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 17 hours ago 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.