I'm retrofitting dormers onto an existing frame. If my math is correct one of the purlins won't support the load of a shed dormer. Question: Can I tie two purlins together to share the load?

[img]http://www.flickr.com/photos/95237704@N04/8681315100/in/photostream/lightbox/[/img]

The image above shows the dormer profile in red. The pink struts illustrate what I'm proposing. [img:center][/img]

Yes, I don't see why that would not work. I see blue connectors in the other bay, will the dormer rest on these? but applied in the dormer bay. Joinery cut into the existing dormer should be considered, as it will weaken the purlins.

Hi Brook, imo, the drawing does not show a sufficient structural scheme. The third purlin up will carry the bulk of the load applied. Not shown in the drawing are crippled rafters that head off the cut purlins and load the third purlin. So the choices boil down to the replacing of or the sistering on to the third purlin.

I wasn't necessarily thinking that the struts would line up with the dormer framing. Do you have an opinion?

Any sense of how to calculate the load bearing capacity of the combined purlins and the effect of mortises?

(And why won't my pic post to the forum?)

I am for sure not the one to answer the picture posting query, may have something to do with the host. I copied the address from your post, minus the [img] things front and back and pasted it into a new window.

I suspect if you connect the purlins with the connectors they may help some in transferring the load but will act as hinge points, too, and not transfer the load directly. By loading onto the connectors you with distribute the load onto both purlins, sharing the load. I am sure I will be corrected if this is far off.

Thoughts on adding new purling, sistered to the older one, beefing up the load capacity at that point, the upper purlin.

Below (hopefully) is a schematic of the dormer with cripples. Are the cripples sufficient to transfer the load to the plate thereby eliminating the need to bolster purlin 4?

[img]http://www.flickr.com/photos/95237704@N04/8681525472/in/photostream/lightbox/[/img]

Hello Brook W.

I have been trying to follow along but I can not see any of the pictures or diagrams you reference. I is there a link to them you can provide?

Regards,

jay

Jay, Sorry about the hassle. I haven't figured out how to post a pic. However, if you copy the link, minus the [img] and paste into your browser you'll see the drawings.

Sometimes TFers need to look at the effect of walls to complete the scheme. The sidewalls of the dormer will transfer the bulk of the dormer roof load to the cripples and hence on to the plates and the third purlin. I would expect that the fourth purlin will be lightly loaded.

That is for sure Roger, those front post on the dormer will carry much of the dormer weight directly to the top plate. I totally missed that at first glance. What is the speced size of the purlins, and span? and the plethora of other things usually called upon to answer such questions?

Try just coping this and paste into new window
http://www.flickr.com/photos/95237704@N04/8681315100/in/photostream/lightbox/

A new image with cripples can be found at dormer w cripples.

Purlins: 6x6 (#2 maybe?) white pine.
Span: 14' 6".

I would extend the new rafters up to the 4th purlin. You need something for roof sheathing to attach to anyway. Sister a bolster to the 4th purling where these new rafters mortice into, tusk or soffit. The bolster need not attach to the principal rafters but should be well fastened to the existing purlin and as full length as possible,tapering the ends with an adze.

Assembly? Stick the tusk tenon into the mortice and slid the foot along the plate aligning pockets to the now cut purling stubs, banging it home.

6x8 bolster? configured 8 inches deep into the living space, deeper than the 6 inches now.