Potential loft conversion - moving collar ties

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I'm considering a loft conversion. Before getting someone round to have a look at it I'm trying to cover the basics of "is it possible/worthwhile or not". I already know headroom is likely to be limited due to the ridge height. Access stairs should still have the required headroom though.

It's a 100 y/o slate roof with horizontal beams between the rafters which are currently in the way. Are these called collar ties? Several have already been moved, presumably to fit the now redundant water tank. Some photos attached.

How would the roof timber be modified to create space if I were to go ahead with it? Simply moving the collar ties higher up? This would limit head space too much. Or removing them completely and fitting in vertical bracing from the joists to the rafters along each side? I've found loads of information on modifying more complicated loft spaces, but for something so 'simple' as this I've not seen much. Any suggestions of what to expect before I invite someone around would be much appreciated. Forewarned is Forearmed etc. Thanks.
 

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I already know headroom is likely to be limited due to the ridge height.
I would start by checking what the head height actually is to the ridge to see whether it is worthwhile. New floor joists deeper than the existing ceiling joists would most likely be required so that would reduce the headroom further.
 
If you have 2.1m to the underside of the existing timber at the ridge as your diagram indicates then even if you bring the stairs bang under the centre of the the roof, I don't think you're going to have the required 2.0m at the edge of the stairs once you've fitted insulation and plasterboard underneath the existing timber.

I suspect once you've handed this over to a structural engineer you're going to find the solution to removing the collar ties is to fit a ridge beam under the existing timber, which leaves you without the headroom for stairs at any point.

I think that leaves you either roof-off and redesign with the smallest build up possible, combined with putting the roof back on slightly higher as the Council won't notice or be able to prove it if they do, or planning permission to raise the roof to do it properly. Perhaps you can lower the ceiling?

From a practical point of view, having the bare minimum at the peak won't leave much useful space once the slope comes into effect, especially as the stairs will be under the 'best' bit of the roof. Can't see your dimensions but on my loft conversion at 6m wide and 8m front to back a 2.7m ridge (reduced to 2.4m with a new glulam under the ridge board) I wasn't left with loads of space, though being able to put a desk, bed, sofa under the eaves does help. I ended up going up to three stories on our rear extension, which created a new roof over the corner to get the headroom to tuck the stairs out of the way and then put a big (2m wide) dormer on the opposing side to get the space really useful.

My instinct based on your diagram is to look at planning permissions for a higher roof as your best option, but once the roof is coming off costs spiral rapidly!
 
Just noticed the 2.1 m vertical dimension, so no, it isn't worth doing because of the limited headroom unless you can include some substantial dormers :!:
 
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Hi all, thanks for the comments. Not what I wanted to hear but there you go... I had planned on adding a 2m wide dormer which would significantly help headroom, but at the other end of the loft from the stairs.

Are there any tricks for getting the stairs to pass building regs with limited headroom if I'm 10-20cm short of space? They will enter the loft too close to the ridge for a velux window to help.

Rasing the roof or dropping ceilings isn't an option, too expensive to justify I think.
 

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