Onduline carport roof

[Stephen]

Hello,

I am thinking or replacing my (leaking) flat carport roof with something like Onduline corrugated sheets. I read the onduline fixing guide, available on their web site, and it requires joists and noggins

2X2 IIRC. This seemed a bit big to me, but I guess it allows for snow on the roof and it is a common size of timber.

Is it best to buy treated timber or since it will be under the roof and dry, doesn't it matter?

I wasn't expecting it to need noggins. What is the best way to fit them? I wondered whether I could mortice the joists, but I now think I may be making extra work for myself! Would it be quicker and easier to just screw/nail them?

I will attach a length of wood to the wall of the to run the joists from. Would a hammer fixing be strong enough or would it be best to use a sleeve anchor? I'm thinking the latter just to be sure.

Any tips on how to attach the joists to this rafter, making sure they are all at the same angle?

Thanks.

[RubberBiker]

High winds can exert a great deal of force, so I wouldn't underbuild - use substantial timbers and a fast and secure fixing systems so that you don't hesitate to add extra anywhere that might need it.

I'd use 100 * 50 mm regularised and treated throughout - it's the standard stuff of stud walls, so cheap and readily available at all builders merchants (=A31-=A31.50/metre). 50 * 50 mm will flex too easily. Noggins are important to stop the joists twisting, and greatly stiffen the overall structure - put them at 1.2m spacings along the joists.

For fixing timber quickly use something like Screwfix Turbogold screws put in with an impact driver or hefty cordless drill - nobody bangs in nails with a hammer any more (pros would use a nail gun mostly in place of screws).

For fixing the wall plate to masonry, multimonti are very good - and can easily be removed again if necessary.

[Kipper at sea]

Just to add to the other mail above. The noggins are also there to catch the fixings holding the corrugated sheets down. The sheets have to be fixed in the top of the roll, the top of the roll doesn=92t always line up with a joist. Therefore horizontal noggins are used. Another way is to lay timbers horizontal across the top of the joists instead of fitting them in between.

[The Medway Handyman]

Thats exactly what I would have said. Great minds etc.

[Cicero]

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You don't actually need the length of wood (wall plate) attached to the wall as you can use individual joist hangers. You'll need to notch each joist to achieve a fall and then put the top row of noggins as close to the wall as the hangers will allow. Flashing will cover any gaps.

I think you definitely need bigger than 2" x 2" timber - 3" x 2" min but

4" x 2" if very long or exposed to high winds.

See:

for joist hangers.

Cic.

[neverwas]

Easy to overlook after all the good comments that it is a flat roof. But I mention it to ask if you are suggesting that the OP switches to a sloping roof? If not, how would one go about preventing all/much drainage from the roof down the wall?

[Cicero]

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The OP is going to use Onduline corrugated sheets which are 2 metres long, so an overlap will be needed to achieve sufficient width to create a carport. Given that an overlap is required the roof will need a slope to prevent water ingress at the overlaps. This suggests that the OP will be making a sloping roof regardless of the existing format. In any case 'flat' roofs usually have a fall (slope) to allow drainage to a gutter so they're not entirely level.

Cic.

[Rob G]

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Definitely use the nominal 4 x2 size of timbers - in fact I did a 2.5 m wide roof with Onduline recently and used 5 x 2 as even that span is approaching the limit for 4 x 2 - and that doesn't allow for the fact that modern timber sizes are well below the nominal. You do have to walk on the roof to do the fixings, which you certainly couldn't do with 2 x 2.

I think if I remember the instructions were pretty comprehensive and I'm not aware of finding anything special to pass on, but if you do have any questions don't hesitate to ask.

I certainly would always suggest Onduine now for any shed roof - so, so much easier and vastly quicker than felt, and really no more expensive.

Rob

[neverwas]

Ah. Ta. I'll get the pointy hat and go sit in the corner now.

