3m flat roof span with 44 × 95 joists

[Ollie78]

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...

Thanks I will give them a bell for confirmation.

To the OP the suggestions about metal plates make sense but I was just thinking why not nail some strips of osb to them. If you can buy those composite osb and timber joists then the principle would be the same. A few strops of osb some PU glue and nails woild be pretty strong.

Ollie

 

[g8dy]

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

I've just spoken to a structural engineer and they advised using 7"x2". I'm tempted to just rip out the roof and start again with 6x2 or 7x2 and lose some headroom internally, and go a little over my total external height

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?

I looked at those, very interesting never seen them before, but to cover my roof it would cost £6-700

Thanks I will give them a bell for confirmation.

To the OP the suggestions about metal plates make sense but I was just thinking why not nail some strips of osb to them. If you can buy those composite osb and timber joists then the principle would be the same. A few strops of osb some PU glue and nails woild be pretty strong.

Ollie

I'm undecided, rip out the roof and build new with 6x2's.
Or make flitch beams from the current 4x2 joists. Although a structural engineer I just talked too suggested not doing this route and go with 7x2 joists for a 3m span

 

[MF1000]

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.

50x50 Angle section on either side of the joists would be much more effective

 

[Jameshow]

What about 1x 4 timber glued and screwed to the top and bottom of the 4x2 that way you have a I beam but only loose 1" headroom and 1" height?

Just a thought.

 

[PDW125]

Although a structural engineer I just talked too suggested not doing this route and go with 7x2 joists for a 3m span

that’s overkill for a shed !! He’s covering his gentlemen’s vernaculars with that comment ..!!

I don’t think you have as big an issue as you think, but it’s going to cost a bit to fix. Fitch plates are the way to go but they need to be full depth of the beam and bolted into place - you’ll need them at 1/3rd and 2/3rd of the full width of the roof. You need 90x5mm steel, 3m long (and it comes in 6m lengths..) and you need to create a sandwich of steel and wood, coach bolted together. Drill the steel with an 8mm bit, every 300mm down the centre line, then put it on the side of the timber you want strengthen then use it to drill your holes for the bolts. Sometimes easier to put a couple of screws in too to hold it. Then offer up the second timber, align everything and clamp it all together. Now drill through the holes in the first timber, starting with the centre hole. Use a decent M8 bolt with square flat plate washers on both sides. Tighten up, and then drill the next one, working out from the middle and ensuring the whole lot stays flush to the steel at the bottom of the timber.

For your remaining rafters, you need to plate them with preferably 18mm ply, OSB will do if it’s all you can get but ply isn’t expensive when you realise you’re getting 52m from a single sheet which is enough for 14 rafters…. Cut the ply down to ~95mm (assuming this is C24 metric 4x2..?) and you need to get some decent foaming D4 glue such as the Egger board stuff. Measure up your 2 pieces to fit the rafter, then glue and push the ply tight with the top of the rafter (OSB on already..?? If so, glue on the top edge too) and then screw in place with staggered 6x50 screws at 300mm centres. Each rafter will ideally have 2 plates, one either side and you stagger the 600mm length top / bottom.

You’ve now got essential a bunch of composite beams - hopefully you glued on the roof boards too as that helps with strength as well, but if not the gluing the ply stringers will help.

Hope that helps !!

 

[MikeJhn]

Amazing how everyone knows better than any Structural Engineer, 7" x 2" at a minimums slope of 4deg and use the current joist as herringbone cross bracing. Square grip nails are in fact easy to get out, you lever out the joist hanger until enough nail is exposed to get a claw hammer on them, then hammer the joist hanger back into place against the new timber, incidentally your joist hangers are superfluous the way they have been used, purpose made right angle brackets for wall plates to joist are available.

 

[HOJ]

I cant use 6x2 because it will take me above my planning permission height.

I have to ask as to why you sought planning permission without considering the roof height ? would a pitched roof not have been a better option and for the sake of compliance, and if you think there is a risk of a neighbours complaint over a height increase, why not make an an amended application, I'm not sure if you could cover that in a Non material amendment though,as you are changing the dimensions, but it has a nominal fee structure.

Added to the fact you intend to:

be suspending various things on a ceiling rack system

Isn't helping with your loading calcs, which is probably why the SE recommends 7" joists, along with the very shallow pitch angle.

