supporting purlins on an angle - 1992 part A table A23

[sm_jamieson]

Low pitch roof diagram with 1992 part A table A23, has purlins (their choice of word) at the pitch angle supporting the roof sheathing. How should such purlins be supported, such that their weight is properly transferred to the walls ? They would have a tendancy to slide. One side will be against a house wall, other a new wall on top of which could be a timber wall plate. Thanks, Simon.

[Hugo Nebula]

On 20 Oct 2005 02:46:02 -0700, a particular chimpanzee named sm [e-mail] randomly hit the keyboard and produced:

They're built it into the walls, both sides. You may be able to use restraint-type joist hangers, as, IIRC, the spans require end restraint.

You're intent on pursuing this idea of using purlins, aren't you?

[Mr Fuxit]

If I understand your query correctly, then the "high" end of the purlins should be notched underneath, so that there is a flat area which would be parallel to the top surface of any brick on which it would rest.

[sm_jamieson]

Haha ! I'm learning a lot fast, thus the profusions of questions to the group. Only using the purlins idea because I saw it in the regs diagram, and it results in shorter spans. I would need purlins anyway, so why not just purlins ...? Cheers, Simon.

[sm_jamieson]

Have you any more info on this. Is the restraint to hold it against the slope, or to hold the walls in ? Simon.

[Hugo Nebula]

On 20 Oct 2005 14:06:53 -0700, a particular chimpanzee named sm [e-mail] randomly hit the keyboard and produced:

Neither. I'm sure Tony Bryer can explain this better than me, but the beam will be better able to resist bending if the ends are not free to rotate (held in place). Try it with a ruler; hold it in place by its ends and press down in the middle. Now try it without holding the ends.

[Hugo Nebula]

On 20 Oct 2005 13:43:14 -0700, a particular chimpanzee named sm [e-mail] randomly hit the keyboard and produced:

OK, let's examine your plans.

As I understand it, you're building a lean-to extension, with the clear span along the slope of 5m, and counter of 4m. You've gone with a low pitch; now correct me if I'm wrong, but this would imply to me that you've got to get the roof to fit under a window or some other obstruction at first floor, or you want to keep the height below a certain height to keep within Permitted Development rules. I'm presuming that you intend doing as much of the work yourself as you can, or employing individual trades rather than a main contractor.

1. "Conventional" cut roof: You would need a purlin at the mid span of the rafters, and as in a previous post, it would be best if it was a steel beam. Without calculating it, you would possibly need a

178x102x19kg UB or a 152x152x23kg UC, giving a weight of ~80-100kg. This will be fitted onto padstones already built into the wall. Unless you're a major league powerlifter you'll need a few guys to help you. This may be in the form of a couple of the brickies for an hour, or a gang of mates with the promise of a few pints after. After this, the rafters can be fitted by one person, either yourself over a number of weeks or a roofer in a day.

1. "Purlined" roof: You're planning 75x200 C24 purlins at 900mm centres. Assuming 590kg/m³ for softwood, this is a weight of 37kg each. For 5m, you're going to need 6no. These will have to be built into the wall as they're installed, so a bit of juggling to get them in the right place while someone else cuts some blocks and mortars around them is needed. Do you want to trust that to your mates? Once that's all done, then it's down to the plywood sheathing, then the counterbattens & felt, and then the battens. This will give you a roof with an overall thickness of 244mm +battens & tiles over

+plasterboard & insulation under, as opposed to the 100mm rafter depth of the "conventional" roof. If ridge height is critical, this would either flatten out your roof pitch even further or reduce the ceiling height inside.