Rafter bolts - stud detectors

[Ramjam]

I have recently decided to stop removing slates and to use rafter bolts. I bought a fancy Bosch stud detector but it doesn't work well enough, how do you do it?

 

[hduz5]

Hilti make a tool called a PX10. It is essentially two sensors. You send one guy in the roof, he holds the sensor on the rafter. The guy outside uses the other sensor and locates the signal from the first one. Repeat this at the top and bottom of each rafter you are going to use and then use a chalk line to mark the roof. Then measure where you want your rails to run, and use the chalk line to mark horizontal lines. Where the vertical and horizontal chalk lines cross is where you want to drill for your hanger bolts.

Personally I'm still not convinced about using hanger bolts. As is well documented on these forums nobody really knows how functional those seals are going to remain in 20 years time, and if you speak to any experienced roofer they are never enthusiastic about drilling holes in a roof! But if you're going to do it (and most companies, including the one I work for do), then this is how to do it quickly and without drilling any unnecessary holes.

 

[Cider]

Any tile should only be drilled on the crest of the tile but its still a sustandard fitting compared to a lapped rafter bracket.
Drilling a slate is asking for trouble as the piercing is defenately going to be exposed to moisture.
Can u imagine having to take all the panels off to replace a cracked slate or tile or even tighten down the backnut on a bolt after a bit of movement. Scaffolding up again the lot. All to save a few extra hours work.
Having said that if the felt is in good order chances are nothing will show.

 

[Ramjam]

I work with a lot of dormer bungalows with artificial slate, in fact I've got a glut of them coming up. Problem is that I couldn't use the Hilti tool because it's converted and whilst I appreciate everyone's concerns I need to remain with the current practice, I can't be wasting an extra day if my competitors aren't. Personally I don't believe that well fitted and suitable goo'd rafter bolts will fail.

I have this detector... Bosch GMS 120 Professional Multi Material Cable Detector
It doesn't seem to be good enough, tested it on a roof where we had access to the inside to verify it's finds but it just didn't deliver, do you have a stud detector that you can rely upon?

 

[Worcester]

@Ramjam, How do you comply with the Building Regs requirement of:

BS 5268: Part 2: 1984. Structural use of timber code of practice for permissible stress design, materials and workmanship. Table 65 Minimum Screw Spacings.

I am at odds to see how any hanger bolt is Bulding Regs compliant based on that.

 

[hduz5]

Worcester, are there any circumstances in which you will use hanger bolts? Eg farm installs on barns? They're the only jobs where I have felt happy with their use, as a minor leak in the future is unlikely to have catastrophic effects (I say this having grown up on a farm - there may be farmers out there who would disagree).

I agree with your concerns about domestic roofs. I guess it is going to be about 20 years before we get a definitive answer on their long term suitability.

 

[Worcester]

@hduz5 I will use them where its right (and that certainly isn't a quick fix into a rafter on a slate roof)

Industrial (and farm) installs where the purlins (which these hanger bolts are designed to go in to) are wide enough, no problem at all, and I happily install them there.

Hanger bolts are NOT designed for UK RAFTERS, they are designed for fixing into purlins - just watch the Schletter videos on youtube.

 

[saint]

I'm a bit confused here???
How is using hanger bolts any different from using the Schuco slate anchors and drilling through the tiles? Or are you saying use the Schuco (or other manufacturer) slate anchors and lead flash over?

 

[Worcester]

@saint
Don't want to hijack Ramjam's thread on stud detectors (most people round here no how strongly I feel about hanger bolts !)

Check the threads on Slate Roof Fixings and you'll get loads of answers:

http://www.electriciansforums.net/p.../22638-mounting-systems-welsh-slate-roof.html

and

http://www.electriciansforums.net/p...energy-forum/35844-fixing-new-slate-roof.html

 

[solarsavings]

@Ramjam, How do you comply with the Building Regs requirement of:

BS 5268: Part 2: 1984. Structural use of timber code of practice for permissible stress design, materials and workmanship. Table 65 Minimum Screw Spacings.

I think you'll find that has been superseded by eurocode 5.

 

[Worcester]

Kind of, but not fully - see my pm

 

[Worcester]

For those that missed the discussion on BS vs Eurocode, here's just one small part of it.

Eurocode 5 is a limit state design approach as opposed to tables, so to be able to know what length of screw you should use, you have to do a series of calculations, just as you do for voltage drop. For edge distances, they do give a table:



An electrician knows all about MCB's, RCD's and Array earthing and most know BS7671 inside out. This is what you should comply with for rafter fixings.

 

[solarsavings]

So how to comply with that requirement using the readily available roof hooks on the market like this Hilti plain tile hook.

 

[MikeXl426]

Is that a 35mm rafter in that image Worcester? If it is, then my maths is breaking down...so, an 8mm screw means 4 times diameter = 32mm from edge....which is 3mm from the other edge.

Is this a senior moment I'm having here? Throw abuse if you like!

 

[Worcester]

@Solar Savings,

Hilti do a special UK Spec hook, still not perfect and the prices are horrific., or for slate the Azur;



That's sitting on a 35mm wide piece of tanalized timber

Neither brilliant, juts the least non-compliant. I haven't come across anything 100% compliant yet...


Minimum width for an 8mm coach screw is 9 x 8 = 72mm (just like the germans use!)