Slate support problem at hip rafter. See photos.

[epsilonGreedy]

My first small trial roofing project with natural slate is going well but now part up a hip rafter I have a wobbly slate as shown in Photo-1. The geometry of the cut slate results in one problematic slate whose top does not reach the batten above, as a result it can flip upwards as demonstrated by the marker pen in the photo. The concern I have is that wind will get under the lower edge of the wobbly slate and lift it until the slate cracks or the fixing nails fail.

 

Photo-2 illustrates one idea I have for a fix. The full slate under the spirit level would be trimmed by about 70mm vertically to the left of the level to free up some unoccupied batten along the top, then the wobbly cut slate would be replaced by a special wide slate-and-a-half that would be wide enough to reach the top batten and hence be supported to prevent the flip up risk.

 

Photo-3 shows another solution. The ruler is positioned along the line of the roll top lead flashing that will be fitted over the length of the hip rafter. The weigh of the code-4 or code-5 lead flashing should weigh down the lower edge of the wobbly slate to prevent the flipping action. I am not so convinced the lead flashing will be snug enough to prevent the wobbly slate from rattling and failing eventually.

 

Swmbo suggested a dab of mastic glue under the lower edge of the wobbly slate.

 

I am looking for a proper solution because I have more hips in the main roof and expect the slate cutting geometry to result in further slates that lack support from a top batten.

 

p.s. my slates are 500mm x 250mm

 

[markc]

Much easier to start at the hip and work away,  but as a solution, add a piece of lathe to side of hip rafter/batten to support the slate, odd angle to cut but it will cure your problem,or add intermediate short battens as required 

Edited November 18, 2021 by markc

[ProDave]

I have never done a slate roof, but I question why the tops of your slates only just reach a little onto their respective battens?  I would have thought the top of the slate would go all the way to the top of the batten with the nail hole positioned to put the nails in the middle of the batten?

 

And could the wobbly slate not just have an extra bit of batten fitted lower down to support it's top edge?

[joe90]

Why not cut the slate to extend over the hip rafter/batten a little?

[Iceverge]

Add a copper slate rivet? 

 

[epsilonGreedy]

2 minutes ago, markc said:

Much easier to start at the hip and work away

 

 

I started at the abutment seen in the background because I thought it was important to commence with full width slates alternating with slate and a halves. Also the eave slate row worked out nicely where it met the the lower end of the hip rafter. The geometry and cuts of the next three rows was pleasing as well.

 

The problem with a 30 degree pitch roof is that at the hips a few courses work out well using regular slates plus a few slate and a halves. Then progressively higher up the pitch the underlying bond pattern of the rows slides relative to the hip rafter then (guess) every 6 rows results in an unsupported wobbly slate. My hunch is that regardless of the ultimate implications at the abutment by giving priority to the hip slate cutting geometry, no left of right adjustment will allow all cut slates to reach their top batten along the hip. The more I think of it, the lower the pitch of the roof the more problem tiles.

 

24 minutes ago, markc said:

... but as a solution, add a piece of lathe to side of hip rafter/batten to support the slate, odd angle to cut but it will cure your problem

 

 

 

A difficult cutting angle for sure and fixing will be a problematic if I don't want to punch extra nail holes through the membrane. Below the green membrane I had already fixed lathes to the side of the hip rafter to ensure the main battens running across over the membrane are well supported below where these battens meet the hip rafter.

 

30 minutes ago, markc said:

or add intermediate short battens as required 

 

 

Ooo now you talking, I like this idea though I will need to lift a few of the already fixed slates to ensure this extra intermediate batten can be nailed to 2 or 3 rafters below. Lucky I went for 300mm rafter centres in the end. 

[epsilonGreedy]

34 minutes ago, ProDave said:

I have never done a slate roof, but I question why the tops of your slates only just reach a little onto their respective battens?

 

 

I did some reading on this and the NHBC or similar say the minimum seat for a slate on the top batten is 8mm. I think my average 15mm, the photos are bit deceiving. But anyhow good point I should be better, I was 30 slates into the job before I realised many of my slates are not cut square along the bottom edge and in the early stages of the job I was too focused on slate alignment along the bottom edge of adjacent slates. I will focus on maintaining a minimum top batten seat going forward.

 

42 minutes ago, ProDave said:

I would have thought the top of the slate would go all the way to the top of the batten with the nail hole positioned to put the nails in the middle of the batten?

