How many rafter sections can be removed for a Velux

[Cory H]

I know a single rafter section can be removed to fit a Velux but how about 2 or 3 rafters for wider windows?

In all cases, rafters, headers and trimmers would all be doubled up.

Thanks

 

[^woody^]

As many as you like as long as the new supports are adequate - which may well be more than just doubling a rafter and chancing it.

 

[Cory H]

Thanks @^woody^, just wondering what is considered safe without involving extra supports over and above doubling-up the framing and supporting rafters?
In my case, I want to remove 2 rafters (5x2 rafters with centres at 360mm with total span of 4.8m) to install a 94wx160h roof window.
Is there any generally accepted limits of how many rafters can be removed without having to resort to additional supports (over and above what I described above)? Thanks again

 

[^woody^]

Typically its double up for a up to 1200 wide window ie one trimmed rafter. But larger than and trimming two rafters would need more detailed assessment - not just the weight of the new window but the loads from the two rafters and whether the existing rafters have the capacity.

 

[Dereekoo]

Logically if you are moving 2 rafters over to the side they would be carrying the same load as before you moved them so would be structurally ok BUT if the roof is being replaced by a Velux this may (or may not) impart more (or less dead load) on the moved rafters, the imposed snow load staying the same. The only other consideration would be if the trimmers changed part of UDL loads on the the rafters to point loads which may affect the calculated bending moment/deflection of the joist and as Woody says the weight of the trimmed rafters/trimmers. Notwithstanding the foregoing 5 x 2 rafters spanning 4.8m seem well undersized?

 

[catlad]

No problem, must be a well built roof if you have 5x2 rafters.

 

[^woody^]

The only other consideration would be if the trimmers changed part of UDL loads on the the rafters to point loads

Which trimmers will do by virtue of being trimmers, and is the main factor

 

[Dereekoo]

Which trimmers will do by virtue of being trimmers, and is the main factor

To an extent depending on the size of the velux which is why an SE or someone with structural nous is best placed to give a definitive comment

 

[dannymassive]

To an extent depending on the size of the velux which is why an SE or someone with structural nous is best placed to give a definitive comment

Yes, I'd agree with Dereekoo - most likely need a structural engineer to look into doing some calculations for you as (in my area at least, others may also confirm) adding a new Velux would need building regs - so building control would want to see the calcs.