increasing headlap on reclaimed slates

[acesumper]

Hi Guys,
I've looked through most of the previous postings on headlap and re-holing slates and I'm still confused. Firstly, my small extension roof pitch is a joke, 11 degrees. But I can't change it, it's been there for 40-50 years and sort of works. It did have 24 x 12 slates, but I've been unable to find these so have got 22 x 11. If I increase the headlap on these, the existing nail holes will not be in the right place. I'm using hooks, but was thinking about all the slates around the edges. If I put new holes into these, 2 cm above the existing holes, will this be OK? I read on a previous posting about using soakers. I know what a soaker is at an abutment or chimney, but how might this work around the nail fixings? A strip of lead fixed by the new nails, but also covering the old holes?
Many thanks

 

[The Makie]

11 is maybe too low for slates unfortunately, you can use this https://www.permavent.co.uk/products/easy-slate/ which can go to 12 or 10 with cement fibre slates

 

[datarebal]

11 is to low , in the past probably the felt keeping the water out..
So what headlap were you thinking of ? Hooks will limit the max headlap you can get.
Probably re holing plus 20 mm will barely give you 90 to 100 hl

 

[acesumper]

Hi guys, thanks for the replies. Datarebal, I'm only replacing the roof because of some broken slates and seriously dodgy work at the top. It is a lean to roof which leans onto a freestanding cavity wall which divides us from the neighbour. When I removed the batons, they weren't too bad. I think it's because the wall faces the predominant south westerly's and the parapet top to the wall shelters the roof itself from most of the driving wind/rain. As I said, it's survived 40 or 50 years. If I can get it to do that again, I'm almost 60 and the wife isn't much younger. Admittedly, if I make 110, and it starts leaking, I'll be a bit ****ed off! But for now, I was planning on a 100mm headlap. So re-holing 20mm higher up might work. But will the existing holes cause more water leakage? It seems like a real problem with 11 degrees is water ponding at the felt tray/fascia board. The bottom baton seems like it might spend too much time wet. Greatly appreciate any advice.

 

[Diver Fred]

I'm no roofer. However 11° does seem much to low an angle to me. I'd not seen the 'Easy Slate' product before but does look to be useful for you.
You haven't said how many courses was on the roof before. With smaller slates I expect you will have more courses not the same or less. I also expect you may need 3x1 (25x75mm) battens to allow for the changed proportion of the slate as headlamp.

What I am confident is that you need new membrane on the rafters with a minimum of 10"/250mm overlap. Hold that in place with counter battens (battens same width and running in the same direction as the rafters) minimum of 1/2"/12mm thick. The felt tray must be underneath the membrane.
You say you believe that there has been pooling of water at the lower edge of the roof; the counter battens will help. I've seen on the use of heavy plastic sheeting on the bottom quarter or less of the roof run instead of membrane to reduce the likelihood of pooling and leaking through the membrane.

 

[datarebal]

Your over looking the fact that before you had 24 x 12.
You should be concerned about head lap and side lap (bond)
Re holing a standard 22x11 by 20 mm won't give you 110 headlap.
Batten size can be the same as they were. 25 x 50 today, you may have 19 X 50 at the moment.
Careful fitting of a quality underlay or two . Might help.
IMHO I'd rethink , your slating prowess might not match the original guys