Roof bodge correction- best way forward

[oldbutnotdead]

Morning all,

Every time I touch anything on this house I uncover interesting things.

Roof Part 2. Place was reroofed about 20 years ago I suspect by a builder (mortar work looked very good but has all cracked with age surprise surprise, roofing work looked a bit poor and I've discovered is not to the manufacturers' specs for Marley Eternit).

I'm going to lift the slates, battens and felt and relay with correct lap, gauge etc but have hit a snag at the eaves. Hopefully the section attached will make sense.
View media item 72341
The rafters sit directly on the penultimate course of bricks on the front wall
This means (at the moment) the felt is virtually flat for half the thickness of the wall which explains a lot in terms of the wall being damp

When I strip everything off (the rafters and purlins are in very good nick by the way- all solid on this side of the roof, even the one I've uncovered here), what is the best way to get some fall into the eaves? I'm thinking either counterbattening the whole roof with 2 x 2 (which will cost and be annoying but at least give me an even roof pitch) or making some sort of double wedge pieces out of 2 x 2 for the lower 200mm of rafter/top of the wall (which will be fiddly and give a pitch change) or I could just run 500mm or so of OSB or ply from behind the bargeboard up the roof (easy but again gives a pitch change plus might give problems with ponding in the membrane). Any other thoughts? (And please don't say knock it down and start again!)

No real hurry on this- not going to chance the scaffold until April!

Cheers

 

[Blagard]

Although it is possible your rafters have been trimmed to sit as you have drawn they should actually be attached to a timber wall plate on the inner brick/block.

If built as you have drawn the pitch of the roof, at least for the bottom courses must change anyway to get over the fascia. The correct way to do this is like your description of wedges. The result will be a sprocketed eaves. You will need to start the sprocket far enough up to get a sensible fall to the eaves so the felt has a fall too.

http://www.uwe.port.ac.uk/roof/sprocket123.jpg

The length of the sprocket will dictate where your fascia is. If you go for a flush fascia as drawn you will need a vent on top of the fascia board as example in the link http://www.timloc.co.uk/products/roof-ventilation/over-fascia-vent/

 

[oldbutnotdead]

Although it is possible your rafters have been trimmed to sit as you have drawn they should actually be attached to a timber wall plate on the inner brick/block.

Mmm, there are many things I'm finding that should be different. It appears the the front half of the house was built well before the rear half (1800 odd possibly)- the construction detailing is very different. The roof on the rear half does have wall plates and other sophisticated modern fittings. The front half......... well. Actually should have made it clear in the first post we're talking solid walls.

If built as you have drawn the pitch of the roof, at least for the bottom courses must change anyway to get over the fascia. The correct way to do this is like your description of wedges. The result will be a sprocketed eaves

http://www.uwe.port.ac.uk/roof/sprocket123.jpg

The length of the sprocket will dictate where your fascia is. If you go for a flush fascia as drawn you will need a vent on top of the fascia board as example in the link http://www.timloc.co.uk/products/roof-ventilation/over-fascia-vent/[/QUOTE]
Ahh, that all makes sense. Once I strip the roof back I'll be able to see exactly what is what but yes that sprocket arrangement makes sense, I'll definitely be wanting myself a table saw to cut the things.
Question though- that pitch change looks fine for tiles but how will it work with slates? (Marley Eternit is what I've got at the moment)

Cheers

 

[Blagard]

Question though- that pitch change looks fine for tiles but how will it work with slates? (Marley Eternit is what I've got at the moment)

Cheers

Yes it will, you just have to make sure the sprockets are a sensible length.
Alternatively you could extend the rafter through and lower the fascia board so you don't have a sprocket, but this means cutting out the bricks to make space for the lapped on rafter extension.

What is the size of the slate?

 

[oldbutnotdead]

Question though- that pitch change looks fine for tiles but how will it work with slates? (Marley Eternit is what I've got at the moment)

Cheers

Yes it will, you just have to make sure the sprockets are a sensible length.
Alternatively you could extend the rafter through and lower the fascia board so you don't have a sprocket, but this means cutting out the bricks to make space for the lapped on rafter extension.

Ta- I prefer the sprockets, don't really want to disturb the wall. Guttering & facia board aren't in brilliant condition so no drama replacing/moving them. And that job will be the perfect excuse for the tablesaw mentioned above- no way I want to mess on doing cuts that fine with my much abused circular. I'll have a play tomorrow on my sample roof (some bits of stick on a baseboard in the front room in the warm and dry), see how much angle I can put on the sprocket before the lift on the slate gets too much (big silly 600mm x 300 mock slates grr)

 

[Blagard]

With the big slate I would suggest you go back something around a metre.

It has been done with the eternit slates but I could not find any photos. But the logic is the bigger the slate the longer and more gentle the sprocket needs to be.

A roofer who has done one may be able to give you a better idea.

 

[oldbutnotdead]

Makes sense. Given that this run of the roof is only 3m from eaves to apex (roof structure is twin apex with a monster valley in between that I've already had fun with. Clue- the old rotted lead got me £400 at EMR) it is very tempting just to whack 2 x 2 up to the ridge. Might do some sums to make sure the ridge width won't get too wide- keep me out of trouble one dark night!

 

[Blagard]

If the change in pitch you already have looks OK then in theory at least you only need the sprockets long enough to carry the felt at a fall into the eaves.

Edit: finally found the detail from eternit for the fibre cement pitch change.
The good news is they do it, the bad news is the detail calls for a very different detail to normal sprockets and calls for leadwork at the change in pitch. In fact the detail is more like a simple link of two roofs rather than the adjustment of pitch on one.

http://www.marleyeternit.co.uk/pure-roofing/index.html#164-165

 

[oldbutnotdead]

If the change in pitch you already have looks OK then in theory at least you only need the sprockets long enough to carry the felt at a fall into the eaves.

The current change in pitch is garbage- there's about 30mm between the underside of the first full slate and the batten. You do get a certain amount of deflection on these artificial slates but there will be a tendency for the nails to pull out so (since I'm going to strip and relay the lot) I need to do something better than the current setup

 

[Blagard]

Please see edit above

Untitled

• Blagard
• 24 Jan 2014
• 1

edit - image added

 

[oldbutnotdead]

Cheers for that bit of research. That does look like joining 2 rooves together and looks like a fair bit of hard fiddly work. All of a sudden, counterbattening the whole roof with 2 x 2 is looking a whole lot more appealing- specially as it'll give me some headroom for insulation AND an airgap.

 

[Blagard]

Before you jump in and do that, you need to think about what will happen if your roof has a valley one side.

There is no doubt the lack of attention to details by the builder/roofer has left you with a big headache but you need to make sure that you just don't end up moving the problem elsewhere!

 

[oldbutnotdead]

The valley on the other side is under control (he says optimistically)- boards replaced, fibreglass down, waiting for a dry day so I can topcoat the stuff then its reslating time. I could counterbatten that side as well but that would leave a fair old drop from slate verge to valley.

 

[Blagard]

What about removing the fascia and fixing it lower to the wall? How would that fit in with the building?

 

[oldbutnotdead]

On the front (where that drawing is from) it would help and be pretty much invisible. I'd still have to build up the rafters a bit (or knock corners off the top course) to get anything approaching a fall on the felt