where rafters cross purlins

[sm_jamieson]

On a traditional cut roof, when the rafter joins the wall plate you cut a birdsmouth to give a good seat onto the wallplate. But where the rafters cross purlins, what is "good form" for the joints here ? The possibilities are

1. Cut birdsmouth at each purlin - gives good seat but very hard so space correctly, and rafter cannot slide and this settle. No builder would do this - too much work, I've never seen it.
2. Chamfer purlin to give a larger contact area - no locking in, rafter can slide to settle
3. Just rest the rafter on the square-ish corner of the purlin. This seems to be very common, but is this good form ? Very small contact area / pressure point. Rafter may settle in and lock to some extent as the corner of the purlin beds into the rafter
4. Angled purlins. I've seen this, but a right pain to build in where the purlins sit in the wall, or not good with joist hangers.

What should I do for my roof ? This is not specified on the plans. Note - in my case roof is very low pitch (12.5 degrees - lowest possible with tiles - redland regent), and the design has several purlins. Sort of cross between a pitch and flat roof in design. For some reason option 3 does not sit easy with my engineering sensibilities, although this is most commonly seen on trad roofs of a standard pitch.

Simon.

[John Rumm]

not seen that

or that

Have seen that - and a variation where a separate tilt fillet it cut to sit on the purlin and match the slope on its top edge.

Probably seen this most often

4 is easy enough if you have prop posts holding the purlins, and works well enough on corbelled out brickwork.

[sm_jamieson]

Thanks for the reply. I'll probably go for three (joist hangers mentioned on plans). A tilt fillet at 12.5 degrees hardly seems worth it. But if using a tilt fillet, how would it be fixed. A couple of nails ?

Do you know the maximum length you can get of rafters ("50mm x 47mm counterbattens" on plans - note lots of purlins !) ? It may be easier to join 2 lengths to make up the 5 metres. If I do this, the joint will be over a purlin of course, but what is the best way of joining them, and the overlap length ?

Also, any idea of the best way to line the purlins up with the rafters ? Seems to be 2 ways:

1. Fix rafters and then push purlins up underneath to align.
2. stretch a string and use this to position the purlins, then fit the rafters over the top

Thanks, Simon.

[Dave Liquorice]

This is the only version I have seen in old roofs. Can't really see the problem with a few bits of cut brick and some mortar at the wall ends. In fcat witha 12.5 deg pitch slope the purlins are not going to be that far off vertical anyway. Couple of bits of broken tile to pack the space if the mortar "look to thick"?

[Tim S]

John Rumm coughed up some electrons that declared:

Mine are all like this...

If following the building practises in my house, simply place two bits of wood to be joined in vague proximity and insert 4" nail :)

I'm really going to have to stop looking at my wood too hard - it's depressing! OTOH, it hasn't fallen down in >30 years...

Cheers

Tim

[Cash]

Simon,

The purlin on a simple, single roof is there to stop the rafters bending - and as such, the purlin rests at 90 degrees to the rafter pitch - and the rafters simply rest on them and these are held by the plumb cut at the ridge and the plumb and seat cuts on the wallplate. You could 'skew nail' the rafters to the purlin as an extra fixing if you wish (normal).

if you are using them on a so-called double roof, then the rafters are 'cogged' (a type of housing joint) to the purlin.

Cash

[John Rumm]

Yup, nail it to the top of the pulin every so often

50x47 seems a bit skimpy for a rafter 100x47 would be more common.

getting 100x47 in 5.4m is easy enough - and ripping one of those lengthways would give you equivalent length 47mm square ish - not sure if you will be able to buy it in that length as a standard size - although a decent wood merchant could rip it to size for you.

The downside of a join is the loss of the extra support you get in cantilever. Perhaps longish overlaps nailed together would restore some of this.

Normally if you are starting from scratch, then you could cut a pattern rafter and make as many copies as required, and erect those with birdsmouth and ridge board (keeping the ridge in balance so it does not get pushed out of line), and add the pulins later (but before felting and battening the roof). Given you have a very low pitch, that is going to encourage the rafters to sag more, and also you will have more lateral thrust at the wall plate just from the weight of the rafters. So having the purlins in place first may be advisable.

[sm_jamieson]

They are more "counterbattens", at 400mm spacing over 225 x 75mm purlins at 730mm spacing. Struct eng passed it, must be OK I guess. The funny roof design is a monopitch with vaulted ceiling, and the smaller dimension (4 metres) was parallel to the slope, thus lots of "purlins", Requirement was to get the thinnest roof possible, including the insulation, without using steel. Lost of glass fibre between the purlins, 25mm celotex under the whole lot. I could have got it thinner with steel, but that seemed a bit OTT.

Yep, I thought that. Its actually more like a flat roof in loadings etc.

Simon.

[sm_jamieson]

Makes sense to have the purlin at the angle of the rafter, I agree. But the last two houses I have lived in had the purlin vertical, and the rafter bearing on the purlin "corner". My parents house had them at the rafter pitch. Simon.