Upgrading rafter ties to proper joists

[SimonD]

I need to create a loft in the existing garage before I build the new one. It's got a traditional pitched roof with rafter tiers every other rafter. The ties aren't of sufficient size to use as rafters across the current span, so I'm considering how to easily add joists across to create my new loft space.

 

Options I'm considering are:

 

1. Cut ends of new joist on the diagonal and then fix to existing wall plate - but with sufficient bearing on the wall plates I don't know if I can slot them in;

2. Install wall plates to the wall just below the ties and fix new joists to these (garage has sufficient height for this);

3. Sister the ties with the new deeper joists - and add new ones to rafters without existing ties.

 

Walls are ashlar faced breeze block.

 

Any thoughts, ideas and experience to help me decide? Any details I need to be aware of?

[Oz07]

Picture would be helpful of the existing situation. Can you put a plate ontop of existing ties or would it not be above existing due to rafters

[SimonD]

54 minutes ago, Oz07 said:

Picture would be helpful of the existing situation. Can you put a plate ontop of existing ties or would it not be above existing due to rafters

 

It's just like this but with ties every other rafter.

[Oz07]

Can you lift the bottom row of tiles and felt and slide the new joists in from outside

[SimonD]

1 minute ago, Oz07 said:

Can you lift the bottom row of tiles and felt and slide the new joists in from outside

 

Not without damage as the felt is properly fixed. I have to say that compared the the old bungalow, this garage was well built with attention to detail.

[Oz07]

Well I don't think you will ever be able to get a full length into the space with enough bearing on both plates. So looks to be leaving you with option 2. I'd be looking to strip the bottom row of felt and replace. 

[Nickfromwales]

I'd just use 2 pieces of timber, put together (sistered) so you can install them 'telescopically'. Clamp, and construction screw these together, and then infill the short pieces to make 2x full lengths per joist.

 

Dead simple.

[Iceverge]

How "good" does this need to be?

 

I'd be surprised if you couldn't wiggle something up into place. 50mm would be enough bearing. 

 

Maybe use 2 plys of longer bendier timber like 150*25 instead of a single 150*50. 

 

You could "truss" the roof by adding vertical support to the existing joists from the rafters.

 

 

 

[Oz07]

Ive Just been assuming this ceiling is boarded. What an idiot. You might get away with poking them thru the inside. 

Edited Sunday at 18:38 by Oz07

[SimonD]

14 hours ago, Oz07 said:

Ive Just been assuming this ceiling is boarded. What an idiot. You might get away with poking them thru the inside. 

 No worries! 😊

 

17 hours ago, Nickfromwales said:

I'd just use 2 pieces of timber, put together (sistered) so you can install them 'telescopically'. Clamp, and construction screw these together, and then infill the short pieces to make 2x full lengths per joist.

 

Dead simple.

 

Yeah, that's one option, but will double up the amount of timber in need to use. I will keep this option in my back pocket.

 

15 hours ago, Iceverge said:

How "good" does this need to be?..

50mm would be enough bearing. 

 

Only good enough to use as loft storage to get things out of the way instead of having to make a shed or get a half container. I was wondering whether I could get away with 50mm bearing and even if it's 75-80mm that should be fine, which means I could get the joists in place with a small amount of bearing on existing wall plates and then bolt additional wall plates to the wall immediately below for the rest of the required bearing.  I think this might be the solution.

[Gus Potter]

14 hours ago, SimonD said:

Only good enough to use as loft storage to get things out of the way instead of having to make a shed or get a half container.

Ah, so you are looking for a temporary solution. 

 

Putting my SE hat on. I need to know what the span of the ceiling joists are, their size (width, depth and spacing) and a photo of the timbers so I can see what grade of timber would be appropriate to use as a basis for a design check. Often I would plump for what is now known as a C16 grade, equivalent to an older SC3 timber grade common in the 1970's say. 

 

Next I need to know just how much weight you are going to put up there. Now when we design this sort of stuff we first need to absolutely make sure it is not going to fall down. The next check is to see if the existing ceiling ties bend (deflect) too much. In your case you have no ceiling that could crack, thus deflection is not an issue. The last check is to make sure that  the deflection is not too wild as this can pull the walls in.. for the techy as SE's we call this a second order effect / large rotational / displacement effects.. for example. It may be that what you have is ok to carry some extra weight as it's a temporary design?

 

Now if it turns out that deflection is a major issue then to control that we can sister the timbers.. but I know from experience that we can often do this by sistering almost to the wall, the sistering often does not have to extend to the wall head support. Crucially often the sistered joists don't have to bear on the wall head!

 

But what is important is that if you sister all the joists all on the same side the cumulative effect is to introduce twisting (a rolling effect) in the roof at ceiling level in your case. To avoid this we sister so they face each other in one bay, then miss one and so on. This technique is used in industrial mezzanine cold form steel floors as standard practice. 

 

Turning back to the forces. The bending force is maximum at the centre span and reduces to almost zero at the wall head. At the wall head the critical check is what we call the vertical shear check. It's likely that if you stop the sistering short of the wall then the existing ceiling joists will pass the shear check unless they have been notched at the wall head. 

 

To conclude. I would first see how much load you want to add, do you need to do anything? As a quick temporary solution.. sistering is probably the easiest, if you can stop them short of the wall head. If not then you can traingulate the roof starting like this. The way this works is an essay.. but before you do this you have to make sure the nails at the ridge and eaves are not rusty for example! What happens in your case is the ceiling joists get almost hung from the rafters.. this increases the axial load in the rafter and if your eaves nails are rusty or connections are ropey you have a problem.

 

 

 

 

 

Edited 1 hour ago by Gus Potter