How hot could a lead valley get?

[epsilonGreedy]

A bit of an armchair question but I am trying to resolve the different advice on laying a regular breather membrane under a lead valley. Some say the lead will heat up on a hot day and damage the membrane. The certified temperature range of one breather membrane I looked at was no more than 100 degrees C but I guess a 30 degree roof at near 90 degrees to the mid summer sun could be heated to an egg frying temp. (p.s. apparently an egg can be fried at 63 degrees C so now I am even more confused).

 

Not laying membrane here under a valley seems like a recipe for leaks.

 

This Tyvek video demonstrates how to lay membrane under a valley, perhaps their working assumption is that a grp valley will be used.

 

 

[Mr Punter]

10 minutes ago, epsilonGreedy said:

p.s. apparently an egg can be fried at 63 degrees C so now I am even more confused

 

I think it is 64 degrees and more like confit than fried.  It was the dish that inspired this place.  https://64degrees.co.uk/ 

[PeterW]

How would you end the membrane without the overlaps in the valleys ..? I allow overlap both ways plus put a piece all the way down the valley so in some places it is 3 layers. 

[Temp]

All I could google was this..

https://www.kcbd.com/story/14930192/how-hot-is-the-playground/

 

Playground surfaces can become very hot during hot weather.  Slides and metal rising to temperatures as high as 175 degrees have the capacity to burn a child's skin, according to Dr. Joe Sasin from University Medical Center. Surfaces can reach temperatures of over 200 degrees depending on the angle of the sun, lack of wind and humidity.

 

Presumably thats 200F = 93C

 

 

 

[Temp]

Seems a perfect black body in the sun can only get to about 90C.. 

 

https://www.quora.com/What-material-will-get-the-hottest-if-it-is-left-out-in-the-sun

 

Quote

 

The Sun is so hot that it radiates most of its light in visible wavelengths, and these photons heat the Earth and exposed objects can absorb photons. First the colour of an object has a great deal to do with its thermal absorption. The colour black absorbs heat while the colour white is more reflective. Let us assume that the material is perfectly black (emissivity of 1.0) and facing the sun which delivers approximately 1 kW of power per square meter at the surface of the Earth (a bit more at the top of the atmosphere). The black surface will heat up until it loses that kW as fast as it is coming in. It will do so by a combination of radiation and convection.

 

To calculate the heat potential of an object one can use the Stefan-Boltzmann law.

 

 

364K =90C

 

Edit: For some reason I cant post the latex calculation at that link but check it out

 

 

Edited August 24, 2021 by Temp

[makie]

Do not use a breathable membrane under lead, it will sweat and damage the lead.

 

Get a non breathable membrane like a1 protect and run a strip down, install the lead and then put your regular breathable membrane on top of the lead to the desired overhang.

[epsilonGreedy]

3 hours ago, makie said:

Do not use a breathable membrane under lead, it will sweat and damage the lead.

 

Get a non breathable membrane like a1 protect and run a strip down, install the lead and then put your regular breathable membrane on top of the lead to the desired overhang.

 

Can I just confirm the the layers you recommend because I don't think you recommend having the breather membrane over the valley lead? I assume what makes most sense is A1 non breather down as a 1m wide strip running along the V base of the valley. Then breather overlaps from either side of the pitch as shown in the Tyvek video above, then the valley lead should be laid over the 3 membrane layers?

 

I can now see the logic of using a non breather membrane because of the sweat problem that I imagine is worst in cold weather when the lead surface might be near freezing point. I will buy a 15m roll of A1 Protect for the valley.

 

A1 Protect is strong stuff and has held up to 6 months of non standard use in my temporary roof cover while being exposed to the sun. I used a single roll of A1 Protect over the rafter ends at the eaves to run water clean off the roof from the tarpaulin that stopped just short of the rafter ends.

 

https://www.protectmembranes.com/public/protect-a1-underlay/p/6

[epsilonGreedy]

4 hours ago, PeterW said:

How would you end the membrane without the overlaps in the valleys ..? I allow overlap both ways plus put a piece all the way down the valley so in some places it is 3 layers. 

 

 

This was my plan until I read that a membrane under a lead valley will fail due to the regular over heating as the lead bakes in the summer sun. I will phone the tech departments of a few membrane manufacturers tomorrow and report back what they say.

[epsilonGreedy]

4 hours ago, Temp said:

Seems a perfect black body in the sun can only get to about 90C.. 

 

https://www.quora.com/What-material-will-get-the-hottest-if-it-is-left-out-in-the-sun

 

 

 

364K =90C

 

Edit: For some reason I cant post the latex calculation at that link but check it out

 

 

All we need now is for Cornwall's most famous mathematician to calculate the point of absorption/emission equilibrium for lead on a roof. @SteamyTea

[epsilonGreedy]

5 hours ago, Mr Punter said:

I think it is 64 degrees and more like confit than fried.  It was the dish that inspired this place.  https://64degrees.co.uk/ 

 

 

Cosmopolitan culture and tasting menus! I remember that before I bought a plot in the middle of nowhere in Lincolnshire.

[SteamyTea]

https://www.bbc.co.uk/programmes/m000kv7j

[makie]

55 minutes ago, epsilonGreedy said:

I assume what makes most sense is A1 non breather down as a 1m wide strip running along the V base of the valley. Then breather overlaps from either side of the pitch as shown in the Tyvek video above, then the valley lead should be laid over the 3 membrane layers?

 

You can do it either way really. I've had some architects want it done over the lead once it is lead on top of the NBM.

I prefer over because it gives a extra layer of protection if any water gets in.
Doing it under would allow water to get in near the nail-holes if any water does get in. (Unlikely if done right)

[epsilonGreedy]

10 hours ago, makie said:

 

You can do it either way really. I've had some architects want it done over the lead once it is lead on top of the NBM.

I prefer over because it gives a extra layer of protection if any water gets in.
Doing it under would allow water to get in near the nail-holes if any water does get in. (Unlikely if done right)

 

 

Hmm that would leave the membrane exposed to daylight assuming an open valley. My concern is that the membrane would rot due to UV exposure after a few years.