoing major works on the room that connects to our house where the beam is located. I thought they had tanked the party wall (where the beam is) but I have since found out that they haven't (not on that wall anyway). They have however laid a concrete floor and have insulated and plastered (not lime) the wall.
The readings from the meter decrease the further I go up the beam. When i first took the readings earlier in the week they were 35% at the bottom and 3% half way up. They are now 39% at the bottom and 6% half way up. I guess this could be down to room conditions?
In the bottom corner of the beam that has the decay, I can scratch the side of the beam and produce powder. I'm just not sure if something like this can take years to spread, whether I can fill/contain with some wax/filler or whether because of the lack of breathability in the floor next door (and maybe even our own) that moisture is going to continue to be directed into beam and cause damage?

 

[Shawld2]

 

[Lime]

I would leave the beam exposed if I could to allow for better ventilation.
However dampness is something that the beam has been exposed to for most of its life and also it may be hygroscopic after all this time.
Who knows what the beam has been subjected to over the years?

Out of interest, what type of moisture meter are you using?

 

[Matt Green]

also it may be hygroscopic after all this time.

All timber is hygroscopic. There may well be salt contamination at the base of the post that is reducing the resistance of the timber leading to a low resistance reading on the meter equal to that which would occur were the post to have a moisture content of 37% in that location. If you see the difference. :wink:

Presumably the moisture reading has been adjusted for testing oak...? The are nominally calibrated to beech as a baseline.

 

[Flyfisher]

Assuming there was no salt contamination and the meter really was giving a true reading of the moisture within the wood, what would be the percentages to worry about, i.e. at what % moisture point does wood wood become an attractive mealtime proposition for something or other?

 

[Lime]

also it may be hygroscopic after all this time.

All timber is hygroscopic. There may well be salt contamination at the base of the post that is reducing the resistance of the timber leading to a low resistance reading on the meter equal to that which would occur were the post to have a moisture content of 37% in that location. If you see the difference. :wink:

Presumably the moisture reading has been adjusted for testing oak...? The are nominally calibrated to beech as a baseline.

I'm aware of that but I was thinking more along the lines of animal fluids.

 

[Matt Green]

Assuming there was no salt contamination and the meter really was giving a true reading of the moisture within the wood, what would be the percentages to worry about, i.e. at what % moisture point does wood wood become an attractive mealtime proposition for something or other?

Around 30% for furniture beetle or death watch beetle to lay eggs on/in it. Once there the larvae can chew through timber at 18% and above with varying degrees of success. Any heartwood would have had to have had the various 'cold water extractives' (phenols, turpines etc) washed out of it before insects would be capable of eating it without dying. It's those extractives that confer the durability of the heartwood.

fungal decay would need about the same levels of moisture: 30%+ for spore germination and survival at anything other 20%, but getting pretty sickly at the lower end. In this case the white rot Donkioporia expansa, oak heart rot, is the one to watch for. It's a specialist 'safe cracker' that can cope with chemical composition of wet oak heart wood a little more readily than other species. It hollows out oak beams if they get wet at one end and lets death watch move in.

On the face of it there there doesn't seem much to worry about although the meter isn't going far into the timber. The chemical composition of old oak and how it changes with age is not well understood. My ex-employer had three pieces of oak aged up to 100 years mounted on a board with electrodes in them: they were bone dry but gave three different readings when they were hooked up to a resistance meter. In an ideal world getting the base of the post micro-drilled with a Sibert drill (or similar) would be the best option to see if it was hollow or softened. It's quite a bit of hassle for a single post though.

 

[Flyfisher]

Thanks for that Matt. Very useful and interesting.