 

[jimbowley]

I don't know what building rules say, but you could notch the ends of 6" or 7" beams to fit the existing mountings and external envelope. Internally you could taper the beams to lose less headroom at the low end, eg 7" tapering to 5". Probably a bit late now but could you have lowered the floor?

 

[flying haggis]

You could fit 6x2 along side each joists and it would only involve removing a small bit of each hanger and wouldn't affect headroom

 

[g8dy]

50x50 Angle section on either side of the joists would be much more effective

Yes thats a possible solution, and very similar to joist plates I've seen online

What about 1x 4 timber glued and screwed to the top and bottom of the 4x2 that way you have a I beam but only loose 1" headroom and 1" height?

Just a thought.

That's a good idea, but would make installing roof insulation a bit of a hassle

that’s overkill for a shed !! He’s covering his gentlemen’s vernaculars with that comment ..!!

I don’t think you have as big an issue as you think, but it’s going to cost a bit to fix. Fitch plates are the way to go but they need to be full depth of the beam and bolted into place - you’ll need them at 1/3rd and 2/3rd of the full width of the roof. You need 90x5mm steel, 3m long (and it comes in 6m lengths..) and you need to create a sandwich of steel and wood, coach bolted together. Drill the steel with an 8mm bit, every 300mm down the centre line, then put it on the side of the timber you want strengthen then use it to drill your holes for the bolts. Sometimes easier to put a couple of screws in too to hold it. Then offer up the second timber, align everything and clamp it all together. Now drill through the holes in the first timber, starting with the centre hole. Use a decent M8 bolt with square flat plate washers on both sides. Tighten up, and then drill the next one, working out from the middle and ensuring the whole lot stays flush to the steel at the bottom of the timber.

For your remaining rafters, you need to plate them with preferably 18mm ply, OSB will do if it’s all you can get but ply isn’t expensive when you realise you’re getting 52m from a single sheet which is enough for 14 rafters…. Cut the ply down to ~95mm (assuming this is C24 metric 4x2..?) and you need to get some decent foaming D4 glue such as the Egger board stuff. Measure up your 2 pieces to fit the rafter, then glue and push the ply tight with the top of the rafter (OSB on already..?? If so, glue on the top edge too) and then screw in place with staggered 6x50 screws at 300mm centres. Each rafter will ideally have 2 plates, one either side and you stagger the 600mm length top / bottom.

You’ve now got essential a bunch of composite beams - hopefully you glued on the roof boards too as that helps with strength as well, but if not the gluing the ply stringers will help.

Hope that helps !!

That was my Dad's reaction too, and I get that they are covering their a** but he didnt like any of the other ideas I suggested like flitch beams etc.

I'm weighing up my options at the moment, i think i'll either go this route and swap out 3-4 joists for flitch beams and then add osb plates to the remaining. It will cost me approx £190 for the steel bar and a sheet of plywood.

OR rip out the whole lot and add in new 6x2 joists, 400mm spacing (even though he suggested 7x2 i think 6x2 will be sufficient). If i see any sagging in future, ill reinforce a few with flitch beams. Cost for 14 c24 6x2 is approx £200

Overall, either route is going to cost be about the same. Second option means I'll lose 45mm internal height as ill remove the top plate of my walls, and go 30mm over my planning permission

Amazing how everyone knows better than any Structural Engineer, 7" x 2" at a minimums slope of 4deg and use the current joist as herringbone cross bracing. Square grip nails are in fact easy to get out, you lever out the joist hanger until enough nail is exposed to get a claw hammer on them, then hammer the joist hanger back into place against the new timber, incidentally your joist hangers are superfluous the way they have been used, purpose made right angle brackets for wall plates to joist are available.

Thanks for that tip, didnt think of using the joist hanger itself to leverage the nails out, that should speed things up.
Re Joist hangers, are you referring to ths type below, are these what would normally be used on a flat roof like mine?

I would have thought joist hangers were better as they wrap around the top of the joist, but i think those right angle brackets would have certinaly been easier to remove, ill have to remember that for future builds.

