Old stable block conversion - roof design

[OscarWilliams]

Just wondering what peoples thoughts are on this roof design...I'm considering sitting a roof on a 30cm stud wall to give more usable height on the second floor, however I haven't seen this method used elsewhere before. The structure will consist of 6x2 rafters with collars. The building in question is an 18th centuary stable block that I am renovating with my brother. We are reluctant to go up any more courses of brickwork in the interest of time and cost...we have done all the work so far and are on a tight budget (famous last words). We've calculated a 45 deg pitch will give us a 30cm eave with the stud wall and will not increase the ridge height too much (if at all) from what it was orginally.

 

Note the odd design - two wall plates, one to bear the rafters and a second wall plate on the inner skin two courses lower to bear the joists which has brickwork ontop of that wall plate between each joist (the picture i've attached should clarify). In terms of building regs the building is an ancillary building to my parents house and we are treating the project very much as a restoration rather than a development...the original tiled roof had rotted and partially collapsed. We are considering putting a corrugated iron roof on but would like to build the frame so that it is strong enought to take tiles if we do decide to go down that route (unfortunately only about 30% of the original tiles are still usable).

 

Any advice is much appreciated, keen to hear peoples thoughts...

 

Cheers

 

Edited August 14, 2020 by OscarWilliams

[Temp]

I think you need SE input on this, mainly to ensure the roof isn't going to sag and push the walls out. I recall reading that collar ties aren't intended to stop the rafters pushing outwards and that a structural ridge beam should be used if you can't otherwise triangulate the roof.

Edited August 14, 2020 by Temp

[scottishjohn]

its a pity you cannot  extend  the original joists out to your roof beams attach your 30cm vertical lifters to them as well -that would be mega strong --then you would have a proper triangle with all load going directly downwards  and very simple to do -

maybe time for new longer joists and use old ones somewhere on the job-

and a nice overhang which could be closed in.

 

Edited August 15, 2020 by scottishjohn

[scottishjohn]

is there a reason to only use 30cm studs to at wall --cos that low will reduce usable space upstairs 

 If you have no planning restrictions i would suggest you think of making that  taller -anything less than 1.3m at wall is really unusable  -make it 1.6 m and you could build in storage behind the wall at same time ?

just means roof will be  flatter 

[OscarWilliams]

1 hour ago, scottishjohn said:

its a pity you cannot  extend  the original joists out to your roof beams attach your 30cm vertical lifters to them as well -that would be mega strong --then you would have a proper triangle with all load going directly downwards  and very simple to do -

maybe time for new longer joists and use old ones somewhere on the job-

and a nice overhang which could be closed in.

 

We are actually planning on driving fixings down through the wall plate and through the top of the joists where the 8x2 covers both courses which im hoping will minimise the chance of any spreading. The plates will also be strapped up every meter. Just to be belt and braces I could add some trianguation from the top of the 30cm lifter to the joist? There will also be a stud wall inside tied from the rafter to the joists.

 

We'd like to go higher but worry it might attract some unwanted attention...30 cm seems to strike the right balance between keeping the ridge height the same but with a slightly flatter pitch which actually seems to make quite a lot of difference to the inside area (at least on CAD mock up anyway). I agree though I think the design would benefit from being better triangulated...

Edited August 15, 2020 by OscarWilliams

[OscarWilliams]

Do you think with the wall plate fixed to the joists and also a stud wall on the inside tied from the rafter to the joist would provide sufficient triangulation?

Edited August 15, 2020 by OscarWilliams

[scottishjohn]

3 minutes ago, OscarWilliams said:

We'd like to go higher but worry it might attract some unwanted attention...30 cm seems to strike the right balance between keeping the ridge height the same but with a slightly flatter pitch which actually seems to make quite a bit of difference to the inside area (at least on CAD mock up anyway). I agree though I think the design would benefit from being better triangulated...

put it on your cad and see how much space there is in upstairs if you you use 1.7m as min height 

you could put you upstands in from wall as well if you extended old joists -so have your  internal walls where they would make sense + get your triangulation ,even if you don,t change the roof height or angle 

 

you seemed worried about  the walls moving  -by using floor joists tied to roof there will be no side load at all so no stress on old brick work 

have you got planning  or this yet ?

[the_r_sole]

If you can't have a bottom chord you'll need to do a ridge beam and rafter arrangement to have a useful space in the roof.

[OscarWilliams]

Triangulate like so....

