Would there be BR issues with this design concept?

[indus]

Hi

I just watched an old episode of Grand Designs and am considering incorporating something I saw. Basically to make the bedrooms seem bigger they ripped out the ceilings hence losing the attic/loft space but ending up with very high vaulted ceilings.

I'm hoping to build a double storey side extension. The loft in my existing house has been converted and so the new loft/attic space is not needed. So in the proposed new section is there any BR reasons I can't do this? ie no conventional ceilings on the first floor?

Thanks

 

[^woody^]

In a word, yes

 

[indus]

In a word, yes

Care to elaborate Woody?

 

[^woody^]

Use larger rafter sections and/or a purlin

 

[tony1851]

How are you going to eliminate the tendency of the roof to spread? Among other functions, ceilings help to tie a roof back. 'Grand Designs' is OK so far as it goes, but they often don't show the technical difficulties of complying with Building Regs

 

[indus]

Use larger rafter sections and/or a purlin

Thanks Woody. Using larger rafter sections is no big deal. By purlins I presume you mean horizontal beams at roughly the height level of where the attic floor joists would have been?

Thanks

 

[indus]

How are you going to eliminate the tendency of the roof to spread? Among other functions, ceilings help to tie a roof back. 'Grand Designs' is OK so far as it goes, but they often don't show the technical difficulties of complying with Building Regs

Hi Tony

I have no idea of how I'm going to eliminate the tendency of the roof to spread

Tbh, I didn't even know it was an issue until Woody and you have pointed it out. That's exactly what I wanted to find out ie what the actual structural challenges are.

I will of course take my technician's formal advice but would really like to find out what the options are first.

So if you (or anybody) has anything further to add about how this should be designed from a structural point of view it would be greatly appreciated.

Thanks

 

[tony1851]

How you effeectively prevent roof spread on a side extension with no ceiling depends on several factors,such as width of extension, configuration of existing roof, the presence (or absence) of cross walls upstairs in the extension etc.

Also, there is not just the issue of roof-spread to consider. Without a horizontal ceiling, the top of the outer (side) wall of the extension is not adequately restrained against horizontal wind loading; you can't just rely on the weight of the roof to secure the wall in position and it will end up acting like an un-propped, vertical cantilever.

The frustating thing is that these open ceilings often work in practice, but it is very difficult to prove it on paper. If this is to be an important part of your design, you really need an SEs input. Building Control will want to see the figures, and the stuctural consquences may be a little more expensive than you might think.

 

[indus]

How you effeectively prevent roof spread on a side extension with no ceiling depends on several factors,such as width of extension, configuration of existing roof, the presence (or absence) of cross walls upstairs in the extension etc.

Also, there is not just the issue of roof-spread to consider. Without a horizontal ceiling, the top of the outer (side) wall of the extension is not adequately restrained against horizontal wind loading; you can't just rely on the weight of the roof to secure the wall in position and it will end up acting like an un-propped, vertical cantilever.

The frustating thing is that these open ceilings often work in practice, but it is very difficult to prove it on paper. If this is to be an important part of your design, you really need an SEs input. Building Control will want to see the figures, and the stuctural consquences may be a little more expensive than you might think.

Thanks Tony.

Once again I had no idea that the ceiling structure provides horizontal support for the external wall, but I presume the purlins Woody mentioned would help with this issue?

Anyway, it seems like there are quite a few structural issues to be taken into account, and as you said these will have cost implications. However I think it would really add a bit of wow factor to the house and so could well be worth the extra money.

I'll get the SE onto it once PP has been finalised.

Many thanks

 

[^woody^]

A purlin, or a beam at ridge or wall plate level will prevent spread and restrain the wall. It can be designed simply

 

[tony1851]

A purlin, or a beam at ridge or wall plate level will prevent spread and restrain the wall.

A 3- or 4 inch wide purlin will reduce deflection of the rafters, but how will it
resist the horizontal component at the top of the wall?
Even more difficult to achieve with a ridge beam.
Also, he would need some rigid connection between the top of the wall and the rafters, to provide support against windload. A beam at wall plate level could do it, but would be clumsy.

 

[^woody^]

A purlin or ridge beam prevents pivot or movement of the rafter, thus can't spread at the plate level

 

[tony1851]

Under load, the rafter will deflect slightly and this downward deflection has the tndency to push outwards on the wall.

If this wasn't a problem, why do truss manufacturers provide glide shoes ffor installation with raised-tie trusses?

Also, why do all the manuals on building defects raise the issue of lack of ceiling ties as a possible cause of walls pushing out at the top?

Untitled

• tony1851
• 5 Aug 2012
• 1

 

[^woody^]

Any movement in a structural member must be via two points, or via rotation around one point. With a rafter, restrain one point and it just cant move outwards. It could rotate about the fixed point, but that wont push a wall outwards

For deflection, this will depend on the elasticity of the rafter, so again, it wont push the wall out because the rafter cant stretch at the end, rather the rafter will just bend until it breaks at the point of load

Without meaning to be condescending, this is very basic mechanics/geometry and can be demonstrated with just a plastic ruler. That's how it was shown to me years ago

Trusses work completely differently and rely on shared loading and restraint of all the members

 

[tony1851]

For deflection, this will depend on the elasticity of the rafter, so again, it wont push the wall out because the rafter cant stretch at the end, rather the rafter will just bend until it breaks at the point of load

When the rafter deflects, it doesn't just 'stretch at the end'. What actually happens is that the bottom of the rafter goes into tension, while the top goes into compression. Logically, this means that the bottom of the rafter increases in length very slightly, while the top of the rafter decreases in length correspondingly. In this way, it assumes the slightly bowed shape I drew with dotted lines on the sketch.

Assuming the end of the rafter stays in position at the purlin, its small increase in length on the underside, where it contacts the wall plate,must mean it pushes outwards on the plate.

I don't mean to be condescending, but this is all basic stuff. Why do all the books on simple traditional structures advise putting a ceiling in? Could you show me any book on building defects which states that a wall being pushed out at the top cannot possibly be due to inadeuate/missing ceiling ties? I doubt it.

As regards trusses, the manufacturers always recommend glide shoes when installing raised-tie trusses. This is because the long, untied length of the rafter will deflect downwards (and therefore outwards slightly) when the roof is loaded with the tiles. The shoes allow for the inevitable slight lateral movement which would otherwise push the wall plate outwards.