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

Roof choices

It would have been easier, quicker, and cheaper to build a flat roof, but I didn't think it would be very authentic. A pitched roof with a single slope would have been a possibility, but this would have complicated the brickwork, and I'm (slightly) better at carpentery than bricklaying. In any event, most of the other porches in the neighbours have apex roofs, so that's what I decided on. As I said earlier, I would have made the roof pitch steeper if I could have done so and stayed within the planning limits, but I could only get away with 30 degrees.

One design choice which needed careful thought was to have a `vaulted' ceiling inside the porch. I wanted to give the illusion that the porch was much bigger than it really was, by putting the ceiling as high as possible, and having no joists across the rafters. Now, the problem with this form of construction is that the joists are usually necessary to prevent the `spreading' of the roof under its own weight. They don't contribute to the ability of the structure to handle the downward force of roof weight, but they contain the outward force acting through the rafters as the weight tries to push them apart.
Concrete tiles are heavy -- mine weigh 3kg each when dry, and somewhat more wet. The worst loading will be under a fall of snow: a six-inch layer of snow will add 30 lbs per square foot to the roof weight. Then there's the weight of the roof timbers themselves. Adding these factors together gives a total maximum roof weight of around 640 kg, or nearly 1000 lbs. Consequently, it was necessary to give considerable attention to the sizing and fastening of the rafters and wall plates. As to sizing, the rafters and plates are all 2"x4" C16 timbers, while the ridge is 2"x6". They are fastened by a combination of 4" screws, steel angle brackets, and various steel wall ties, as described below. The rafters at the house end are bolted to the house wall using 6" bolts, to prevent the roof moving in the front-back direction. The rafters at end furthers from the house are secured to the wall plate to resist spreading.

The completed roof frame. Note that the rafters rest on timber wall plates, not on the brickwork. A brick wall will stand the weight of the roof, but not the lateral stresses caused by the weight of the roof or the action of the wind

Roof construction

The standard method of attaching a roof to the walls is to lay timber wall plates on the top course of bricks, and fasten them to the brickwork with steel ties. The rafters (and joists, where used) then attach to the wall plates. The wall plates resist deflection more than a brick wall can and, even if you're using joists to control deflection forces, you've still got to content with later forces that arise in strong winds, for example.

Once you've decided on the roof pitch, you can work out the cutting plan for the rafters. Some care is needed here if you want the relevant parts to but up against each other neatly. Conventionally a `bird's mouth' cut-out is made where the rafter rests on the wall plate, both for tidyness and to increase the contact area. Working out the position and shape of the bird's mouth is a bit fiddly. If you have decent CAD software you can figure out the rafter cutting by drawing and measuring; if you don't (and I don't), scale drawing on graph paper works reasonably well.

The roof is supported by four pairs of indentical rafters. The `bird's mouth' at the left hand end locks into the wall plate timbers on top of the brickwork

It's fiddly to assemble the rafters and ridge timber because you need at least five timbers to be in place all at the same time (four rafters and the ridge) before you can fasten any of them. I got around this problem by putting temporary joists across the wall plates, and resting the ridge on a stack of scrap timber on these joists. With the ridge in the right position, I could screw the rafters to the ridge and to the wall plates.

`Hurricane ties' fasten the rafters to the wall plate and the brickwork, improving the resistance of the rafter to being torn away from the wall plate by lateral forces

Ceiling

On top of the rafters I fastened two panels of 9mm exterior plywood. These are not structural, they merely provide the ceiling of the porch as seen from inside. The rafters will remain visible from inside, which is why I've stained them to make them look more interesting.

Seen from inside, the absence of joists and high, white ceiling makes the porch feel much larger than it really is

The use of a `ceiling' in this location is unconventional, and does lead to complications. Normally the roof felt would lie directly on the rafters, and the tiling battens would then be screwed or nailed through the felt and into the rafters. However, with a solid ceiling you can't really do this, because the horizontal battens would tend to trap water between themselves and the ceiling, and not allow it to drain away. So it's necessary to fasten counter-battens on the ceiling, the felt on the counter-battens, and then the tiling battens on the counter-battens and felt. The use of the counter-battens adds about an inch to the height of the roof, which is not a problem in itself, but does make it harder to make the gable end tidy. More on that later.

Approximate costs

2"x4" treated timbers, 15 off 8'	£80
Wood preserver	£20
Plywood	£10
Brackets, bolts, etc	£10

Total	£120