By how much can I notch a 6" x 6" purlin as it goes across the top of bent half way between ridge and plate whilst still retaining its full structural strength

Greener,

Rule of thunb is 1/3 max, so it is 2" for 6" deep purlin. If you run the engineering math you might get more than that in some cases, depending on load conditions (snow, wind) and purlin support/overhang.

Michal Zajic
TFDesign

So if I understand correctly - the notched purlins bearing on the bent/frame has been reduced to a depth of 4" even though the purlins structural required depth has been say calculated to be 6", my understanding of structural engineering is limited could you explain to me why if a purlin which needs a 6" depth between supports can be reduced to 4" at the bearing points?

God knows I've notched many a purlin, most of them 6x6, and like Michal said, they were taken down to 4", with a 4" lap (of course, the lap length depends on the size of the principle rafter). But these were full 6x6, not 5.5 x 5.5; keep that in mind. And from what I understand about wood is that a 2" notch in a 6" timber still holds its weight distribution properly - the weight is transfered onto the principle rafter at the joint.

Also, what length are the purlins spanning? This wiil make a difference in whether or not you will need a center notch, and thus a rafter to notch it to.

Keep in mind, I am not an engineer...I've just dealt with A LOT of purlins!

Good Luck!
*K

If I follow the question correctly - you need the full strength of the purlin in mid-span for reasons of stiffness against bending. The depth required at each bearing has more to do with absolute failure in shear.

Most timbers would have excessive flex or mid-span failure long before they break in shear at a tip - therefore the mid span needs more "meat" than the bearing at tenons or laps.

Oversimplified, but I hope it makes sense.

Pete

Yes that does make sense thanks.