Bargeboards, undercloaking and water ingress

[Bob49]

Hello,

I've noticed that cement fibre board undercloak on verges can get damp during rain, sometimes there are damp patches the full width of the visible part of the undercloak.

In a roof where there is an outer rafter covered with a uPVC bargeboard, like:

what's to stop the undercloak passing water behind the bargeboard and onto the outer rafter behind and eventually rotting it?

Should care be taken to angle the undercloak? Or should there be a bead of silicone between the bargeboard and undercloak?

But why does it get damp in the first place? Is it not water resistant?

This is a good example of what I am talking about:

Hope that makes sense!


Thanks,

Bob

 

[Mw Roofline]

Yep, you’re right. Water does backtrack a little onto the gable outer rafter and then it dribbles down said rafter and congregates at the bottom. Whenever we strip off gable fascias, the bottom of the rafter is always mashed, although a lot of rot comes in via the box end as well.

On some houses, back in the day the roofers used to nail the undercloak to the top of the timber fascia. This sealed it tight and together with painting the fascia made the gable timbers last forever. However newbuilds where a plastic fascia is nailed to the gable are never as good as the old way as there is something about the way it fits up to the undercloak, it just never sits tight enough.
And, yes, a silicone seal all the way along is a good idea as long as it’s gokd quality and really fed deep into the corner as you don’t want a huge long string of snot dangling down after a few years cause it’s come away.

 

[Bob49]

Thanks for replying. But would silicone stop it, if the undercloak is soaking up water then silicone would only stop water on the surface of the undercloak.

It wouldn't stop water that was soaked up by the undercloak. If you see what I mean.

Why is something that can soak up water used as an undercloak. Seems wrong to me.

But maybe I have this wrong and it doesn't soak up water as bad as I am imagining.

Bob

 

[Mw Roofline]

I think it just gets wet and looks damp.

 

[palaceray]

For a good job use natural slate for the undercloak, and it will not soak up moisture.