Help!! Clarification on roof detail

[AFH_J]

Morning all,

 

Long time reader of the forum, and joined yesterday to get some advice. I am going to cut a long story as short as possible:

 

I have been self renovating a house for the last 3 years which has involved architects, structural engineers, planning, going back to bare brick, new electrics, new plumbing, insulating etc etc. The final part of the 2nd extension (double storey side) is near completion and the roof now needs doing,...this was going to be something that I pay someone else to do and just hand the plans over, but I have unfortunately been made redundant and as such am currently on leave for the next 6 months so I am trying to use the time wisely,...

 

So thats the background, my query is regarding the plans and how to join old to new:

 

- My existing house is of solid brick construction at 225mm wall thickness, which means the wall plate will sit on the outer edge of the 225 brickwork. However, with a cavity wall setup, the wallplate needs to be on the inner leaf meaning that the inner leaf brickwork will have to be built higher to accommodate the birds-mouth, correct? This means that the restraining / joining horizontal timbers that join the bottom of the rafters together will sit higher? The challenge is that there is a loft conversion going in too. The new side extension is set back 225mm to be subservient, but this also had to be minimised it to retain the head height in the dormer at the back - The roofline was originally planned to be in line with the existing, but the planners requested the roof be set back too, so the plans were adjusted and the new steel cranked beam was offset.

 

Regardless whether I go with the 'set back' roofline or break the rules and go with the original plan of the continued roofline, I am still struggling to understand how the construction is setup and the floor height will be the same? Because, in my head the restraining timbers will need to be correctly planned and they will go 'through' the blockwork for the inner leaf? The plans detail the connections, wall plates etc but I assume that a roofer would know this when I handed the plans to them, but as that's not happening now, I am struggling to understand how the floor height is maintained with a different wall construction unless the floor timbers go through the blockwork?

 

If you've not already figured it out, I overthink!!

 

Looking forward to some advice, and apologies for the lengthy message. I can share plans etc if required.

 

Thanks in advance.

[Iceverge]

Get a pencil out and do a sketch please for us. 

 

I think I have your issue visualised. 

 

My suggestion would be to have 2 separate wall plates. 

 

 

You can set the raised top plate back (left in this picture) to achieve your required roof hight. Regardless of the wall construction below and you can have it identical no matter what the situation is with the walls below. Very useful on wobbly buildings. 

 

It carries the additional advantage of allowing continuous joining of insulation between the wall and roof insulation in the cavity wall situation. 

 

Have you a required timber depth figured out and a targeted U value? 

 

I could do a sketch if you have plans. 

 

 

[AFH_J]

@Iceverge Thanks for the swift reply.

 

I have attached the my dodgy sketch (apologies, it may not be very clear but it is in my head), and the plans. I have included both the inline roof and the set back roof amendment - I think the set back looks good but the lone skylight doesn't work for me, the clean roofline looks good too but with 2 skylights inline with the below windows, however this wasn't agreed with the council unfortunately - Anyway, added both for reference.

 

The timber joist size as detailed in the plans is 47 x 170 C24 however I cannot find / see a targeted U value, but the requirement is 100mm PIR between rafters, and then 50 underneath - These plans are a couple of years old now and am aware the regs have changed, and although no requirement to comply with the current regs, I have advised my BC Officer that I will (where possible) adhere to the current regs,...so the 50mm over the top is likely to increase - In addition to this, if I sister the rafters with 47 x 150 C24 as per plans, I will have 'less' insulation so I may extend the existing rafters with strong 3x2 timber (current rafters are 75 x 50), poly glue and screws as I understand this will also satisfy BC - And also means I can go for a bigger depth with less risk of cold spots.

 

The current ceiling rafters run front to back but the way I read that plans is to change that and have them run left to right, over the various supporting walls (makes sense as its now going to be a structural room), so I will sit the new joists just above (perpendicular) the existing timbers that sit on the existing wall plate (my dodgy sketch doesn't show the direction, just the perceived problem) - regardless of how I do it, the new and old wall plates wont line up correctly so this is where I need the advice!!

 

Thanks again!

[Iceverge]

There we go. 

 

 

Build the inner leaf to the same height as the old wall, Add a new wall plate just for the joists. 

 

Then add a second wall plate for the new rafters on top of the joists. 

 

 

This would be my preferred buildup. A hybird warm roof but everyone here seems to be terrified of them. You'd need raise the level of the old roof too. 

 

 

 

 

 

Here's a version with mineral wool between the rafters. Better than PIR in many ways. 100mm PIR below as it's good value at that depth. 

 

 

Or a full fill mineral wool with ventiation above the membrane. 

 

 

 

 

[ETC]

Er…….ask your engineer. Don’t make it up yourself.

[AFH_J]

@Iceverge Outstanding response, thank you vey much - Really appreciate it

 

@ETC 100% agree, but my engineer is in the fortunate position to holiday for 6 months of the year so is currently not contactable. Wanted to jump on here to get some ideas as I'll no doubt need to get someone else to look into it for me and confirm that a proposed solution it is up to spec,...but wanted to have an idea of what I may need before another SE wants to completely re-design and re-spec everything,...especially as its someone else's work.