Box profile sheeting, minimum pitch

[Pantryman]

I have a single storey, single pitch garage that needs a new roof. Someone suggested using box profile sheeting which seems a fairly inexpensive and straightforward option. Only problem is the pitch is very low just 1.5 degrees or about 1 in 40 fall. All the sheeting suppliers I've found recommend a minimum pitch of somewhere between 5 and 10 degrees. Can anyone offer any advice or does anyone have any experience of using box profile sheeting on a very low pitch?
Cheers.

 

[GmB]

What is the run length? Not having any joints is important at a low pitch I would have thought. High snowfall area? Shallowest angle I have done is 5 degree with no problem.

 

[Mur Huwcun]

If you can do it in one sheet runs it will be ok. Start at the end opposite to most of the wind to get your sheet overlap right. What’s your perlin size and spacing?

 

[mo!]

If you are putting anything in there that needs to be kept dry then avoid box profile, the condensation will drive you mad. Put a pitch on it and do the job properly.

 

[Pantryman]

Thanks for the replies.
Snowfall not a problem, run length about 6 MTR or 20ft, purlin/spacing as suppliers recommendations, probably something like 4x2 every 3 ft. I can do one length but I'll be alone so thought I could split length in two with something like 2ft overlap. The suppliers I've spoken to all say the issue is to do with leaking through the fixings more than anything. Personally, I would think if it's going to leak at 1.5 degrees then it would probably leak at 5 degrees. Could just be suppliers covering their backs I guess but maybe there's a legitimate reason.

 

[Steve welder]

I'd try and do it in one sheet if you can.

 

[7610 super q]

As above, one sheeeeeet. The next problem is fixings. The modern way is to use short Tec screws in the troughs. Probably best using longer screws through the peaks, but this risks deforming the sheets if screwed too tight. I've got a large roof to do, and can't make up my mind which is best.
Oh, and silicon under the edge ( lengthways ) joints ? Not needed for steeper pitches.....but ?

 

[TheTallGuy]

Below 5 degrees & you really do want to be aiming for doing it in single lengths as capillary action can draw water a long way if having a joint. If you do go for a joint, make sure that the end of the sheet is protected as it will tend to hold moisture & will rust.

As above, one sheeeeeet. The next problem is fixings. The modern way is to use short Tec screws in the troughs. Probably best using longer screws through the peaks, but this risks deforming the sheets if screwed too tight. I've got a large roof to do, and can't make up my mind which is best.
Oh, and silicon under the edge ( lengthways ) joints ? Not needed for steeper pitches.....but ?

The problem with going through the tops is that over time the tin flexes with heat etc - the screw prevents it going "up", so it must go downwards. Eventually this leads to a permanent deformation that leaves the tin a bit loose and able to rattle against the fixing, thus damaging it more. With conventional corrugated tin the profile is better able to expand and contract without permanent deformation, although it does happen if you put too many screws in.

 

[Happy hillbily]

Try and get more of a pitch on the roof and do it in one sheet. The roof of the milking parlour here is at a similar pitch but done with corrugated sheets, the joint between the two lengths was the first to start to leak. Then we would get debris sitting at the end of the sheet which would act as a mini dam holding water on the roof, which has rotted out the sheets at the bottom of the run. Also when we get snow the heat from the cows thaws the snow on the roof, but the end of the sheet at the bottom is insulated from the heat from the cows by the purlin, so the snow and ice acts as a mini dam, holding the water on the roof, then the water goes over the ridge of the corrugation and drips/pours down inside.
Don't say to yourself, ah it will be all right it's less hassle to just bang some sheets on, it will do ! It won't I'm speaking from experience, If your replacing the roof sheets, do it properly once, then you will have a maintenance and leak free roof for years.

 

[TheTallGuy]

Try and get more of a pitch on the roof and do it in one sheet. The roof of the milking parlour here is at a similar pitch but done with corrugated sheets, the joint between the two lengths was the first to start to leak. Then we would get debris sitting at the end of the sheet which would act as a mini dam holding water on the roof, which has rotted out the sheets at the bottom of the run. Also when we get snow the heat from the cows thaws the snow on the roof, but the end of the sheet at the bottom is insulated from the heat from the cows by the purlin, so the snow and ice acts as a mini dam, holding the water on the roof, then the water goes over the ridge of the corrugation and drips/pours down inside.
Don't say to yourself, ah it will be all right it's less hassle to just bang some sheets on, it will do ! It won't I'm speaking from experience, If your replacing the roof sheets, do it properly once, then you will have a maintenance and leak free roof for years.

I think the phrase is "Do it right, do it once, or do it cheap and pay the price to do it twice..."

 

[Pantryman]

Many thanks for all of your replies, much appreciated. Cheers folks.

 

[Bruce Almighty]

Below 5 degrees & you really do want to be aiming for doing it in single lengths as capillary action can draw water a long way if having a joint. If you do go for a joint, make sure that the end of the sheet is protected as it will tend to hold moisture & will rust.

We built a 30' lean-to at about 7 degrees. To stop the capillary action we had to separate the sheets by unscrewing the teks, squirt expensive silicone (cures in water) and then screw teks back down. Lesson learned !

 

[Warp Land Farmer]

1.25 degree, no down slope laps, crown fix, double lap across slope.