[dennis]

You need a large overlap on a very shallow slope. I did my carport using 4m long sheets. It has a very little fall.. about 1 inch in 13 ft. It works OK but unless you put some sort of drip strip on the end the water will flow under the sheets rather than drip into the gutter. I used a bead of silicone and that has worked well for 20 years. It looks like its getting close to replacement time though.. it has gone brittle in places.

[PeterC]

Don't feel bad :-> almost all roofs are flat - only the level ones give trouble with drainage (horizontal ones aren't flat).

[PeterC]

Very true - I used 2x2 for a 10x8 shed, so when I was on the roof I put a prop under each timber as I was working above it and a 8x4 sheet of ply on top. With only about 8 deg. slop, the nailheads downhill held me in place.

Yes, the info on Onduline's site is v. good.

Agreed. Last time I used felt (chicken coops, so not big or critical) I went to B&Q, tried the 'extra' version and could rip it with fingers and thumb of 1 hand. Went to a proper roofer, paid about 10 - 20% more and got some BSH stuff. Onduline is OK - just wish that the sheet were 1m wide, as on a 2.4m shed there's only just enough overlap.

[Stephen]

Sorry for the slight delay in replying. Thanks for all the help. I will definitely use 4x2. You mention 4x2 having a limit: what is it and where would I find other limits for future reference?

I hadn't appreciated that I would need to walk on the roof. I thought you worked from the side of the last sheet. I presume you put a board across to spread your weight a cross a couple of joists?

Thanks.

[Stephen]

I had planned to make sure everything was fixed well but hadn't realised wind could snap 2x2. I have taken your advice on board and will now use 4x2 instead.

I had not realised that. I thought it was strong stuff.

Thanks.

[Stephen]

Thanks. I hadn't realised that.

If building from scratch, shouldn't I make sure the joists line up?

Good idea. I will think about that.

[Stephen]

Isn't it easier to fit one piece of wood with a spirit level, rather than have to worry about getting several joist hangers at the same height though? Or are you thinking of the hangers that go into the mortar, in which case the mortar line should already be straight?

The holes in the joist hangers look very small, but I am looking a at photo, not one in real life. Are they big enough to get screw through and into the wall in one direction and the wooden joist in the other?

I see the instructions give a wide range of fall angles; is any one angle optimum?

There seems to be two types of metal work: joist hangers and truss clips. The only difference I can see is that the truss clip is open and the joist hangar has a piece across the bottom. Does it matter which I use?

You mentioned notching. Can't I just cut each joist to make a triangle and slip that into the hanger?

\ |\ | \ | \ | \ | \ | \ | \ | \ \ \ \ \

Or do I need to notch it too:

\ |\ | \ | \ | \ | \ | \ ___ \ | \ | \ \ \ \ \

to fit over the hanger?

[Cicero]

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I've just seen your late post so here goes if you're still watching.

1. The wooden wall plate will be quite heavy (4" x 2" x 15 feet approx.) so it's much easier to fix individual joist hangers even if you can get help to support the wooden wall plate. This is why I suggested joist hangers. There's nothing wrong with a wall plate but it's easier to fix joists into hangers rather than to a wooden wall plate.

1. The best joist hangers for this purpose are the ones that go into a brick joint held in with mortar. You might also need to secure on the front face with one or more screws so that there's no risk of them pulling out. A mortar course in bricks may not be absolutely level but if it's good enough for the house it will do for a car port. Use the type with a bottom plate rather than open - gives more support. Have a look for joist hangers with a solid back plate with a central fixing hole - easier to fix with one big screw - use hex head coach screws.

2. Notching will be needed but when you do this the effect will be to bring the end of the joist slightly out of plumb so you may need to cut the end of the joist to compensate for the notching (the notching is called a 'bird's mouth). Decide on the required angle for the end of the joist BEFORE cutting the notch, otherwise it will be out of position.

1. Angle of fall depends on several things - what slope looks best, strength of prevailing winds, etc. but it's rarely critical. Best to look at neighbouring houses for guidance.

I hope that covers all your questions, but if you need more information it would be best to start a new thread as your original may disappear from sight.

Cic.