The K-Zone: A porch too far: Tiling the roof

Some good luck

I didn't plan it this way but, as luck would have it, the size of the tiles I used meant that a whole number of tiles would fit in such a way as to give just the right overhang at the gable end. I would have been prepared to accept a few centimetres difference, but if the last tile had protruded, say, 20cm from the gable end, I would have had to cut them. And cutting tiles is a horrible job.
In addition, because I was constrained to use Marley interlocking tiles (to match the main roof), I didn't have to interlock the tiles, as they're designed to lock together in straight lines. And this meant that I didn't have to use one-and-a-half-size tiles at the ends either.

Battens and more battens

Counter-battens on the ceiling plywood allow the felt to `sag' slightly, creating a vertical channel for any water that gets under the tiles to run away

A tiled roof is typically constructed from a layer of breathable felt, on top of which are horizontal battens. The tiles `hang' from the battens, with occasional nails to secure them. The felt forms a continuous sheet (except where nails or screws have to puncture it) from the ridge right down to the gutter.

In an ideal world, rainwater would never get underneath the roof tiles, so there would be no need to allow it a drainage channel over the felt. In practice, tiles are not very resistant to driving, horizontal rain -- it just blows right into the gaps between tiles. So some trouble has to be taken to ensure that any water that does get under the tiles can drain. The use of a rigid ceiling complicates that, because it prevents the felt sagging. In the conventional construction water can run between the battens and the felt; here it cannot. So it's necessary to install counterbattens, and fasten the battens and felt to those.
It's worth bearing in mind that the
entire weight of the roof (including that pesky snow) will be carried by the fastenings that secure the counterbattens to the joists. So screws or decent-sized nails are in order.

I felt that

It's difficult at this stage to visualize exactly where the gutters will be, so I left the felt much longer than it needed to be, and trimmed it to size later. Felt needs to be installed from the bottom up, so that each new piece covers the edge of the lower piece, and thus prevents water being able to ooze between the felt edge. Ideally the felt needs to be continuous across the ridge, because this is a place where wind-blown rain droplets can otherwise get under the felt.
When installing the felt, I made sure it butted up against the house wall as firmly as possible, and then I sealed any small gaps with a really thick layer of bitumen. Ideally, water would never get into that edge because the flashing will keep it away; but where water ingress is concerned you can't be too pessimistic.

The felt is laid on the counter-battens. It doesn't really need its own fastenings, because in the end it will be held firmly between the battens and the counter-battens

Tiles go up at last

If all the preparation has been done carefully enough, the actual tiling is simple enough, particular when you don't have to do any cutting. Although I'm generally not a big fan of nails (they're too hard to get out if you make a mistake), tiles need to be secured by nails, not screws. The reason for this is that if you need to get a single tile out, because it's been damaged, for example, this will be very difficult if it's screwed down -- there will be another heavy tile resting on the screw head. With nailing, you might be able to wiggle the nail out by wiggling the tile.

The tiling itself is straightforward, provided that the batten spacing is accurate

Rather than using ridge tiles, I covered the small gap at the ridge with flashing. Not only is this much quicker and cheaper, it doesn't require another 50 lbs or so to be added to the already substantial weight of the roof.
After getting all the tiles in place, I sealed up the edge nearest the house with bitumen again, just to be on the safe side (and I had some to use up, anyway).

Not very flash

I won't dwell on the subject of flashing, because I'm not very pleased with the self-adhesive aluminium flashing I used, for reasons of cost and time saving. It doesn't look bad as such, it just doesn't look authentic. And I have a feeling that it will peel off in time. With the proper primer applied to the tiles and masonry it seems to form a reasonably strong bond. But we'll see what effect a British winter has on it.

Approximate costs

Roof tiles	£80
Timber battens and counterbattens	£50
Aluminium flashing and primer	£20
Felt	£10
Sealant, nails, etc	£10

Total	£170