Roof rafters for dormer conversion.

12 Aug 2021
Reaction score
United Kingdom
Hi all, just looking for help with my dormer conversation. I’m installing new roof rafters 150x50 and sitting these on the load bearing eaves to which the original rafters from this to the wall plate are to remain but I have a couple of problems.

-replacing the whole rafter will make the roof higher than next doors if I birds-mouth this new 150x50 to the wall plate.

- how to I join the 70mm original rafter to the new 150mm rafter when this is being birds mouthed onto the load bearing eaves.

the photos might make more sense.


  • 4F43D898-8D58-4094-B85D-BD8B8FB10901.png
    354.6 KB · Views: 80
  • 087A8DD7-AECD-4058-A8A9-EB903E4332F2.png
    480.4 KB · Views: 87
Sponsored Links
The drawings are not the greatest but my understanding is as follows.
Existing 70mm rafters stay in place. New 150 rafters glued & screwed to the side of existing rafters from ridge down to the new Eaves closure wall where they will be birdsmouthed. Top of new rafters will be level with tops of existing rafters and bottom of new rafters will be about 80mm below bottom of existing rafters. The new rafters don't appear to go all the way to the existing wallplate but could do if required with a seat cut to wallplate.
If in doubt ask your designer/engineer to clarify.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links