Shed roof construction

[11Tommy11]

Hello,

Im going to redo my shed roof as it is currently sagging in the middle and pooling therefor leaking under heavy rainfall.

I’ll put a couple of images up of the current roof construction which is 6 x 2 with joist hangers off of a centre beam (2 x 6 x 2s bolted together). The span is 4.6m. I’m assuming 6 x 2 isn’t man enough for this span which is why it’s sagging through the centre beam?!

What should I replace the timbers with? Thinking 8 x 2’s but getting mixed info as to whether or not that will strong enough.

I have also considered dropping a support beam under the middle (perpendicular to the rafters) for them to sit on to prevent any future sag. Is this a good idea / needed?

Main roof measures 4.6m x 4.5m and will have 18mm OSB and EPDM finish with celotex in between rafters.

I’ve attached a couple of images of what I’m thinking of doing.

Be great to hear some advice
Thanks.

 

[Harry Bloomfield]

What should I replace the timbers with? Thinking 8 x 2’s but getting mixed info as to whether or not that will strong enough.

Where the proposed new red beam is, it looks as if you already have a continuous timber beam/joist across there, and that beam is dropping? A steel might be needed, under that beam, with timber spacers between the steel and the existing timber, to gradually wedge it back up to level. Another way, might be to try raising it with a temporary acro jack, then build a pier as a more permanent support for that timber, in the middle.

 

[11Tommy11]

Thanks for getting back to me.

I did consider repairing the existing roof with a beam jacked up some how but after inspecting that roof, it’s in quite a bad state so I’m going to strip all that off and start again with some new timbers.

I could do with some advice on what timbers would be adequate and if I’d still require a beam through the middle.

Thanks.

 

[noseall]

Thanks for getting back to me.

I did consider repairing the existing roof with a beam jacked up some how but after inspecting that roof, it’s in quite a bad state so I’m going to strip all that off and start again with some new timbers.

I could do with some advice on what timbers would be adequate and if I’d still require a beam through the middle.

Thanks.

Keep the 6" x 2"s in place, prop the roof timbers and graft a timber filleted steel beam up between them (red line, last image).

 

[Harry Bloomfield]

I could do with some advice on what timbers would be adequate and if I’d still require a beam through the middle.

Sight along, under the beam - how much does it dip down?

An acro jack, would be a quick way to see what could be done by adding a pier, with minimum effort.

 

[11Tommy11]

Would the pier not then be in the middle of the room?

 

[Harry Bloomfield]

Would the pier not then be in the middle of the room?

Obviously, but a much simpler, quicker, more certain fix than the alternative ways to fix the sag in the middle of that beam - always assuming that a pier there, would not be an issue?

 

[stevie888]

If you don't want a prop somewhere near mid span and you don't want to lose headroom by undermounting a steel then the next option would be to either totally replace the whole thing (which seems a waste as the joists look ok as far as we can tell and even the osb is showing no obvious problems) or inset a steel to directly replace the inadequate double. It's a shed so you don't have to slavishly follow TRADA tables in fact I suspect if your 6x2s were continuous you wouldn't be making this post.

Peesonally I'd just sort out a steel to go under what's there and accept a small local headroom loss. With a bit of low engineering cunning a 100 wide RHS laid flat should be adequate. Not terribly structurally efficient use of steel but far and away the easiest option. They are available either at either 150 or 200mm and a variety of wall thickness. This approach has got me out of a few tricky problems over the years. If you crunch a few numbers even a smaller section might work.

 

[11Tommy11]

Thanks for your reply stevie888

If I was to replace the whole thing what would you recommend?

If I was starting again should I have added a steel / wooden beam / purlin through the centre under the roof structure for it all to sit on and if so what size would this be?

Should I have used 8 or 9 x 2s?

Thanks.

 

[noseall]

If I was starting again should I have added a steel / wooden beam / purlin through the centre under the roof structure for it all to sit on and if so what size would this be?

Prop the joists and slot one in, I.e.....
Buy a beam (with holes drilled in the web centre) and fill out the web with timber. crush the bolts up flush. Cut off the excess.
Leave the existing midspan beam in place, but cut a slot for the new beam (after propping) just this side of the existing, with a recip and multitool.
Slot the new beam up inside the joists and fix the existing timbers to the new web timbers with jiffies. Screw the existing midspan beam to the new one.
Make sure you have propped the sag out before fixing.

 

[stevie888]

Thanks for your reply stevie888

If I was to replace the whole thing what would you recommend?

If I was starting again should I have added a steel / wooden beam / purlin through the centre under the roof structure for it all to sit on and if so what size would this be?

Should I have used 8 or 9 x 2s?

Thanks.

If you do start over then 8x2 at 400crs will match TRADA deflection limits, but in the real world 7x2 would be fine for a shed.

If you go with Noseall's suggestion take the existing centre timber completely out: you'll struggle to get things straight and you'll have a sketchy connection.

 

[11Tommy11]

Ok thanks, that’s what I wanted to hear!

Say I use 8 x 2 and putting 100mm celotex in between rafters but need to have a 50mm ventilation gap would that noggins have to be 6 x 2 to allow for continued ventilation.
Am I right in thinking if the noggins were also 8 x 2 it would block the 50mm air gap?

 

[noseall]

If you go with Noseall's suggestion take the existing centre timber completely out

Eh? I explicitly said to leave it in and fix it to the new beam (after propping out the sag).

 

[noseall]

Ok thanks, that’s what I wanted to hear!

Say I use 8 x 2 and putting 100mm celotex in between rafters but need to have a 50mm ventilation gap would that noggins have to be 6 x 2 to allow for continued ventilation.
Am I right in thinking if the noggins were also 8 x 2 it would block the 50mm air gap?

How are you going to vent every joist run at each end?

 

[11Tommy11]

Sorry, I’m not sure what you mean?

Im putting a vented soffit around the perimeter and from what I have researched for a warm roof there needs to be a 50mm ventilation gap above insulation before the OSB.

So I was then thinking if the noggins (spaced at every 1500mm or so) are tight under the OSB is this not going to stop the ventilation gap and form cold spots?