Damp and Thatch

[Flyfisher]

Since rain has stopped play today, how about 'daft question time'? Is there any evidence, anecdotal or otherwise, that thatched properties have less damp problems than others?

This thought occurred to me whilst watching the rain splash off our new scaffolding onto walls that would normally be protected from anything other than driving rain by the 12-18-inch overhang of our roof. I've no experience of thatched roofs but they often seem to have a 3-4 feet overhang, which must keep the walls almost completely out of the rain as well as, I imagine, preventing splashes onto the base of the walls. Also, assuming the ground is level, hardly any rain would get to within 3 feet or so of the foundations, which must surely be a good thing.

The 'umbrella effect' of thatch seems like the ideal roof design, keeping water off the top, sides and ground around the building - so why are non-thatched roof overhangs not designed to have similar overhangs?

 

[middi]

The thatched roof was designed to keep away rain from sides of building , the tiled roof relying on gutters to take the water away from side walls.

 

[FamilyWiggs]

A larger overhang would also presumably cut out light to upstairs windows.

 

[Flyfisher]

The thatched roof was designed to keep away rain from sides of building , the tiled roof relying on gutters to take the water away from side walls.

Agreed, but this difference must surely result in more rain on walls (since gutters provide only minimal overhang less wind is required to push the rain onto the walls), rain falling closer to the foundations and splashes being within range of the base of the walls. Taking all this into account, are thatched properties typically more resistant to damp than those with tiled roofs? (compared with other period proerties that is - modern construction with DPCs and cavity walls etc would, presumably, make the differences irrelevant).

Fair point about the light reduction. I can see that would be an issue these days.

 

[middi]

I don't think that anybody gave a real thought ( or cared ! ) about normal rainfall splashing on the walls when guttering is (or was) used, The main concern would be the cost and impracticality of a 2/3 foot overhang. It is one of those situations where "progress" results in " evolution" of building methods.
This fact is highlighed in situations where thatch has been replaced by tiles and the 1/2 foot overhang is still evident by the rafters sprockets with gutter hangers situated on the rafter face. The thatch would normally overhang the rafters by another foot or so past the sprockets.

 

[Penners]

Well, here's a completely inexpert view.

Yes, thatched roofs do tend to have deeper eaves than tiled roofs, thus providing more shelter from the rain to (at least the upper parts of) the walls. However, they also tend not to have gutters (with some exceptions). So instead of the surface water being carried away to a downpipe, and thence to an underground soakaway some distance from the building, it all drips into the ground near the base of the walls of the house.

For this reason, I have a suspicion that thatched roof buildings may have a tendency to suffer more damp than tiled roofed ones.

Be gentle with me.

 

[Moo]

And what if the soil is of the sandy persuasion? It could get washed away over time and have a nasty effect on the building's stability.

A.N.Othernon-expert.

 

[skier-hughes]

it all drips into the ground near the base of the walls of the house.

For this reason, I have a suspicion that thatched roof buildings may have a tendency to suffer more damp than tiled roofed ones.

Be gentle with me.

A big problem if the house uses lime mortar as well then?

 

[Flyfisher]

Hmm, I can see your reasoning Penners but aren't drains/soakaways necessary with guttered roofs for the very reason that all that rainfall is concentrated into one or two downpipes? Would a thatched roof dripping four feet away from the house footprint be any worse than rain falling directly onto the ground next to the walls? Perhaps it depends on the intensity/duration of the rain?

Are there any thatched property owners out there that can confirm or deny any of this conjecture?

 

[MdB]

I don't know about thatch, but I can confirm that if your house collects all the water from the roof and then dumps it at the bottom of the down pipe with no soak away at all that you end up with a damp wall, green on the outside and black on the inside.

 

[middi]

The way I interpret it is simple my dear "watsons".

During the stone age, as the human populus increased and thinking began to evolve there was a need for more housing . The think "tanks"of the day arrived at the conclusion that as there were no more pre-dug caves they had to dig their own, " sod this for a laugh" they thought and began to put up housing on the cheap, round sticks and a thatched roof ( a few hours process and no special tools needed) this kept the rain away off their heads.

