Rafter Braced Joist Strengthening

[Robert B]

Hi all,

I am looking into either strengthening our existing loft joists to have more storage, or even possibly constructing a new floor for an eventual full loft conversion if possible. I'm trying to get a structural engineer out to see it, but it would be good to have some ideas as I'm going mad trying to figure this out! I am aware of the need for buildings regs by the way.

We have what I think is referred to as a rafter braced roof i.e. the ceiling joists (2x3" roughly 12" apart) do not sit on the front and back walls and are instead attached higher up to the rafters (2x3"), giving sloping ceilings to the upstairs bedrooms. Span of joists is 5.30m.

If constructing a new floor, the usual way of doing it is adding beams from gable wall to party wall and hanging joists of it. The trouble is our party wall is only half a brick width!

We have a structural wall about half way in the house (party wall to gable).

So questions are:

1. Does anyone know of a way of strengthen the existing joists whilst keeping the roof strong?
2. Does anyone know if half a brick wall has ever been known to support 2 beams and new floor?

Many, many thanks for any help you can offer.

Rob

 

[Notch7]

sounds like you have rafter ties acting as collars instead of ceiling joists as such, giving you skeilings in the bedrooms

 

[Robert B]

Hi Notch7. Yes, that sounds like what we have.

 

[Robert B]

I should add that we have purlins (3x3.5"), and struts coming from middle structural wall. I wonder if strengthen the purlins would help if strengthening the joists - I am not sure how to do this though. I'd like to remove the struts for more space. Believe it or not the roof is supported by old apple tree trunks!

 

[John D v2.0]

1. Does anyone know of a way of strengthen the existing joists whilst keeping the roof strong?
2. Does anyone know if half a brick wall has ever been known to support 2 beams and new floor?

I think all these are going to be for a structural designer. This kind of roof we have too and it's tricky.
Off the top of my diy head, options without changing the roof line would be lower the ceilings or get some cranked(bent) beams made up to go onto the wall. Alternatively you could run things the opposite direction.
If you literally just want to reduce the deflection on the existing setup, you can sister the existing joists along most of their length with something thicker. But you'd never get that signed off for anything official by an SE.
Half brick walls can easily support beams and anything else. The weight can be spread by a padstone, the main problem would be stability of the wall. Half our roof is held up by a 2 inch cinder block wall, but there are metal posts attached to the ends of it to stop it crumpling

 

[Robert B]

Hi John, thanks very much for your reply. It's nice to hear from someone who has the same roof as ours. All of the information I can find is for 'normal' traditional roofs!

I am having difficulty finding a structural engineer willing to take a look! They all seem booked up, I'll keep on trying though.

Thanks for taking the time to come up with some ideas. I think lowering the ceilings would be too much work and hassle and also bring the height of the rooms down too low. Cranked beams may work but I have no idea how to go about this - I wonder if I could make similar things out of timber. Running things in the opposite direction would cause the head height in the loft to be too low I think.

If the joists are sistered, I imagine that would put more load on the rafters, so do you think the purlins also need sistering/beefing up, whilst also adding collars under the ridge-beam? And then would this be enough to enable the struts supporting the purlins and the ridge-beam to be removed, do you think?

Many thanks,

Rob

 

[John D v2.0]

I doubt you can remove the struts easily, remember for most of this the deflection is the issue, it would only be of you try to use timber where steels are needed that there would be a strength issue.

 

[Ian H]

Before we had our roof done I visited one that the builders were doing and that had a single brick party wall.

The BCO or SE had them chemical anchor an angle iron at loft floor level and build a block wall off that, tied into the existing wall. Then on top of that they had a huge padstone and RSJ ridge beam.

I'm not sure if the RSJ's for the floor also came off it though.

 

[Robert B]

Hi John, I am sorry but I'm a bit confused as to what you mean.

Hey Ian, thanks for your reply. That sounds mental!

 

[John D v2.0]

Cranked beams may work but I have no idea how to go about this - I wonder if I could make similar things out of timber.

I don't think you could make a strong enough connection, timber doesn't have a lot of strength in the first place. These beams would hold up your whole floor.

If the joists are sistered, I imagine that would put more load on the rafters, so do you think the purlins also need sistering/beefing up, whilst also adding collars under the ridge-beam? And then would this be enough to enable the struts supporting the purlins and the ridge-beam to be removed, do you think?

Doubt it, an SE would be qualified to redesign your roof, but generally removing things without replacement would be difficult.

 

[Robert B]

Thanks for clarifying, John. Sounds like this'll be difficult then.

 

[John D v2.0]

Thanks for clarifying, John. Sounds like this'll be difficult then.

An SE would be able to tell you better. But for normal storage you needn't worry, just do what you can and board it over.

 

[Robert B]

Thanks John. Cant help but keep thinking of solutions though:

Surely beefing up the purlins should help? I imagine the struts are there to stop the purlin from bowing, so if the purlins are stronger by being sistered, then less likely to bow? no?

I agree an SE would be great, but I'll be damned if I can find one around here!

 

[John D v2.0]

Surely beefing up the purlins should help? I imagine the struts are there to stop the purlin from bowing, so if the purlins are stronger by being sistered, then less likely to bow? no?

maybe, but adding the sistering adds weight. it may be surprising at first, but once you get to a certain span, the self weight of the section used actually is too much. So you can increase the depth up to a certain point, but for standard timber floors you can't even get to 5m span.
Switch to engineered timber I joists or trussed setups and you can go further as they're lighter, but there are still limits well under, say 10m or so. Then finally steel takes over as the only option.

 

[Robert B]

I can see that sistering the purlins would add weight, but I imagine that weight would go through the walls, which should be ok, no?

I do appreciate your patience with me John, thanks for your help!

I'll add some photos on the off chance that some amazing solution comes from them!

The dimensions of the roof space is 4.53m (gable to party wall) x 5.20m. The floor I want to create will sit in the middle and be 2.70m x 4.53m.

20180523_071111

• Robert B
• 23 May 2018

20180523_071205

• Robert B
• 23 May 2018

20180523_071154

• Robert B
• 23 May 2018

Tree trunk!

20180523_071148

• Robert B
• 23 May 2018

More tree trunks! I guess no SE would pass these but they've held the house up for 150 years!

20180523_071122

• Robert B
• 23 May 2018