What's Firfix?

[Anonymous]

Plans for a friend's new building specify that the roof structure must be diagonally stabilised by nailing "Firfix metal strapping" in an X shape across the tops of the rafters on each face of the pitched roof. I have googled "firfix" and all I seem to be able to find is a system for tying new extension buildings to the original structure.
Can anyone tell me what Firfix metal strapping is, please?

 

[Anonymous]

Is it this?

 

[Anonymous]

Firfix is usually used to mean what are often called 'extension starters'. They are metal strips fixed to a wall that you then slip metal brackets into (a bit like a wall shelving system) so that the brackets are in the bed joints of the extension you are building. This creates a physical link between new and old without having to 'tooth in' brickwork. You can even get them with slip membranes/joints to create a moveable/flexible joint.

However, in the situation you refer to, all that is really needed are some galvanised mild steel lengths (usually with a series of holes along them. The ends can be bent to an angle (or bought with the angle formed) so that they can be fixed to corners, over walls/timbers, etc. You simplythen nail or screw through the holes.

I think you will find Firfix is a manufacturer's name and the word has been used in a way that could confuse because of its more common usage.

 

[Anonymous]

Thanks for that, Stephen. Do you think the stuff in the picture I posted above is what my friend should use? It's a Screwfix product.

 

[Anonymous]

I think what you're after is Furfix.



http://www.strongtie.co.uk/telechar/tel-img-57-en.pdf

 

[Anonymous]

Thanks for your response, John. The name certainly looks like the source the word used by my friend's architect, but the product is obviously designed for tying in extensions to the main body of a building, rather than as a simple bracing strap for roof trusses. Or do you think the architect might have intended the vertical element of the system to be used for this purpose without the lock-in masonry ties? Have you ever seen this done?

 

[Anonymous]

Ah, my fault for not reading your original post properly.

OK, start again.

I think the metal roll in your photo is what the architect means. I've never seen metal strapping used for diagonal bracing before though. Much more common to use timber bracing in a 'W' pattern. I wonder why the architect would specify that?

 

[Anonymous]

John - I think the reason for this spec is the fact that there will be a vaulted ceiling, plasterboarded on the underside of the rafters, so there's no way of bracing the structure "invisibly" other than with these steel straps nailed on top of the rafters in an X configuration to each face of the roof. There are four principal rafters which have trusses, midpoint collars and queen posts supporting purlins.
It's an extension to a listed building in Norfolk - built as a small separate structure with a link to the main house.

 

[Anonymous]

The style of bracing suggested has been used before but as the strip is thin it only works in tension so requires a large amount of longtidudinal timber bracing in the roof to work against.
Why can't timber diagonal bracing be used with packers of the same thickness used on the underside of the rafters to give a flat base for the plasterboard?
I'm also not overhappy with pre-galvanised steel which is punched with holes and slit to width leaving a large amount of uncoated steel in the holes and edges being exposed to possibly high humidity under the felt.

 

[Anonymous]

Third attempt anyone else getting file errors?
I can think of two other ways to brace this type of construction.
1;Sarking, plywood fixed to the top of the rafters this will eliminate the need for any other bracing at rafter level but will need counter battens before the felt and tile battens.
2lywood panels fixed to battens attached to the faces of the rafters if the rafters are a reasonable depth it will still allow room for insulation and a ventilation gap.
You could use OSB in place of plywood but although it's ok when kept dry I'm not so sure if there is any water ingress, maybe showing my age and lack of confidence in some newer materials!!

 

[Anonymous]

Thanks for that, David (yes - I get loads of File Error! reports - patience is a virtue!).
I like your idea of sarking with plywood (not OSB?). What thickness of plywood would be appropriate?
If I understand you, the correct techique is: rafter, sarking board, vertical batten (aligned with each rafter?) membrane, horizontal batten, tile. Is that right? Presumably the insulation would then be stuffed into the cavity between the sarking board and the plasterboard that is fixed to the underside of the rafter?
It will of course be more expensive than the proposed solution, but I like the feeling of solidity and stability about it.

 

[Anonymous]

David,
It will depend on rafter centres starting at 9mm for 400mm and going up from there.
Sarking originally with softwood boards is traditional construction for Scotland and with plywood for North America. Never understood why we don't/didn't use it in England & Wales.

David.

 

[Anonymous]

These will be at 400mm centres, so 9mm. Thanks for that info, David.
Sarking also seems to be universally used in Scandinavia.

 

[Anonymous]

Scandinavia is a bit different as rafters are often spaced at 1000 or 1200mm centres with softwood boards about 40mm thick with a double t & g joint used as sarking.
It's interesting that sarking boards are normal in areas/countries where there is likely to be driven snow in winter, Scandinavia/Scotland/Canada etc, I assume they got fed up with the drips & draughts and used boards to prevent them.