[scottishjohn]

12 hours ago, OscarWilliams said:

I'm considering sitting a roof on a 30cm stud wall to give more usable height on the second floor, however I haven't seen this method used elsewhere before.  The building in question is an 18th centuary stable block that I am

look at any design for a modern TF 1/2 storey house and it will be built as i am suggesting 

bottom cord which is the ceiling joist of ground floor 

 then studs set in and attached to bottom  to give upstairs walls of approx 5ft  at the ridge or close to it there will be another cross beam -what you fix ceiling to ,which also gives it all the strength it needs 

 to go fully vaulted you need a ridge beam that goes full length of building to attach rafters to each side of it

[scottishjohn]

9 minutes ago, OscarWilliams said:

Triangulate like so....

that would work  but would use 6x2 for the vertical brace instead of 4x2,seems alot more to do than just using extended joists which would attach to end of rafters ,

 the 30cm upstand would not be needed or the short secondary brace.

 you could knock the outer row of bricks of and put joist through at same height as now and extend rafter down to meet it,but if not wanting to move any of old joists then yes it should work as you show,probably you could replace the short brace with 15mm osb triangles 

 

Edited August 15, 2020 by scottishjohn

[Russell griffiths]

You originally said that building more block work will be a lot of hassle, I think you will find messing about trying to reconfigure this to have the correct geometry even more hassle. 

 

By by the time you have messed around trying to work it out you could have run 4 courses of blocks around and be done with it. 

[Russell griffiths]

29 minutes ago, OscarWilliams said:

Do you think with the wall plate fixed to the joists and also a stud wall on the inside tied from the rafter to the joist would provide sufficient triangulation?

Nope. 

[Russell griffiths]

Do a full drawing including the ridge. 

[Temp]

14 hours ago, OscarWilliams said:

In terms of building regs the building is an ancillary building to my parents house and we are treating the project very much as a restoration rather than a development...

 

So no Building Control involvement? 

 

 

[Temp]

Whats wrong with the structural ridge beam approach with rafters "hanging" from that? The ends either bear on pad stones in the gable wall or posts. Sometimes also a wall mid span. Sometimes assisted by purlins. 

 

[OscarWilliams]

35 minutes ago, Temp said:

 

So no Building Control involvement? 

 

 

No, as all within permitted development. Even if it wasn't, the building is very much off the radar; only visible to dog walkers, covered by trees in ariel photos etc...not that I am planning on taking any real liberties.

[Onoff]

5 minutes ago, OscarWilliams said:

only visible to dog walkers

 

They just leave snotty, anonymous notes if they don't like something you're doing. Just ask @nod.

[the_r_sole]

4 minutes ago, OscarWilliams said:

No, as all within permitted development. Even if it wasn't, the building is very much off the radar; only visible to dog walkers, covered by trees in ariel photos etc...not that I am planning on taking any real liberties.

 

What are you planning on using the space for? I'm not sure how structural alterations like this are outside of building control involvement, avoiding the correct permissions at this stage is just building up future problems ime 

[OscarWilliams]

36 minutes ago, Temp said:

Whats wrong with the structural ridge beam approach with rafters "hanging" from that? The ends either bear on pad stones in the gable wall or posts. Sometimes also a wall mid span. Sometimes assisted by purlins. 

 

There is an interior wall running laterally through the middle of the building with a joist running along the top of it - I could put a vertical support (4x5) running from the joist to the ridge board so that is supported in the middle (would have approx 4m ridgeboard either side), if I used 8x2 for the ridge board it should be fairly structual if only supporting 4m either side, hopefully removing the need for a steel or flitch beam?

 

Thank you for all the feedback btw...

Edited August 15, 2020 by OscarWilliams

[OscarWilliams]

2 minutes ago, the_r_sole said:

 

What are you planning on using the space for? I'm not sure how structural alterations like this are outside of building control involvement, avoiding the correct permissions at this stage is just building up future problems ime 

Work space during the week, perhaps spend the odd night in there...I'm careful what I say, the canvas is blank at the moment. Fact of the matter is the old roof had collpased and we're putting a new one on...nobody else needs to know anymore.

 

Perhaps naive of me to think that way but it's the only way the project will get done...my brother and I are both in our early 20's and using our own funds for the project, bottomline budget is 7k.

[Simplysimon]

could you not do the collars traditionally

[Temp]

As I understand it collars are not designed to stop the load pushing the walls out but to stop wind loads pulling the roof apart at the ridge. For this reason they should be in the top third of the roof height (whereas to stop spread lower is better).

 

 

[Temp]

2 hours ago, OscarWilliams said:

No, as all within permitted development.

 

Humm. Repairs are within permitted development but raising the roof more than 150mm needs planning permission. Building Regulations are totally separate to planning and a building control application or notice is normally required for anything structural (and for other things).

[Temp]

2 hours ago, OscarWilliams said:

There is an interior wall running laterally through the middle of the building with a joist running along the top of it - I could put a vertical support (4x5) running from the joist to the ridge board so that is supported in the middle (would have approx 4m ridgeboard either side), if I used 8x2 for the ridge board it should be fairly structual if only supporting 4m either side, hopefully removing the need for a steel or flitch beam?

 

I'm not an SE but that doesn't sound big enough.  

 

Google found an online service that will calculate the ridge beam steel for £50...

 

https://www.beamcalculation.co.uk/start-calculation/steel-ridge-beam/

 

PS Straps are also needed to connect the rafters together above the ridge beam. 

Edited August 15, 2020 by Temp