Problem With Roof Load onto Steels in loft conversion

[eglwysman]

I have attached a picture, This shows the opening we have for the stairway and directly above is where the truss meets the floor, You can see from this angle however that the truss is connected to the binder and not a supporting wall underneath which is the same for all 4 trusses, So my point in making supports within the new loft area as per my diagram above going direct down to the binders is where I plan to put the new supports we want to put them directly over the internal walls fopr extra support so why is that not better than what is their at the moment?

 

[eglwysman]

1st Pic shows where stair access is going to be.

2nd Pic shows the water tank moved to the corner that will be boxes in the fitted wardrobes.

3rd Pic shows the steels cemented into wall on padstones

4th Pic shows how far I have joisted up the 9x2's thus far

5th Pic shows joists inserted into steels you can't see it but they are 3" above the binder

6th pic shows the the 9x2 joists and you can see the gap between the binders and the new flooring.

 

[RonnyRaygun]

I have attached a picture, This shows the opening we have for the stairway and directly above is where the truss meets the floor, You can see from this angle however that the truss is connected to the binder and not a supporting wall underneath which is the same for all 4 trusses, So my point in making supports within the new loft area as per my diagram above going direct down to the binders is where I plan to put the new supports we want to put them directly over the internal walls fopr extra support so why is that not better than what is their at the moment?

I can see dowels into the binders, and I would imagine that there is also a connection from the truss to the existing ceiling joist.
What you will find is that the truss isn't being supported by the binder, but is actually supporting it, due to the effect of triangulation. The binder will in turn give further support to the ceiling joists.
So you won't be able to transfer load to the binder once the truss is gone.

The other thing I've noticed is that it's not just the top purlins which are given support by the trusses, but the bottom ones too. Remove the trusses, and the lower purlins sag, imposing load to the new steelwork...but the steels are at maximum capacity already, so you can't allow that to happen...

You need to get support to all four purlins, once the trusses are removed, and that involves more steelwork. One option would be to splice through the steels already in place with more steels (at 90 degrees to them), and send props up to the purlins.

Your SE has cocked up, pure and simple...

 

[eglwysman]

I have attached a picture, This shows the opening we have for the stairway and directly above is where the truss meets the floor, You can see from this angle however that the truss is connected to the binder and not a supporting wall underneath which is the same for all 4 trusses, So my point in making supports within the new loft area as per my diagram above going direct down to the binders is where I plan to put the new supports we want to put them directly over the internal walls fopr extra support so why is that not better than what is their at the moment?

I can see dowels into the binders, and I would imagine that there is also a connection from the truss to the existing ceiling joist.
What you will find is that the truss isn't being supported by the binder, but is actually supporting it, due to the effect of triangulation. The binder will in turn give further support to the ceiling joists.
So you won't be able to transfer load to the binder once the truss is gone.

The other thing I've noticed is that it's not just the top purlins which are given support by the trusses, but the bottom ones too. Remove the trusses, and the lower purlins sag, imposing load to the new steelwork...but the steels are at maximum capacity already, so you can't allow that to happen...

You need to get support to all four purlins, once the trusses are removed, and that involves more steelwork. One option would be to splice through the steels already in place with more steels (at 90 degrees to them), and send props up to the purlins.

Your SE has cocked up, pure and simple...

Many Thanks for the reply. Ok according to the calculations the steels we have support the floor load only as it is assumed the steel is unsupported through it's entire length, so if this steel was supported underneath from the internal walls that would make it's capacity to handle weight greater thus being able to transfer more weight to it?

I only state this as their is a guy who lives around the corner from me who has the exact same loft as me, He did not do his loft to reg's just did it His steels are only 5'8" depth whereas mine are 8.8" his joists are only 6x2 mine 9x2, He supported his steel by building up from internal walls and transfered the weight of the ceiling direct to those steels and 6 years later after the bloody winter we had last year no problems, So i'm just wondering if their is a way without the expense of an extra £2000 for more steels when I have done everything right thus far getting plans drawn up and structual calcs etc submitting them and yet now I got all this, My house is turned upside down I got the kids squashed into a room until it's done and now it looks like it won't be done

 

[RonnyRaygun]

Hmm, if the wall under is sufficiently supported and strong enough then you should be able to use it as a central support for the steels.
That way you would be able to take the load of the lower part of the roof onto the steels.

Then you need to get a calc done to see if your existing rafters will span between the ridge and the lower purlin with the trusses and upper purlins removed. If not (and probably not) get stronger rafters installed between ridge and lower purlins, that are capable of spanning that distance.

Go back to your SE and suggest the above to him, and get him to do the calcs to prove it all works.

 

[eglwysman]

Hmm, if the wall under is sufficiently supported and strong enough then you should be able to use it as a central support for the steels.
That way you would be able to take the load of the lower part of the roof onto the steels.

