3m flat roof span with 44 × 95 joists

[g8dy]

Looking at span tables; C24 @ 44 × 95 will give me just under a 2M span. My flat roof is at a 2-degree slope and spanning 3M.

I don't want to use 5" or 6" joists because it will take up too much head space in my shed.

I've already put in the roof joists and OSB and walking on seems pretty solid. It will have a EPDM sheet going on top and maybe a 600x600 skylight. Sadly I run out of timber, ideally, i wanted to have the roof joists 400mm spacing so they would be the same as the wall joists.

I added in 2 extra joists in the middle of roof to try and give more support. Just wondering if there's anything else I can do. Its all sheathed in OSB and nailed in the place now

For those recommending using 6x2" joists instead:

The reason I opted to avoid 6" joists is because I want AS MUCH headroom in the shed as possible. I'm 6ft4 and will be suspending various things on a ceiling rack system. Secondly, I hit my head on far too many things while doing DIY in my small shed and utility room, I managed to give myself a concussion a few months back after one incident, I'm fed up with everything in my house being too low so when building this new shed I decided from the start that headroom is THE most important thing.

 

[Kayen]

A 2 degree slope is IMO the equivalent of zero slope. Asking after you've done it doesn't really make any sense and if you run out of timber you surely buy more . . .

Water will undoubtedly pool and add weight, which will cause your roof to sag, which will then gather more water and add more weight etc. etc.

Sorry if I'm being harsh, but I think you've gone about this in the wrong way.

.

 

[imageel]

I think you're asking for trouble - with a 2° slope the potential snow loading will cause significant flexing of those joists at those spans and spacing - you may even have problems during heavy rain since that shallow a run off will result in significant amounts of standing water building up before it runs off.
If you add glazing this will add significantly to the static load and just exacerbate the issues previously mentioned

 

[Jameshow]

Much better off with 6" joists.

It won't affect your headroom at all as they are on top of your walls.

I hardly think planning are going to worry about 2" tbh.

 

[ChaiLatte]

If the headroom loss is absolutely not possible, buy some 90 x 6 steel strip and bolt it to the joists. Drill some exta holes in it so the fasteners for the noggins can pass through it.

 

[ChaiLatte]

If you add glazing this will add significantly to the static load

I think 'significantly' is a slight exaggeration. What is the extra-over weight of a 600mm square lexan skylight plus its upstand?

 

[deema]

The old timers rule of thumb for sizing joists was the half the width in ft, plus an inch. So in your case they would have used 6” X 3”. I cannot see the logic in building a substandard roof.

 

[johnnyb]

Look it will probably be OK So don't stress. If it isn't your redoing it anyway!

 

[g8dy]

A 2 degree slope is IMO the equivalent of zero slope. Asking after you've done it doesn't really make any sense and if you run out of timber you surely buy more . . .

Water will undoubtedly pool and add weight, which will cause your roof to sag, which will then gather more water and add more weight etc. etc.

Sorry if I'm being harsh, but I think you've gone about this in the wrong way.

.

You're not being harsh, i appreciate your views I was building over xmas so couldn't get anymore timber sadly. I wanted to get a roof on because of the weather, but I agree, I think I've gone about it the wrong way and didnt realise 3M span is actually quite a lot for 4x2 timber. I'm now trying to figure out a way to remedy it. I'll list my options below, please give your thoughts

I think you're asking for trouble - with a 2° slope the potential snow loading will cause significant flexing of those joists at those spans and spacing - you may even have problems during heavy rain since that shallow a run off will result in significant amounts of standing water building up before it runs off.
If you add glazing this will add significantly to the static load and just exacerbate the issues previously mentioned

I'm in SouthEast so not much snow but i agree with your thoughts, the rubber roof weighs a lot, plus a skylight, its all going to add to the sagging over time.

Much better off with 6" joists.

It won't affect your headroom at all as they are on top of your walls.

I hardly think planning are going to worry about 2" tbh.

