Doubling up rafters - how to support

bpj

We would like to add two Velux windows to a catslide roof, each requiring a section to be removed from one rafter. The remaining rafters (50x100 mm) will be doubled up with new 47x150mm rafters (increased size specified by structural engineer as a supporting stud wall is also being removed). We are aware of the need for building control sign off.

The new rafters will be supported from below by a new timber frame wall, will be bolted along their length to the existing rafters, and connected laterally to the cut rafters. However, I am not clear on the best way to support the new rafters at their top. Here's a diagram:

The main roof rafters overlap with the rafters to be strengthened:


For two of the rafters this is the only accessible side (the wall interferes with the other side), and so the only option seems to be to somehow splice to the main roof rafters. They are 50x100 mm vs the new 47x150 mm, and the accessible length is only ~200-300 mm. Our structural engineer didn't seem too worried about this as the load is transferred via its other connections and is well supported below. However he didn't want to give any specific advice. (Possibly the best advice - employ an experienced builder - does not seem to be feasible at the moment, as we need the job finished one way or another before Christmas). If anyone reading can suggest an approach we'd be very grateful.

Thanks very much in advance!

grumbler

How thick is the wall? Is it possible to cut the main roof rafter in the middle of the wall to free the space for the new one?

Alternatively you can cut the rafter flash with the wall and add a wall plate fixed with anchor bolts to the wall to support the new rafter(s).

Mutton_Geoff

You don't need to support the ends as you are not adding any load, just cut the new rafters and bolt them to the existing ones using 13mm wide headed bolts spaced approx 40 or 50cm. A structural engineer can run some calcs for you in a short time to give you peace of mind.

grumbler

Mutton_Geoff said:

 you are not adding any load

You are. These rafters now support two noggins, i.e. a part of the load of the rafter cut for the window.

I agree, the extra load is probably not big enough to justify extra support for the new rafters at the end - when the weakest part of any rafter is the middle.

bpj

Thanks very much for the input. These plans are from a structural engineer. His advice on the question was:

"on specific detailed connection elements, in our experience it's often much better leaving  an element of leeway for the contractor so they can also use their experienced judgement etc as they will be used to this sort of thing.

Structurally the main principles are the rafters are strengthened along the main length and supported by the lower timber frame wall."

I think the second paragraph matches with what you're both telling me. It sounds like the bolts to the existing rafter are going to be doing all the work, so any kind of splice to the existing will be sufficient.

Thanks again!

Mutton_Geoff

grumbler said:

Mutton_Geoff said:

 you are not adding any load

You are. These rafters now support two noggins, i.e. a part of the load of the rafter cut for the window.

I agree, the extra load is probably not big enough to justify extra support for the new rafters at the end - when the weakest part of any rafter is the middle.

I was thinking total snow load for the roof spread across the walls rather than localised increases due to a missing rafter.