 

 

I cannot see that working. The pre drilled nail holes are about 200mm below the top of a slate hence a slate is fixed to the batten below and not the batten where the top of a slate rests. As a consequence of this the top half of each batten cannot be covered because the next row of slates up the pitch will need to be nailed into the uncovered top 50% of each batten,

[epsilonGreedy]

46 minutes ago, joe90 said:

Why not cut the slate to extend over the hip rafter/batten a little?

 

 

I am keeping my options open for fitting a wood "mopstick" batten along the top of the hip rafter around which the roll top lead flashing will be formed. I could follow your advice if using conventional capping slates down the hip, even so the square sides of hip top lathe sit a few mm higher than the main battens that intersect with it. For the cut hip slates to sit level on the hip-top lathe I would need to route a generous radius along the edge.

[epsilonGreedy]

1 hour ago, ProDave said:

And could the wobbly slate not just have an extra bit of batten fitted lower down to support it's top edge?

 

 

I am leaning towards this solution.

 

I might amend @markc's advice for starting at the hip as follows:

1. Start at the abutment and fix the eave row all the way to the lower end of the hip.
2. Mark a reference line on the battens starting between  two eave row slates upwards a point close to the end of the ridge/hip junction.
3. Fix a triangle of slates up towards the ridge aligned to batten marks.
4. As the triangle of fixed slates gets closer to the hip, loose fix a few slates to identify the problem cut slates and then fix any intermediate battens before the extending triangle of fixed slates covers the rafters the intermediate batten will be nailed to.
5. If the height of the intermediate batten does not support a cut slate snugly tack a shim of code-3 or code-4 lead around the batten to raise its height.

 

Edited November 18, 2021 by epsilonGreedy

[epsilonGreedy]

49 minutes ago, Iceverge said:

Add a copper slate rivet? 

 

 

Not seen one of these before.

 

Would I drill a hole towards the lower edge of the problem wobble slate and secure this rivet to a batten below? I guess is this is covered by the roll top lead flashing apron there is less of a concern about a leak through the hole.

[makie]

1 hour ago, epsilonGreedy said:

 

Not seen one of these before.

 

Would I drill a hole towards the lower edge of the problem wobble slate and secure this rivet to a batten below? I guess is this is covered by the roll top lead flashing apron there is less of a concern about a leak through the hole.

 

Don't add a copper rivet, these are made for cement fibre slates.

 

3 hours ago, ProDave said:

I have never done a slate roof, but I question why the tops of your slates only just reach a little onto their respective battens?  I would have thought the top of the slate would go all the way to the top of the batten with the nail hole positioned to put the nails in the middle of the batten?

 

This is the English method for slating also so even in Scotland you wouldn't have seen it. Hell I've been roofing for 14 years and only just done my first slate on battens a month ago.

They go to roughly the middle of the batten and then that leaves the top half for the nail from the next slate.

[ProDave]

2 hours ago, makie said:

 

Don't add a copper rivet, these are made for cement fibre slates.

 

 

This is the English method for slating also so even in Scotland you wouldn't have seen it. Hell I've been roofing for 14 years and only just done my first slate on battens a month ago.

They go to roughly the middle of the batten and then that leaves the top half for the nail from the next slate.

Like I say, I am not a roofer, but the few slate roofs I have seen have all been nailed at the top, just like you do with concrete tiles.    Every day is a learning day.

[makie]

3 hours ago, ProDave said:

Like I say, I am not a roofer, but the few slate roofs I have seen have all been nailed at the top, just like you do with concrete tiles.    Every day is a learning day.

 

If you have seen someone slate on battens and they have put the slate to the top of the batten, they have either done the roof wrong or put double the amount of battens on the roof.

[SuperJohnG]

My roofers....(who have done an A1 job) just used CT1 to stick the slates in where the irons had been....don't be too precious and getbsone CT1 in there.

[makie]

15 hours ago, SuperJohnG said:

My roofers....(who have done an A1 job) just used CT1 to stick the slates in where the irons had been....don't be too precious and getbsone CT1 in there.

 

Hate to tell you mate but that's frowned upon by most roofers.

 

They should have clipped the top corners of the two slates above the iron and then single nailed them to allow them to swing. That way the slate where the iron was can be double nailed and then the two above can be both glued and have a single nail.

[Mr Punter]

Could you just fix the wobbly ones additionally with a slate hook, which will secure the tail?

[ETC]

Just add in extra laths where you need then. It’s not rocket science.

[SuperJohnG]

3 hours ago, makie said:

 

Hate to tell you mate but that's frowned upon by most roofers

Such is life. They might have added some nails in too...but nothing has slid out yet!