Garage mezzanine floor advice

[Steve Blackdog]

High folks,

I wonder if I could borrow your brains for a mo.

I want to put a small 8 x 4' floor over my up and over garage door, so I can store some stuff. In effect this will be a mezzanine.

The garage has a pitched roof. This will be in the pitch of the roof adjacent to the gable end. The garage is just over 3.5m wide.

I was thinking of bolting four 6x2" joists to the bottom of the purlins, packing them out underneath so the walls share some of the load.

Is there any reason why i can't 'hang' the 6x2s from the purlins?

Cheers

Steve

 

[deserter]

You'll need to check, but I'm pretty certain, that the law states if the mezzanine touches the existing building other than the floor, you need to get building regs involved, if you make it free standing inside your building you don't.

Deffinately worth a thought.

 

[finneyb]

Is it a mezzanine floor or a shelf? I would argue that at 8ft 4ft its a shelf. You would not be standing on it but only storing material its not a floor and building regs doesn't come into it.

Just my 2p

Brian

 

[Steve Blackdog]

You'll need to check, but I'm pretty certain, that the law states if the mezzanine touches the existing building other than the floor, you need to get building regs involved, if you make it free standing inside your building you don't.

Deffinately worth a thought.

Mmm. Thanks for this. I guess I ought to call building control, but I am loathe to do so, if I get sucked into an expensive and complicated job. The total cost of wood and boards is less than the notification fee!

I can hire a storage crate for under a tenner a week, so I don't want to go over the top with this.

@ Brian - you are spot on - it is a shelf - at the highest point it will be around 4'. I can only get boxes etc in the area. There's a limit to the weight of stuff I can lift up there anyway.

Any thoughts about my "how to" question? Is there any reason not to hang my "shelf" off the purlins?

Cheers

Steve

 

[Phil Pascoe]

Possibly batten around the walls and put a joist across with a couple of corbels to carry the outside edge? As said, it's only a shelf, albeit a possibly very heavily loaded one.

 

[finneyb]

I wouldn't use 6x2's - too heavy you are just putting unnecessary load on the roof/purlin. I would keep the self weight as low as possible.

If you used something like http://www.diy.com/rooms/finnforest-fib ... 586_BQ.prd stiffened underneath with something like http://www.diy.com/departments/metswood ... 230_BQ.prd. Glue and screw the batons to the sheet to get the floor stiff.

Hang floor from purlins using 8mm polyprop rope. Drill floor pass rope thro around baton and back thro floor and tie to purlin.

HTH

Brian

 

[Steve Blackdog]

The reason I thought I would go with 6 x 2 is that the span is 3m and with any weight on the shelf, it would sag. Actually to be fair, I was going to buy 4x2's and the guy at Travis Perkins said I needed 6x2's for a 3m span.

I can probably pack out under the 6x2s with something to put the load onto the wall.

I guess I could also rip the 6x2's into 3x2s, but I would rather it be over-engineered than the other way around.

Cheers

Steve

 

[petermillard]

As I understand it, purlins are there to prevent the rafters from sagging under its own weight or e.g. snow load on the roof, so I wouldn't want to add anything that could reduce their effectiveness, personally; hard to judge though, without knowing the approximate intended load on your 'shelf', which presumably, you don't really know yourself yet??

There are span tables online that give max. recommended spans for certain sections of types of constructional timber (C16, C24 etc...) but again, without knowing the intended load it's hard to judge - though the one I found with a quick Google suggested a 2.9-odd metre span for 6x2 under minimum load, so the TP guy was pretty much on the money, by luck or judgement.

Could your 6x2s be cut so the load is shared between the rafters and the purlins? Just trying to think of the stresses involved under load??

Cheers, Pete.



edited for clarity

 

[Steve Blackdog]

Thanks for that, Pete.

Try to imagine the purlins forming the sides of a triangle or A shape and the new 6x2s forming the bottom of the triangle very close (say 150mm) to the bottom of the A. I would have thought that adding a new member tying the opposing purlins together, would actually make the roof above the 6x2's stronger, but add load to where the purlins meet the supporting beams/walls.

I think I need to support the 6x2s from below as well as bolting them to the purlins.

Cheers

Steve

 

[wobblycogs]

Personally I wouldn't hang the shelf from the purlins. You've said you won't be putting much up there but in my experience the weight quickly adds up and I think there's a risk you could overload the roof timbers.

If I'm understanding your garage correctly I think you'd be better off attaching some 2x4 to the walls either wide with concrete / sleeve anchors and then spanning the gap with 2x6 (don't forget to brace with noggins). A bit of T&G chip over the top would leave you with a nice shelf. If the walls aren't high enough to support the span maybe consider building towers out of 2x4 on each side to support the span. The towers could be used a shelving.

 

[Phil Pascoe]

Yes. That's much what I said.

 

[Steve Blackdog]

DOH! I don't mean purlins, I mean rafters!

Sorry folks. Does this change your views?

 

[Brentingby]

Nail a 6x2 to the wall just below the top plate and use joist hangers to support the timbers spanning the garage.

If your rafters are heavy enough, you could tie into them at the center. Then you could probably get by with the 4x2s. You would want to attach to the collar ties. Are there collar ties?

 

[wobblycogs]

Do you have side to side to beams low down on your roof, at the height of the walls (usually called something like roof ties, rafter ties, ceiling joists, etc - Google an image)?

It sounds like you don't if you are able to get a useful mezzanine floor up there. If that is the case then I'd assume you have just collar ties higher up and the roof is birds mouthed into the top of the wall. This is fine but it does mean additional load low down on the rafters will be more inclined to push the walls apart than if the the ties were lower down. I wouldn't hang anything substantial from the roof timbers. Of course you have to also account for the new timbers you are adding acting a ties themselves if fitted correctly. Much easer to fit a wall plate I think.

 

[paulrockliffe]

If your garage isn't a habitable space, you don't need building control sign off. Even if the garage is habitable, it can't be a mezzanine because the head room won't be enough to make the new space habitable. So again, no building control required. What you describe is storage, not a new room and there's quite a few things where I'd expect it to fail a BC inspection. So basically, ignore BC.

Personally I would always avoid adding any loading to the roof structure that is directly supporting the tiles, ie purlins and rafters. I'd consider using any tie beams if they look suitably strong, but a wall plate and joist hangers will be more than enough and take all the load directly to the brick structure. Resin anchors are pretty highly regarded too, but unlikely to be necessary.

 

[Steve Blackdog]

OK. Decision made. I will sit the joists on blocks on top of the wall plate on either side, but still bolt them to the rafters for stability.

The reason I can't use joist hangers is that the bottom of the joists need to be higher than the top of the up and over door. The tops of the wall plates are lower than the lintel over the garage doors.

I have also screwed a support timber onto the (wooden) lintel, which will share the weight.

When the job is done, no doubt I will fill it with empty boxes and bubble wrap!

Thanks for your help everyone. It has really guided my thinking.

All the best

Steve