Exposed truss roof: can I remove diagonal braces and replace them with full OSB boards? (PICS INSIDE)

[bigbob764]

I have an disused outbuilding (approx 10m x 3m) which I'd like to turn into something useful like a workshop/office/gym/etc. The building has a truss roof and currently there is no internal ceiling i.e. the trusses are all exposed.

I could put a regular flat ceiling in, but the height would be pretty low and it would make the place feel cramped and boxy. So ideally I would like to leave the trusses exposed as a feature and fit plasterboard around them to create a vaulted ceiling effect.

However, currently the trusses are strapped together (as they should be) with diagonal and longitudinal braces like this:

These braces would obviously get in the way of any attempts to create a nice vaulted ceiling, and they are very ugly to boot.

My question is: is it structurally safe to remove the braces if they are replaced by another rigid frame between the trusses?

Could I add sheets of OSB (or even regular framing timbers) between the trusses? Would this be enough to keep them rigid without the braces?

This is the kind of exposed truss effect I'm going for btw:

 

[Wayners]

I thought you could fit boards on the outside then batten and tile
No need for strapping on inside then.
Maybe. Not my area but I remember we couldn't as it would not lineup with neighbours roof

 

[stuart45]

I thought you could fit boards on the outside then batten and tile
No need for strapping on inside then.
Maybe. Not my area but I remember we couldn't as it would not lineup with neighbours roof

That's the way it's done in the USA. When trussed rafters started being used here they didn't realise the problem with racking without the boards. Early roofs didn't have the diagonal bracing.

 

[bigbob764]

I thought you could fit boards on the outside then batten and tile
No need for strapping on inside then.
Maybe. Not my area but I remember we couldn't as it would not lineup with neighbours roof

That's the way it's done in the USA. When trussed rafters started being used here they didn't realise the problem with racking without the boards. Early roofs didn't have the diagonal bracing.

My roof is [unfortunately?] in great condition so I can't realistically take that approach. Or at least it would be a huge amount of work if I did. Glad to know that boards are a structurally sound way to brace a roof though.

Edit: just found a relevant section in the NHBC standards:

• diagonal and longitudinal bracing should be provided at rafter level (this may be omitted where rigid sarking boards are used, e.g. chipboard, plywood or OSB, which are fixed to each trussed rafter with 3mm x 50mm galvanised round wire nails at 200mm spacing)

So it looks like OSB can be used instead of bracing, although presumably they would normally be fitted on the outside face of the trusses as per the discussion above. In my case, i.e. with an existing roof, I guess I'd have to find an equally strong way of securing boards between the trusses internally on the side faces of each truss, i.e. here:

Angle iron? Angle brackets? Some kind of channel fixing?

I'll consult a structural engineer if I decide to go ahead.

 

[^woody^]

The concept is sound, but I doubt OSB can self support 1200mm

 

[bigbob764]

The concept is sound, but I doubt OSB can self support 1200mm

Thanks woody, I've seen your replies on lots of similar threads before.

I wondered if I'd get away with self-supporting spans of OSB because it won't be under any real load since it's not supporting roof tiles etc. but you're probably right in that it's too much of a distance.

Instead of OSB could I use framing timbers beween the trusses to provide rigidity and then fix the plasterboard ceiling directly to those?

 

[JohnD]

A roofer told me that the bracing is only needed during construction. Once the roof is smothered with battens, and especially if there are gable walls, it can't go anywhere.

 

[^woody^]

A roofer told me that the bracing is only needed during construction. Once the roof is smothered with battens, and especially if there are gable walls, it can't go anywhere.

No. There are many system builds from the 60s and 70s with no bracing and the whole roof moves sideways, opening the tile spacing, increasing/reducing the verge projection and pushing the gable out of plumb.

 

[^woody^]

Thanks woody, I've seen your replies on lots of similar threads before.

I wondered if I'd get away with self-supporting spans of OSB because it won't be under any real load since it's not supporting roof tiles etc. but you're probably right in that it's too much of a distance.

Instead of OSB could I use framing timbers beween the trusses to provide rigidity and then fix the plasterboard ceiling directly to those?

It would be the same principle for the plasterboard you proposed - that would need support battens.

So perhaps if you fitted a suitable number of say 4x2s that would support the plasterboard, they would also deal with bracing the trusses.

 

[catlad]

Are they 600mm centres these trusses?

 

[thomp1983]

How deep are the trusses?

 

[robinbanks]

No reason why a set of diagonal braces between each truss couldn't be made decorative? The trusses already use diagonal bracing in their own formation - what's a bit more? (and a good way better looking than OSB)

 

[bigbob764]

A few updates:

1. Turns out the roof is already boarded with OSB on top of the trusses!

2. The trusses are at 600mm centres.

3. Here are a couple of pics showing the trusses and some bracing:

Are they 600mm centres these trusses?

Yes they are.

How deep are the trusses?

They're about 4m across if that's what you mean?

No reason why a set of diagonal braces between each truss couldn't be made decorative? The trusses already use diagonal bracing in their own formation - what's a bit more? (and a good way better looking than OSB)

Yeah it would probably be possible to create nice-looking braces but I'm wondering if standalone bracing is even necessary if I create an internal frame for plasterboard etc. between the trusses.

 

[JohnD]

Are you in Scotland?

 

[robinbanks]

I'm wondering if standalone bracing is even necessary if I create an internal frame for plasterboard etc. between the trusses.

If the OSB were not there then I would say no, unless you were putting diagonal members in. That the OSB is there however, should mean that it will provide racking resistance

I presume you want to essentially plasterboard the OSB so that you can look up and see the trusses and in between them a nice smooth painted surface - IMO it would look odd if you hid the angled parts of the trusses (that the osb is attached to) behind plasterboard so you want to keep those on show..

..but my question here is "is this roof insulated above the osb?"

If it ain't, it would be a massive mistake to omit insulation, so you'd likely be looking at insulating under the OSB, then possibly attaching timbers as fake angled parts of the trusses so it doesn't look like the truss bracing just disappears into the plaster, then boarding and plastering after

If it is insulated above the osb, great; just prime and mesh the osb and plaster it direct, forget the plasterboard or if you have a lot of joints and cracking is a concern you can look to screw thin plasterboard directly to the OSB, or perhaps just thick lining paper to hide the grain and paint the paper

Also.. gang nail plates ain't the prettiest things; perhaps cladding the trusses with hardwood would be a better look (and a painstaking task)