But you have just given me a great idea if I do put in new joists. Use the right angle brackets and leave the bottom part of the joist hanger in the timber, saving me having to remove so many nails

I have to ask as to why you sought planning permission without considering the roof height ? would a pitched roof not have been a better option and for the sake of compliance, and if you think there is a risk of a neighbours complaint over a height increase, why not make an an amended application, I'm not sure if you could cover that in a Non material amendment though,as you are changing the dimensions, but it has a nominal fee structure.

Added to the fact you intend to:

Isn't helping with your loading calcs, which is probably why the SE recommends 7" joists, along with the very shallow pitch angle.

I did consider roof height, my main goal was to maximise internal height, but i thought it would be ok to use 4x2 and in the design stage i just assumed i could use 4x2 like I had been for the floor and walls. Then a few things happened; I run out of wood and wanted to get it done over xmas so had to space the ceiling joists by 500mm and thats when i realised it was a bit bouncy when walking on and started to research if 4x2 was acceptable or not.

You could fit 6x2 along side each joists and it would only involve removing a small bit of each hanger and wouldn't affect headroom

I cant see how it would need only a small part of the hanger to be removed. I think you mean, cut the joist hanger and unwrap from top part of joist so you can slot in a 6x2 next to it right??

Fitting 6x2 in the current setup with how my walls are built would mean i'd go over my planning permission height by 2 inches.
Still unsure whether to just risk it and stick new 6x2 or remove my wall top plate (you can see in the image above) and lose some internal height

 

[Woody2Shoes]

Yes thats a possible solution, and very similar to joist plates I've seen online
View attachment 174774

That's a good idea, but would make installing roof insulation a bit of a hassle

That was my Dad's reaction too, and I get that they are covering their a** but he didnt like any of the other ideas I suggested like flitch beams etc.

I'm weighing up my options at the moment, i think i'll either go this route and swap out 3-4 joists for flitch beams and then add osb plates to the remaining. It will cost me approx £190 for the steel bar and a sheet of plywood.

OR rip out the whole lot and add in new 6x2 joists, 400mm spacing (even though he suggested 7x2 i think 6x2 will be sufficient). If i see any sagging in future, ill reinforce a few with flitch beams. Cost for 14 c24 6x2 is approx £200

Overall, either route is going to cost be about the same. Second option means I'll lose 45mm internal height as ill remove the top plate of my walls, and go 30mm over my planning permission


Thanks for that tip, didnt think of using the joist hanger itself to leverage the nails out, that should speed things up.
Re Joist hangers, are you referring to ths type below, are these what would normally be used on a flat roof like mine?
View attachment 174775

I would have thought joist hangers were better as they wrap around the top of the joist, but i think those right angle brackets would have certinaly been easier to remove, ill have to remember that for future builds.

But you have just given me a great idea if I do put in new joists. Use the right angle brackets and leave the bottom part of the joist hanger in the timber, saving me having to remove so many nails
View attachment 174776


I did consider roof height, my main goal was to maximise internal height, but i thought it would be ok to use 4x2 and in the design stage i just assumed i could use 4x2 like I had been for the floor and walls. Then a few things happened; I run out of wood and wanted to get it done over xmas so had to space the ceiling joists by 500mm and thats when i realised it was a bit bouncy when walking on and started to research if 4x2 was acceptable or not.

I cant see how it would need only a small part of the hanger to be removed. I think you mean, cut the joist hanger and unwrap from top part of joist so you can slot in a 6x2 next to it right??

Fitting 6x2 in the current setup with how my walls are built would mean i'd go over my planning permission height by 2 inches.
Still unsure whether to just risk it and stick new 6x2 or remove my wall top plate (you can see in the image above) and lose some internal height

"....would mean i'd go over my planning permission height by 2 inches..." Seriously, nobody's going to enforce against that - really!

 

[The Bear]

You don’t need any joist hangars, brackets, straps etc on your roof. Save your money. The only metal you need is your nails.
Mark

 

[imageel]

You don’t need any joist hangars, brackets, straps etc on your roof. Save your money. The only metal you need is your nails.
Mark

Umm, I'd disagree. Am Essex based and had open fields both front and rear and the house front faces west - so the wind is either blowing front to back or the vice-a-versa. When we have storms blowing stepping outside via a side door and mug of tea usually results in half the tea being lost in the wind. When I questioned my BC as to why a pitched roof EDPM covering my rear extension required 95 x 150 x 6mm thick I-beams over 2 picture windows he replied wind loading...
During one storm a neighbour lost 4-5 concrete ridge tiles 2 of which ended up in my front garden... they weigh approx 6.6kg each ignoring any attached mortar....