Ive already had planning permission approved to take it to 2.7m and I'm already hitting that with 4x2 ceiling. 6" would take me close to 3m and I don't want to risk pissing off the neighbours.
Reason i opted to avoid 6" joists is because I want AS MUCH headroom in the shed as possible because I'm 6ft4, and will be suspending various things on a ceiling rack system.


Thanks for your input guys, I really don't want to do it but i think im going to have to rebuild the ceiling, how on earth im going to remove 300+ twist nails in the joist hangers I have no idea .

What do you think is the best option, or do you have any other suggestions? (I cant use 6x2 because it will take me above my planning permission height. Ideally i want to stick with 4x2 joists in ceiling)

Option 1: Split the ceiling into smaller sections using 2 flitch beams, reducing span of joists to under 2m

Option 2:

Double up current 4x2 joists (still unsure if this will be sufficient)

Option 3 (thanks to @ChaiLatte):

Attach steel plate to a few joists to add reinforcements.

 

[g8dy]

If the headroom loss is absolutely not possible, buy some 90 x 6 steel strip and bolt it to the joists. Drill some exta holes in it so the fasteners for the noggins can pass through it.

I hit my head on far too many things while doing DIY in my small shed and utility room, I managed to give myself a concussion a few months back after one incident, I'm fed up with everything in my house being too short so when building this new shed I decided from the start that headroom is THE most important thing.

Your idea is similar to a flitch beam, and I was thinking about the same thing of just attaching flar bar to some joists. The question is how many, 3m 80mmx6mm steel bar is £36 a pop

I think 'significantly' is a slight exaggeration. What is the extra-over weight of a 600mm square lexan skylight plus its upstand?

From looking online I think the average is 20-30kg for 600 x 600

 

[imageel]

Unsure where in Essex you are however IS&G steel are local and in my experience quite competitive on pricing so worth a call.
They stock stuff in 4.6m lengths but do offer a free torch cutting service if you can then fit it in your car - or if you have a battery powered angle grinder that would work too.
I'm based in Kelvedon and the comment wrt snow loading was for real and based upon experience - we do occasionally get heavy snow and as others have commented, scrimping on a roof with all the attendant consequential damage is really not worth the risks IMHO.

 

[Kayen]

I haven't had the time to go through all the answers, but another option would be to take the OSB off and add another 4x2 (5x2) on top of the existing joists. Prop them from below first to give them a positive bend, then glue and nail the next set on top. If you taper them it would give you a greater slope, which I think you will need.

 

[ChaiLatte]

how on earth im going to remove 300+ twist nails in the joist hangers I have no idea

Maybe you can reduce the work by more than half. Cut through the existing hangers with a grinder and leave the bits on the wall plate where they are. Put the new ones in a place that does not clash with the old ones. Swap the joists end for end and then the bit that was poking out into free air will be clear to attach the new hanger. That means you only have to remove half a hanger from one end of each joist. Grind off the nail heads with a flap disk.

 

[Jameshow]

How about screw and gluing 2x2 on top of the 4x2?

 

[g8dy]

Unsure where in Essex you are however IS&G steel are local and in my experience quite competitive on pricing so worth a call.
They stock stuff in 4.6m lengths but do offer a free torch cutting service if you can then fit it in your car - or if you have a battery powered angle grinder that would work too.
I'm based in Kelvedon and the comment wrt snow loading was for real and based upon experience - we do occasionally get heavy snow and as others have commented, scrimping on a roof with all the attendant consequential damage is really not worth the risks IMHO.

Thanks, I don't drive so I usually have to stick with the online stores that offer delivery. Local builder/steel merchants are usually cheaper than the online stores but I always find you either have to pickup or their delivery options are far too expensive

I haven't had the time to go through all the answers, but another option would be to take the OSB off and add another 4x2 (5x2) on top of the existing joists. Prop them from below first to give them a positive bend, then glue and nail the next set on top. If you taper them it would give you a greater slope, which I think you will need.

