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Author: Subject: Garage roof joist strength
gingerprince

posted on 9/2/20 at 10:28 AM Reply With Quote
Garage roof joist strength

Hi

Bought an electric hoist from Lidl a couple of weeks ago to save my back when lifting the engine amongst other things.

Does anyone know how to calculate strength of joists? I intend to lay some of that slotted metal bar (framing channel?) over a number of joists to spread the load, and would like to know how many.

Joists are 35x70mm with about 5300mm span. Obviously also supporting the garage roof (double).

Motor is rated for 125kg, or 250kg if used as block and tackle, and book says mount for 500kg. Planned use is for bike engine so nowhere near those loads, but I'd like to understand what my rafters can and can't take.

If they won't take it safely, then what's the best way?

Thanks.

[Edited on 9/2/20 by gingerprince]

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coyoteboy

posted on 9/2/20 at 10:43 AM Reply With Quote
Rafters are made of what? Mounted to the wall how?





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gingerprince

posted on 9/2/20 at 10:57 AM Reply With Quote
quote:
Originally posted by coyoteboy
Rafters are made of what? Mounted to the wall how?


Sorry my bad, I meant the joists not rafters, doh! They're wood and supported at either end by the walls.

I've changed the question and subject to reflect this.

[Edited on 9/2/20 by gingerprince]

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snapper

posted on 9/2/20 at 11:18 AM Reply With Quote
What I did was to use a joist that is over the brick peers either side which gives the joist a 2 brick width support.
At this point I wanted to add a bit of adjustment so I used 2 short lengths of scaffold tube across 2 hoist, to give for & aft movement.
A third short length of scaffold above the two for & aft poles with scaffold pole clamps to secure them gave a very strong structure to hang the hoist from.
Longer scaffold poles would allow you to have multiple location points along the length of the garage and further spread the load across multiple joists.





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steve m

posted on 9/2/20 at 12:03 PM Reply With Quote
I made a triangle in metal that went from both sides of the garage and joined in the middle of the joist,
no calculations, it was just there for piece of mind,





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Bluemoon

posted on 9/2/20 at 06:34 PM Reply With Quote
Sound like small joists to me span is large..if its part of a roof trusses they are normally only designed for roof loading...

Their are on line calculators...

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Mr Whippy

posted on 10/2/20 at 07:21 AM Reply With Quote
Sounds about the same roof as my garage, not even close to being strong enough to hang an engine off. Personally I wouldn't even risk it, if it fails you trash the roof, damage the car & engine or yourself... Hire and engine lift or buy one from Gumtree, most folk want rid of them as they take up space. If you need to have the winch suspend it from a steel section properly mounted on top of the garage walls.
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loggyboy

posted on 10/2/20 at 07:57 AM Reply With Quote
They should be designed for allowance of storage, maintenance access and for snow loading. Add a timber that spreads the load over several trusses and lifting a couple of hundred kilos for a short period of time shouldnt be an issue. (IMO)

[Edited on 10-2-20 by loggyboy]





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jacko

posted on 10/2/20 at 07:34 PM Reply With Quote
I used to put two props from the floor to a joist with a car in between the props after lifting the engine /gearbox out I pushed the car back so I could put the engine on a trolly
You could bolt a steel angle to your joist to fasten the winch too first
Jacko

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Charlie_Zetec

posted on 10/2/20 at 07:48 PM Reply With Quote
Having recently gone through building control, I was made to double-joist every other joist on a 4.1m span between 300mm tall metal I-beams using 8"x2" C24 grade beams. And that was just for a loft area.

Whilst I understand that it's probably overkill (and the inspectors probably just want to cover themselves), I would suggest the size of beams you've got aren't up the job.

If you're looking for long-term and feel inclined, I would consider inserting a pad stone on the side walls (assuming brick walls) and spanning an I-beam between which would allow for significant loads to be lifted if needed, otherwise have a look at gantry frames for use with block and tackle such as THIS, or if you're really not fussed, then a basic engine crane will do a pretty good job.





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pigeondave

posted on 12/2/20 at 10:30 AM Reply With Quote
quote:
Originally posted by Charlie_Zetec


Whilst I understand that it's probably overkill (and the inspectors probably just want to cover themselves),


Theres no probably in that statement. They're Tw@s the majority.


@gingerprince Flat roof joists will only be designed for a load of 0.75kN/m which is about 75kg/m, or a point load of 0.9kN say 90kg.

Although your joists do sound a little small for the span, its also a garage so will be designed to a lesser standard.

The main issue is deflection, when designing timber its the deflection which gives the main issues.

what you could do is get some rolled steel angle drill some holes in it and bolt it to the side of the joist, although this will only give you a static hoist position.

Alternatively you could try to find a small one man band structural engineer, someone who works off his kitchen table. You want an old boy as theyre going to give a realistic solution, the kids nowadays over engineer everything.

the more info you can provide (little sketch) the more likely they'll no come out to look (time is money after all)

Just an idea

you could also look for the Trada timber span tables they might have some advice

[Edited on 12/2/20 by pigeondave]

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SJ

posted on 12/2/20 at 01:03 PM Reply With Quote
I made a timber A frame to lift my engine using some chain blocks.

Did the job but I had to lift the engine then move the car.

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coyoteboy

posted on 12/2/20 at 05:24 PM Reply With Quote
That's too small for my liking, I'd just weld up a separate frame or plough in some posts internally. Cheaper than rebuilding the roof and car below it.





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gingerprince

posted on 12/2/20 at 10:25 PM Reply With Quote
Thanks for views all. Some varying opinions but enough in the negatory to tell me that it needs more thinking about and not just stick it up there (yub yub!). Certainly don't want the roof coming down on me! It is an attached double garage, but given the size of the joists it seems it's specced for light use. Plus, there is already some boarding up there for storage.

The hoist was more of an impulse buy (we've all done it at Middle of Lidl!), not likely to lift lots, just bike engine in the first instance. Think I'll return the hoist and go with plan A of just carry it, but get some help (have done it myself in the past but back trouble means I should really get assistance).

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ragindave

posted on 12/2/20 at 11:39 PM Reply With Quote
I have fitted one no problem lifting 250kg just spread the load over 5 beams.

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[Edited on 12/2/20 by ragindave]

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