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Repairing a Garage Roof; How to Re-bed the Verge and Ridge

Postby tamarisk » Sat Jun 27, 2015 10:11 am

Apologies for the length of this posting, I never know how much info to give - some like a lot others like minimal to start with.

I'm trying to do a favour for a friend by carrying out some repairs to there garage roof, which, going by other issues, appears to have been bodged. The problem is re-bedding the verge and ridge which has broken up and crumbled and in places gone back to sand. There appears to have been a lack of cement in the bedding mix but I'm also wondering if the width of the undercloak overhang might have added to the problem.

Some background info:-
    The roof is covered with Sandtoft Double Roman concrete tiles pitched at approx 20 degrees.
    The eaves have no soffits.
    The fascias which are fixed directly to the wall plates overhang the end walls by between 80mm to 105mm
    The gables have no bargeboards.
    The gables are timber clad directly to the side of nominal 34mm x 90mm timber rafters.
    The original 300mm wide undercloak overhangs the gables by between 80 to 105mm and relies solely on being pinched between the tile batten and gable cladding for support (and I guess the fact that it also overhangs inside)
    The tile battens overhang the gable cladding and extend to within 10 to 20mm of the undercloak outer edge.
    Verge clips have not been used.
    The gable ends face east and west along the Thames Valley.
    Some of the undercloak is broken and will need to be replaced.
    One fascia is 5735mm wide i.e. 19 x 300mm tile width plus 1 x 35mm tile overlap.
    The other fascia is 5760 so I'll have to stretch the tiles to fit or trim that fascia.

I've read in a CTMA (Construction Tile Manufacturers Association) document that the bedding mortar should be about 40/50mm wide and not come into contact with the ends of the tile battens (which it did) and the tile battens should be cut so they do not overhang the outer face of the gable (which they do). In the same document it says that the roof covering (and thereby the undercloak) should over-sail the walls by 38 to 50mm.

I've also read that in order to reduce shrinkage the sand used for the bedding should not be ordinary soft building sand but sharp sand, though not the stuff used for screading. However, the sand supplied from the local roofing suppliers specifically for the job appears to be soft building sand. When I checked with the suppliers they told me that is what they supply for bedding verges. The also said there undercloak was the standard width of 150mm.

So my questions are:-

Should I replace the undercloak again with 300mm material or the standard 150mm?
Is an unsupported overhang of 80 to 105mm for the undercloak acceptable, if not what should I do about it? The obvious thing would be to cut the tile battens back to give more room for the bedding and then fix the undercloak to the battens where they still overhang.
If I don't fix the undercloak to the tile batten should the battens be trimmed off flush with the outer face of the gable cladding?
Should I use sharp sand or the sand already supplied by the roofing suppliers for the bedding mortar?
Should I install verge clips?
I understand that the bedding should all be done in one go from the same mix. So I don't end up mixing insufficient or way to much what’s the best way to gauge how much mortar is needed?

Any help would be much appreciated many thanks.
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