Unfortunately, I cannot add any height to the joist, either on the top or bottom of the joist. The maximum I can work with is 4" joist (reasons mentioned above). The simplest method so far seems to sister a metal bar along a few joist to give some extra reinforcement

Maybe you can reduce the work by more than half. Cut through the existing hangers with a grinder and leave the bits on the wall plate where they are. Put the new ones in a place that does not clash with the old ones. Swap the joists end for end and then the bit that was poking out into free air will be clear to attach the new hanger. That means you only have to remove half a hanger from one end of each joist. Grind off the nail heads with a flap disk.

Thanks for tip, i think i understand what you are saying. I'll probably pick a choice that doesn't require taking off any joist hangers

How about screw and gluing 2x2 on top of the 4x2?

Thats a possibility but I cannot add any height to the joist, either on the top or bottom of the joist. The maximum I can work with is 4" joist (reasons mentioned above)

 

[The Bear]

Cant add much more than has already been said but I agree you're going to get a sag and pooling on that roof. Personally I'd redo it properly but the steel could work (I think you'll still get pooling at 2 degrees but it might resist some of the sag). If the old timbers do come off you can ditch the joist hangars (which you are using as tie down straps and not for hanging joists), save some cash and skew nail instead.
Regarding the height depends on how anal you think your neighbours/the planners are. I doubt anyone would notice an extra 2 inches in height outside. If you think they would, then take off the second wall plate and make sure every ceiling joist is on top of a stud, but you'll lose 2 inches of height inside.
Hop you find a satisfactory solution.
Mark

 

[Cobbs]

Could you just get rid of the rafters completely? Assuming the roof is to drain from one long wall to the other, it's easy enough to get hold of insulated roof sheets that would be able to span the three metres in one go, with no woodwork needed other that the walls.
You'd have to raise the higher side to get a 4 degree slope, but if you were to do that with a 4x2 on the higher side and let the lower ends of the sheets just sit atop the wall, would it get you there?

Or have I misunderstood the problem?

 

[Ollie78]

Could you just get rid of the rafters completely? Assuming the roof is to drain from one long wall to the other, it's easy enough to get hold of insulated roof sheets that would be able to span the three metres in one go, with no woodwork needed other that the walls.
You'd have to raise the higher side to get a 4 degree slope, but if you were to do that with a 4x2 on the higher side and let the lower ends of the sheets just sit atop the wall, would it get you there?

Or have I misunderstood the problem?

Can you let me know which sheets these are. When I looked into the insulated sandwich panels for a low pitched roof the supplier told me they must be supported every 600mm.

Ollie

 

[Cobbs]

Can you let me know which sheets these are. When I looked into the insulated sandwich panels for a low pitched roof the supplier told me they must be supported every 600mm.

Ollie

Every two foot sounds a bit extreme - 2 metres I could understand. I got mine from a firm called Cladco, who are only just down the road from me but I'm pretty sure they deliver nationwide. The sheets I have on my roofs are 50 mm thick, supported at just over 2 meters, but I get up and walk around on them a few times a year to clean leaves and sticks and so on and I'm fairly average weight - they don't even flex, so I would guess they'd be fine at 3.

Sheets

Having just looked, they don't seem to have the 50 mm ones anymore and the colour is different too, but it'll give you an idea what you're looking for, they won't be the only people able to supply such things...

 

[Molynoox]

No point taking the roof down, adding the metal plates is the best option in my view, it will be like a retrospective flitch beam.

Need to drill and bolt them to the timber, probably every other joist. Metal fab guys will drill them for you.

If you are running electricity think ahead as it will need to run through ceiling normally, those holes will need to be in place for electrician as he probably won't drill though metal for you. Also, holes in timbers that small will reduce the strength more than in 6x2 so that needs factoring in.

I think you know mistakes have been made.... but it's not a disaster fortunately

Martin