What size of rooflight requires more than doubling of flat roof rafters?

[Jml]

We have been temporary “repairing” part of our flat roof, thanks to a site member who recommended “black gloop”which did the trick.  Now even that is not working, so time for a replacement.

 

 Having got a number of quotes, we now think adding two flat rooflights to the roof would add a bit of wow, for us.  We started looking at 1200x900mm but having marked them out on the ceiling they look a bit small.  We have come to the conclusion that 1000x2000mm would best suit.  
 

Our roof has joists at 400mm centres, so 2000mm long rooflight, means cutting quite a few of these.  Whilst I understand that doubling of roof rafters is common to support the rooflight, is this going to be enough given the size?  Is there a maximum sizeof rooflight or number of cut rafters that doubling the rafters is sufficient, before getting a structural engineer involved? Is structural engineer involvement necessary anyway, none of the roofers have mentioned it?  Many thanks in advance for any thoughts.

 

 

[markc]

All depends on which way the joists/rafter run and which way around the roof lights are.

Much more involved than just doubling

[Jml]

Rafters run at 90degrees to the long length of proposed rooflights, so would mean cutting maximum number of rafters, probably 6.

 

Easy to measure and see which way they run, through collapsed ceiling!

Edited August 29, 2021 by Jml

[markc]

Right, now cutting 6 rafters would put up and extra 3x load on adjacent rafters.

this may require them to be made deeper, not just doubled or trebled. Plus weight of roof light itself also needs factoring in.

sounds like this job could do with SE input. Cutting one rafter for a velum etc on a pitched roof is much more straightforward.

[Jml]

Many thanks for your response. As an existing roof,  increasing depth of rafters is going to be a problem.  We may need to go smaller!
We don’t mind double or triple rafters, I don’t suppose there is a guide as to how many rafters can be cur for each of these options?

[markc]

Too many variables to have a guide.

if you cut one rafter you double the two adjacent ones. After that it gets more complicated.

Even the position of the roof light/load within the rafter length has a big impact on loads.

[Temp]

37 minutes ago, Jml said:

Many thanks for your response. As an existing roof,  increasing depth of rafters is going to be a problem.  We may need to go smaller!
We don’t mind double or triple rafters, I don’t suppose there is a guide as to how many rafters can be cur for each of these options?

 

Put some steels in?

[Jml]

2 hours ago, markc said:

Too many variables to have a guide.

if you cut one rafter you double the two adjacent ones. After that it gets more complicated.

Even the position of the roof light/load within the rafter length has a big impact on loads.

Many thanks.  Ok, nothing is ever simple!

 

May need a bit of a rethink.

[Jml]

2 hours ago, Temp said:

 

Put some steels in?

Thanks for your response.

 

We were just hoping for something relatively simple, quick and straightforward.  Ceiling has already partially collapsed so needs done before winter.  As I said above we may need a bit of a rethink.  

[SteamyTea]

7 hours ago, Jml said:

increasing depth of rafters is going to be a problem.  We may need to go smaller

Can the rafters go upwards  rather than downwards into the room.

It is the depth of the beam that gives it strength, not the width.

[Jml]

23 minutes ago, SteamyTea said:

Can the rafters go upwards  rather than downwards into the room.

It is the depth of the beam that gives it strength, not the width.

Many thanks for the suggestion.  Would be very difficult as flat roof has small section of pitch roof on two sides, another flat roof next to it on the third side and the main house roof on the last side.

[Dave Jones]

SE should be less than £200 and may save you that in timber. Doubling up is normally fine.