Then you need to get a calc done to see if your existing rafters will span between the ridge and the lower purlin with the trusses and upper purlins removed. If not (and probably not) get stronger rafters installed between ridge and lower purlins, that are capable of spanning that distance.

Go back to your SE and suggest the above to him, and get him to do the calcs to prove it all works.

Thanks for the reply again, Funny enough I have just emailed across this idea to him and hope to get a reply. I am not planning on removing any of purlins the middle purlins we were simply going to rub down and paint to make a feature as I understand the support the give to the roof. As per my drawing earlier in the middle part of the attic we were going to pack again from the internal walls to the new floor joists which we were going to have double so 2 x 9x2 joists and stud direcly to those middle purlins on each side of the loft, The idea was as the space is large the bed would be at one end and have a kind of walk in wardrobe area the other as the purlins on each side are 10ft apart it would not be a narrow walkway and thus also add support to those middle purlins, We were also going to do this at each end of the loft area as one end we are having wardrobes made off one gable end so again the frame work would be supported underneath from internal walls packed upto the joists and then stud work upto the purlins and at the other gable end over the bed to create storage but done in the same way, So long each middle purlin it would be supported 3 times once in the middle and two at each end, Plus the braces across the ridges and noggins from the steel upto the lower purlin does this sound ok?

 

[RonnyRaygun]

Do the internal walls that you propose to use go down to ground level and do they have foundations?

Were you to remove the purlin and strengthen the rafters you would have a lot more freedom in your room layout.

 

[eglwysman]

Do the internal walls that you propose to use go down to ground level and do they have foundations?

Were you to remove the purlin and strengthen the rafters you would have a lot more freedom in your room layout.

Yes they are brick build all the way down black mortar lovely it's a 1940's semi. If I remove the purlin though even strengthen the rafters would that not make it as stong? As I would have thought that purlin is providing a good deal of support to those rafeters?

 

[RonnyRaygun]

The upper purlin is giving support to the rafters because they aren't capable of spanning from the ridge to the lower purlin.
But if you put in rafters that are capable of spanning that distance (or sister the existing ones) then you have no need for the purlin...just a suggestion as it won't be very expensive to do.
Any extra timber will easily be covered by the insulation that you have to put in the roof anyway.

 

[ibby]

The upper purlin is giving support to the rafters because they aren't capable of spanning from the ridge to the lower purlin.
But if you put in rafters that are capable of spanning that distance (or sister the existing ones) then you have no need for the purlin...just a suggestion as it won't be very expensive to do.
Any extra timber will easily be covered by the insulation that you have to put in the roof anyway.

I would have also suggested that, but wouldnt sistering tje joists lower the height of the overall loft.

But right now he has no option. unless he removes the steel and replaces with bigger ones!

Normal rafters are 3x2s, If you put 6x3s or 6x2s side by side that would make the rafters hardcore

 

[RonnyRaygun]

I would have also suggested that, but wouldnt sistering tje joists lower the height of the overall loft.

But right now he has no option. unless he removes the steel and replaces with bigger ones!

Normal rafters are 3x2s, If you put 6x3s or 6x2s side by side that would make the rafters hardcore

I could be wrong as it depends on the spans, but I reckon that 50x150s would do the job by themselves, or 50x100s or possibly 50x125s working in combination with what's already there.

Isn't it something like 165mm of insulation plus an air gap these days to comply with regs, so there's around 200mm to play with.

 

[eglwysman]

I was not really planning on removing the purlins as yes sistering the rafters would lower the space we have in the loft so don't really want to go down that road.

If that is the only option is it possible to sister and leave the purlins in?

 

[RonnyRaygun]

It won't lower the space you end up with as you are going to lose near 200mm due to insulation and airgap if you are doing it to comply with buiding regs.
As I said, you will probably only need 50x125 deep joists alongside the existing ones.

 

[ibby]

I was not really planning on removing the purlins as yes sistering the rafters would lower the space we have in the loft so don't really want to go down that road.

If that is the only option is it possible to sister and leave the purlins in?

Why dont you sister them next to the existing joints?

But as pointed BCO regs you need x ammount of insulation

 

[eglwysman]

Insulation is not a problem as I have the space blanket insulation and not kingspan due to keeping as much head height as poss. I believe that middle purlin is carrying the majority of the middle section of weight and just believe I need to support that purlin at differnet points as much as poss we have a stud wall coming off a brick internal wall from the first floor upto the purlin in the middle of the loft thus seperating the loft into 2 large sections. We are alos doing hte same off internal birck walls at one end as we are having fitted wardrobes the problem is the other end as their are no walls underneath to pack off to go upto the purlin.