Couple of garage questions

[ChrisZx]

Hi all,

Looking to start building my 6x5m single skin block garage soon, need to order materials. Just wanted to check a few things!

Want to have an EPDM roof on 175x75 C16 joists across the 5m length (4.8m span) on 400 centres. Will 100x140 concrete lintels be alright for openings of 2.2m and 1.2m in the 6m side? One either side of the pier in the middle. Both near the top so almost just roof weight. By my calculations would be more than enough. They alright to go directly on 3.6N lightweight blocks?

Got a 4.6m roller door to go in the 5m front, will be fitted directly between the piers so the opening is also 4.6m. Beams will be running parallel to this opening, so do I need a steel across this or just another joist? From the way the (rusty and out of practise) mech eng graduate in me looks at it the only difference to any of the other beams is that there isn't another beam the other side of it to help take the load, but I'm not sure that's much different to the beam running nearest the back wall.

I would like the roof fall to be along the 6m length despite the joists running the other way to this. Can I run firrings perpendicular to the joists? My worry is the ones at the back would be far too thin. How far apart would you space them? And also all the material in the middle of the joists would be wasted! Can you get firrings cut the other direction if that makes sense? So the angle is on their short length?

Hope some kind soul(s) can help me out here!

Thanks

 

[^woody^]

For the opening, you can use timber just like the rest of the roof and no steel - probablly a double joist though to fill the gap. Assuming that there is no loading from the doors.

You can run you furrings counter to the joists. Spacing would be to suit the boards so 400mm centres or so to prevent any board sag unless you are using 18mm T&G boards or 25mm plyboards where spacing could be 600mm. Work out the required height based on the fall required (about 120mm?), but at the thin end, cut them to 5 or 10mm and not down to nothing. Also make sure the furrings run to the very edge of the roof to the drip, and not just to the joist edge - so you dont end up with a flat section of roof at the gutter

 

[geraldthehamster]

If the furrings will go counter to the joists, and if you're ripping them yourself, isn't it easier just to rip timbers of decreasing thickness and fix one to each joist?

 

[ChrisZx]

Thanks woody, by "double joist to fill the gap" I assume you mean to fill the depth of the opening so it would just be aesthetic? There would be very little/no gap between the back of the joist and the front of the door housing to maximise internal space, so by the sounds of it doubling up won't be necessary? The door will be fixed sideways into the pillars so it can go wherever I want in between the pillars.

Furrings perpendicular sounds like a plan then, though I guess gerald decreasing height flat timbers would work as well. The height difference needed across a 75mm beam would be less than 1.5mm so I doubt it would cause any issues.

Thanks again!

 

[ChrisZx]

Hmm now I think on it more not sure about a few things, one of them height. Something like one 75mm, 2.1m door and sill, 140mm lintel, 175mm joist and I'm already at the 2.5m limit before furrings and the roof itself. Even if it did fit, how would I fix the joists to the wall over the 1.2m and 2m openings? Could hang the joists inside the walls by 140ish which might give me the extra height, but still have the same problem of how do I fix the hangers to the lintels/ultimately walls.

Might have to look at running the joists over the 6m length with an rsj in the middle and a lintel over the roller door.

 

[geraldthehamster]

The most recent flat roof I did had the fall perpendicular to the joists, with decreasing furrings on top of the joists. If you fix tapered furrings perpendicular to the joists, the boards are only supported directly on the joists where the furrings intersect. That might only be an issue if you're planning to walk around on the roof, but seems more solid to me. Woody correct me if I'm wrong please.

 

[^woody^]

If you can make the roof fall both ways from the centre, then you half the furring height

 

[^woody^]

The most recent flat roof I did had the fall perpendicular to the joists, with decreasing furrings on top of the joists. If you fix tapered furrings perpendicular to the joists, the boards are only supported directly on the joists where the furrings intersect. That might only be an issue if you're planning to walk around on the roof, but seems more solid to me. Woody correct me if I'm wrong please.

I dont think it is less solid. Even at the thin end of the furrings, the boards are still bearing on the joists, so if they are spaced properly, then the boards should be just as rigid

 

[ChrisZx]

If you can make the roof fall both ways from the centre, then you half the furring height

Good point, especially as I would do that over the 5m span so each run would only be 2.5m, guess I could squeeze the extra height down to 40mm if I needed to, bit more would be better though.

Still need to think about fitting to the lintels though, guess I could use a steel if it would make it easier

 

[ChrisZx]

Ah looks like these will save me the height of the lintel/steel so still might be possible

http://www.jji-joists.co.uk/images/interface/downloads/hangers/sst-steel.pdf

 

[geraldthehamster]

I dont think it is less solid. Even at the thin end of the furrings, the boards are still bearing on the joists, so if they are spaced properly, then the boards should be just as rigid

But where a board passes over a joist, it is only in contact with the width of each furring. It's not supported on the joist between the furrings. If your furring is 2 inches deep, most of the board is 2 inches above the joist, with thin air beneath. I don't see how it can be as solid.

I still think you'd be better calculating the fall across the joists, and adding full length parallel-sided strips to each joist, decreasing as you go.

 

[ChrisZx]

I might argue the gap has made the structure deeper and therefore more resistant to bending but I guess less resistant to point loads, the boards anyway

 

[geraldthehamster]

OK, diagram A below shows, in plan view, joists (175mmx75mm) with parallel furrings laid on top (say 50 deep at the start of your fall, and 10mm at the end of the fall). Board fixed on top is in contact with, and supported by the joist-with-furring-on-top for the full length of the joist (in red).

Diagram B shows joists (black) with furrings laid across (red). Boards fixed on top are only in contact with, and supported by the joist-with-furring-on-top where the red and black intersect. The furrings (red) taper from (say) 50mm to 10mm and do not provide additional support. The boards can sag above the joists between the support points - not significant at the 10mm end but more so at the 50mm end.

Unless you're planning on walking about on the roof, or have significant snow or heavy pigeons, it may not be an issue. But I don't see why the parallel method isn't both more solid, and easier.

View media item 100136
As always, I'm happy to be shown wrong.

 

[ChrisZx]

In general I would agree. What I said about the extra resistance to bending is probably true because you have the same mass of material further away from the centreline of the joist (only not true if the furring buckled I think), but was meant tongue in cheek just to highlight the different factors at work here.

Guess the only reasons to lay them perpendicularly is if they're strong enough anyway it's a moot point, and that they're easier to buy. But as I said earlier for the slopes we're talking about I personally think flat timbers of decreasing height would be fine. A <1.5mm difference across 75mm is not going to bend 18mm ply appreciably

 

[geraldthehamster]

It's a lot easier to rip parallel than tapering, on a table saw. Especially an Aldi table saw.