 

[The Bear]

No worries on the disagrement but this is a shed not a house and needs perspective. A cut roof on a house is only going to be nailed to the plates.

 

[PDW125]

Fitting 6x2 in the current setup with how my walls are built would mean i'd go over my planning permission height by 2 inches.
Still unsure whether to just risk it

Build the ground up 2” around it as it’s measured from the highest ground level and just go with 6x2 then !!

 

[MikeJhn]

No worries on the disagrement but this is a shed not a house and needs perspective. A cut roof on a house is only going to be nailed to the plates.

And then a few tonne of roof tiles or slates to help prevent lift, screw nailing is the old way of attaching joists to the wall plate, but then the joists where bird mouthed to the wall plate and a much thicker section.

To the OP the clue is in the name joist hanger the right angle plates you have shown above are correct tor that application, but don't forget the herringbone cross braces, very important, joist fail by twisting first they do not shear straight through.

 

[Jameshow]

Yes thats a possible solution, and very similar to joist plates I've seen online
View attachment 174774

That's a good idea, but would make installing roof insulation a bit of a hassle

That was my Dad's reaction too, and I get that they are covering their a** but he didnt like any of the other ideas I suggested like flitch beams etc.

I'm weighing up my options at the moment, i think i'll either go this route and swap out 3-4 joists for flitch beams and then add osb plates to the remaining. It will cost me approx £190 for the steel bar and a sheet of plywood.

OR rip out the whole lot and add in new 6x2 joists, 400mm spacing (even though he suggested 7x2 i think 6x2 will be sufficient). If i see any sagging in future, ill reinforce a few with flitch beams. Cost for 14 c24 6x2 is approx £200

Overall, either route is going to cost be about the same. Second option means I'll lose 45mm internal height as ill remove the top plate of my walls, and go 30mm over my planning permission


Thanks for that tip, didnt think of using the joist hanger itself to leverage the nails out, that should speed things up.
Re Joist hangers, are you referring to ths type below, are these what would normally be used on a flat roof like mine?
View attachment 174775

I would have thought joist hangers were better as they wrap around the top of the joist, but i think those right angle brackets would have certinaly been easier to remove, ill have to remember that for future builds.

But you have just given me a great idea if I do put in new joists. Use the right angle brackets and leave the bottom part of the joist hanger in the timber, saving me having to remove so many nails
View attachment 174776


I did consider roof height, my main goal was to maximise internal height, but i thought it would be ok to use 4x2 and in the design stage i just assumed i could use 4x2 like I had been for the floor and walls. Then a few things happened; I run out of wood and wanted to get it done over xmas so had to space the ceiling joists by 500mm and thats when i realised it was a bit bouncy when walking on and started to research if 4x2 was acceptable or not.

I cant see how it would need only a small part of the hanger to be removed. I think you mean, cut the joist hanger and unwrap from top part of joist so you can slot in a 6x2 next to it right??

Fitting 6x2 in the current setup with how my walls are built would mean i'd go over my planning permission height by 2 inches.
Still unsure whether to just risk it and stick new 6x2 or remove my wall top plate (you can see in the image above) and lose some internal height

Osb plates Either side if the beams will make little difference tbh.

 

[flying haggis]

just to bung in another tuppence worth, how about cutting off one side of the hanger only leaving the other side to hold the timber down and adding stepped 6x2 alongside the 4x2. i have tried to picture what i mean ie cut away the red bits and add 6x2 timber shaped like the black bit alongside the 4x2 where the red bits of hanger have been removed. no nails need to be removed and in fact if you just cut the bottom of the marked triangle and fold it back the new joist will sit alongside happily

 

[LeeAkeroyd]

Could you not sit a 47 x 50 or 75 down to 0mm firring piece on top of the existing rafters?

 

[pip1954]

Hi I would grind down each side of bracket and move new joist to new positions (no nail removal) if you go with 6x2 I would not cut bottom out of it because it could split. Could you'd save 4x2 timber for bench making and future projects, I would not space new joists to far apart false economy.
Do the job once do it right hope you get it sorted