Is this flat roof structurally sound

[Surfer-Rosa]

We have just had a flat roof light installed, and flat roof re-felted.

 

The company we used made a lot of mistakes during the project that they had to correct. Since they left we've removed the plasterboard to install new lighting, and are concerned about the structural integrity of the rafters.

 

The builders cut the rafters going left to right, and attached the upstand to doubled up original rafters with joist hangers..

 

One main concern is that the new rafters used to double up the original rafters aren't in contact with the brick of the supporting wall, meaning that while they do provide bracing for the original rafters, they do not help distribute the weight of the new structure.

 

Is this safe? Any thoughts appreciated.

[Mr Punter]

Your bigger problem is that the way it is set up with the insulation and shower room it will rot out within a few years.  You have a cold flat roof and there should be a continuous air gap above the insulation so that there is an easy airflow path all round.  You don't have this, so any warm moist air from, say, the shower, will find its way up until it meets the cold decking and roof joists where it will condense keeping them coated in water, with no way to escape.  Unless you have insulation above the decking this is a disaster.

[Surfer-Rosa]

12 hours ago, Mr Punter said:

Your bigger problem is that the way it is set up with the insulation and shower room it will rot out within a few years.  You have a cold flat roof and there should be a continuous air gap above the insulation so that there is an easy airflow path all round.  You don't have this, so any warm moist air from, say, the shower, will find its way up until it meets the cold decking and roof joists where it will condense keeping them coated in water, with no way to escape.  Unless you have insulation above the decking this is a disaster.

Thanks for your post, you're right, there is no way for air to circulate currently, above the pir. We are planning to make ventilation gaps through some of the firring and have two vents at (almost) opposite ends to encourage air flow in the cavity wall around the building. Thanks again for your comment.

 

Do you have any thoughts on the original issue?

[kandgmitchell]

So new joists have been added to beef up the original joist which acts as a trimmer for the new opening. Probably not critical that the new joist bears onto the brickwork as it's shear forces at the bearing that becomes important and it always surprises me how joists turn out to be very capable when you think they shouldn't. However, it's simply good practice to have all the members properly packed up off the loadbearing walls so get some shims into those gaps. 

 

What is not clear is how the new and existing trimmer joists are joined along their length - there appears to be a gap between them. If they are going to share the loading imposed by the trimming joists, then they need to be properly connected. I'd be bolting those together with spiked disc washers between so they acted as a single unit, simply bunging a few screws or nails in at random is just not enough.

 

And echo comments above, this is a very risky cold roof arrangement.

[Kelvin]

We have a similar rooflight arrangement. Our trimmers  bolted together as defined in the structural engineering drawings albeit I had to add them after the fact as they were forgotten about. 
 

 

[Temp]

While the plasterboard is down I think I would fit a vapour barrier before replacing it (in addition to adding ventilation above the insulation)

[Spinny]

Have you got an insulated upstand on the rooflight ?

[SteamyTea]

On 06/08/2025 at 08:52, Surfer-Rosa said:

One main concern is that the new rafters used to double up the original rafters aren't in contact with the brick of the supporting wall, meaning that while they do provide bracing for the original rafters, they do not help distribute the weight of the new structure

It is the depth of the joists that stop the bending, not the width (that reduces lateral movement). So if the new joists are not attached properly at the end, they are not doing much.

Get a vapour control layer in, best practice to fit one anyway.

Also check the condensation risk on the skylight and the associated upstand. 

 

As you have a felted roof, I take it no one will be walking on it, ever.

 

No surf here today.

Edited August 9 by SteamyTea