As we have evolved the " think tanks" of the modern day have decided that they wanted to keep the rain off their feet as well ( it has now taken a few million years to make this step forward) this brings us to guttering and soakaways.

Yes it is true, accumulated water from a roof and directed at the base of the building will make it wetter and damper. Give us a chance, we are at an evolving
stage. All we have manged to achieve so far is come up with the conclusion that if we take the rain away from our buildings too quick and too far we create flush floods that can result not only in substantial wetness to our feet but actual drowning humans in their building is some extreme cases.

Just think, we want to save the human race by colonising other worlds. Any other advanced race in some corner of the universe can rest at ease.

 

[Penners]

Would a thatched roof dripping four feet away from the house footprint be any worse than rain falling directly onto the ground next to the walls?

Well, surely yes. The rain dripping off the edge of a thatch is an accumulation of all the water that would have fallen on the area of ground covered by the house. So all that water is concentrated into a line at the dripping edge of the roof.

Whereas, with gutters and a soakaway, it's carried well away from the house (I think the regs say minimum 6m, don't they?) As Middi has explained in his fascinating history lesson, it's all about evolution. :wink:

 

[Flyfisher]

Would a thatched roof dripping four feet away from the house footprint be any worse than rain falling directly onto the ground next to the walls?

Well, surely yes. The rain dripping off the edge of a thatch is an accumulation of all the water that would have fallen on the area of ground covered by the house. So all that water is concentrated into a line at the dripping edge of the roof.

I'm not entirely convinced, but neither am I entirely sure of the counter argument. Yes, the rain dripping off the edge of the thatch is the accumulation of all the water on that face of the roof, but it is dripping (say) four feet away from the wall and will therefore only get to the wall by percolating through the ground. For a downpour of a certain duration it won't make it to the wall before the rain stops falling, thus the ground at the wall remains completely dry. Whereas with no roof overhang the rain will always fall directly at the base of the wall no matter how short the downpour. There's probably some fancy algebra that would explain it all, but life's too short.

But it's disappointing not to hear from any thatch owners out there.

 

[Penners]

the water...will therefore only get to the wall by percolating through the ground

But are you saying that can't happen? Given the constant dripping that will carry on throughout the rainfall, I reckon it's very likely.

 

[Flyfisher]

Yes, certainly, given enough time the rainfall could definitely soak through the ground to the base of the walls. But, starting from 4 feet away it will take longer to reach the wall than on a guttered roof (i.e. rainfall falling directly at base of the wall). So, depending on the duration of the rainfall, water may not actually reach the wall at all before draining away downwards or evaporating, whereas for a guttered roof any duration of rainfall will always reach the base of the wall. Thus, overall, I think I'd expect a thatched property (or any with large roof overhang) to be generally drier.

(I'll just get my anorak . . . . . )

 

[jocelyn plummer]

Well now, in a Bedfordshire Village there are some brand new houses for sale, they have been thatchedwhich is interesting. They are probably still on Rightmove, (village of Pavenham ). There are probably some interesting construction conundrums using modern day materials!

Anyway, Flyfisher, whilst researching a thatched property a few weeks back, I spoke to Keith Quantrill http://www.thatchconsultant.co.uk.
and found him very helpful.

 

[fjpdesigns]

I can confirm that under the overhang of our thatch, which is probably about 30 inches each side, the ground is very dry, much drier than around a modern tiled extension.

 

[Penners]

(I'll just get my anorak . . . . . )

Which colour? :wink:

But actually I think this is an interesting debate, and one which is certainly of significance to anyone who's thinking about buying a thatched property.

However, I acknowledge that we've probably covered the subject, now!

 

[Flyfisher]

I have a whole range of colours for every conceivable occasion :wink:

Thanks for that info Jocelyn, although I'm having enough 'fun' with my pantiles at present without worrying about going back to the original (allegedly) thatch.

Glad we got a final word from a thatch owner though. Thanks fjp.

 

[Penners]

I'm having enough 'fun' with my pantiles at present

There's one of those phrases that you have to type v-e-r-y carefully. Just one missing "L".... :shock: