Roofing material for a carport: corrugated plastic or felted exterior ply or what?

[RJS]

My (painfully slow) carport rebuild now has all the joists in place and I a= m wondering whether to roof it with corrugated plastic - as before - or som= ething different.

The original structure collapsed under the weight of snow two years ago whi= ch caused the joists to sag sufficiently for their ends for slip off the su= pporting posts. That design flaw has been remedied in the rebuild but I am= wondering if it would be prudent to build a roof that can be walked on for= the purposes of snow clearance.

What do the team think?

Richard

PS a side window in the kitchen does look into the carport but it is not an= 'in front of sink so that one can look at the view window'.

R

[Brian Gaff]

well that plastic goes brittle and the wind cracks it off on anything I've used it on.

Brian

[newshound]

wondering whether to roof it with corrugated plastic - as before - or something different.

caused the joists to sag sufficiently for their ends for slip off the supporting posts. That design flaw has been remedied in the rebuild but I am wondering if it would be prudent to build a roof that can be walked on for the purposes of snow clearance.

'in front of sink so that one can look at the view window'.

Perhaps not quite enough information. I assume you are talking about clear plastic rather than the bitumen fibre stuff. Can't you clear snow with a long-handled broom, or is it very wide or obstructed? How much slope have you got? In my experience plastic clears itself OK if there is a reasonable slope (but I don't get two feet of snow in a night). But if you are talking about walking on it presumably the slope is less than

15 degrees. How much benefit would you get from the light? If it is a big window this may make more difference than you would think.

[brass monkey]

The original structure collapsed under the weight of snow two years ago which caused the joists to sag sufficiently for their ends for slip off the supporting posts. That design flaw has been remedied in the rebuild but I am wondering if it would be prudent to build a roof that can be walked on for the purposes of snow clearance.

What do the team think?

Richard

PS a side window in the kitchen does look into the carport but it is not an 'in front of sink so that one can look at the view window'.

---------------------------

Ours uses that corrugated stuff, its been there ~ 10 years, fine. It rattles a bit in high wind but I've never lost any. I do worry about the snow but it'll only collapse about 3" onto the camper :D

[RJS]

Mine collapsed onto the MIL's car! Pictures here:

[RJS]

Yep, I'm deliberately sparse with info to prevent overload with irrelevancies and would rather provoke questions.

Carport is approx. 7.6m x 3.5m and has a slope of 5 degrees. Not much I know but they are all the same round here - which I know is no excuse for me not to have increased the slope. It collapsed under 0.6m of snow in 2010. Kitchen window is 0.8m x 0.8m

[brass monkey]

[RJS]

Hi,

The original material was corrugated plastic. I have a specialist supplier of plastic sheeting in various weights so wouldn't be using the rubbish from the DIY sheds.

Carport is approx. 7.6m x 3.5m and has a slope of 5 degrees. Not much I know but there all the same round here - which I know is no excuse for me not to have increased the slope. It collapsed under 0.6m of snow in 2010.

Kitchen window is 0.8m x 0.8m Lights are almost always required in the kitchen.

Pictures here:

[Grimly Curmudgeon]

surprised; it didn't look as if it was adequately supported at all. Otoh, when done properly, corrugated plastic sheeting is pretty strong stuff.

[RJS]

Correct. The joists were skew nailed to the 4"x2" wall plate and to the 4= "x2" plate siting on top of the four posts. Where the joist reached the po= sts there was a steel lip of about 1/3" supporting the end of the joist. T= he weight of snow made the joists sag and pushed the posts away from the ho= use, resulting in the ends of the joists falling off the steel 'lip' puttin= g the whole weight of snow plus roof onto the skew nails. Add to this woo= d that turned out to be pretty poor and you have a recipe for the inevitabl= e. Remarkably there was no damage to the car.

The replacement structure is slightly different insofar as I have supports = for each new pressure treated 4" x 2" joist made from 70mm RSA bolted to th= e wall and the far end of each joist is supported across the whole of the t= op of the post - about 100mm in all. There are coach screws up through the= posts and wall supports into each joist. What were single plates fixed to= the wall and across the tops of the posts are now sections between each jo= ist, fixed to wall and steel supports or to the 50mm RSA linking the tops o= f the posts. Hopefully that will take the weight of more snow than the end= wall of the house can bear!

Jokes aside, is the consensus of opinion to go for corrugated plastic? It= would mean no access to the roof. Bear in mind that it is 7.6m x 3.5m.

[harryagain]

The original structure collapsed under the weight of snow two years ago which caused the joists to sag sufficiently for their ends for slip off the supporting posts. That design flaw has been remedied in the rebuild but I am wondering if it would be prudent to build a roof that can be walked on for the purposes of snow clearance.

What do the team think?

Richard

PS a side window in the kitchen does look into the carport but it is not an 'in front of sink so that one can look at the view window'.

R

The only other clear plastic I can think of is multiwall polycarbonate, very expensive and requires a completely different structure. Won't break though.

conservstory above. (Aluminium rails) There are kits for fixing it both on to timber and metal.

[Kevin]

wondering whether to roof it with corrugated plastic - as before - or something different.

caused the joists to sag sufficiently for their ends for slip off the supporting posts. That design flaw has been remedied in the rebuild but I am wondering if it would be prudent to build a roof that can be walked on for the purposes of snow clearance.

I think that, given the proximity of the neighbour's house, it would feel very gloomy in the kitchen if the roof were entirely opaque. What about a compromise - a mixture of solid and translucent? You could then bridge the translucent sheets off the solid for access for maintenance.

I'd look at box section steel for the solid , and either matching profile grp or cheaper corrugated pvc between - making sure it's over the windows, of course.

If you've had problems with condensation on the underside of the sheets in the past, and are bothered by it, go for insulated box section steel

- not too expensive if you shop round for seconds or surplus sheets - and either air-gap translucent sheets or multiwall polycarbonate.

[RJS]

Hmmm. Whatever goes up, it's not going to be expensive multiwall polycarb= onate or similar. Until the snow incident I managed without being able to = cross the corrugated plastic. Having said that, much to my surprise the co= wboys who painted the house placed scaff boards on the corruagted plastic a= nd then ladders on the boards to reach the top of the side wall. They didn= 't crack a single sheet!

And I'm not adding more joists to suit a fernickety panel spacing system :=

-) Hence wavy plastic or felted ply. =20

OTH there is this stuff:

th= at a friend has used to good, but probably good, effect.

Rihard

[NT]

carbonate or similar. =A0Until the snow incident I managed without being ab= le to cross the corrugated plastic. =A0Having said that, much to my surpris= e the cowboys who painted the house placed scaff boards on the corruagted p= lastic and then ladders on the boards to reach the top of the side wall. = =A0They didn't crack a single sheet!

=A0:-) =A0 =A0Hence wavy plastic or felted ply.

What's the point of insulating a car port roof?

NT

[Grimly Curmudgeon]

I would suggest drips, but the insulation will all be at the same temp top and bottom, so might not make any difference to condensation. Otoh, uninsulated material can drip well in the right circumstances and it's all at the same temp all the way through, so it might be worth trying to see if it works.

[Kevin]

But it won't be at the same temperature in the circumstances that lead to severe condensation under uninsulated roofs: a clear winter night sky. The hoar frost phenomenon. If the roof is insulated, the underside will probably not get a frost buildup.

The OP, having apparently dismissed my suggestions, then gives a link to the stuff I'd suggested as the deluxe option. I don't understand the reference to altering joist spacings: surely there are purlins that the sheets themselves are fastened to? Especially corrugated plastic or that awful Onduline stuff.

But I'm not a great fan of built-up felt roofing. You don't see it on new warehouses. Lay it on Stramit for a real disaster.

[RJS]

Sorry Kevin

Didn't mean to appear to dismiss your suggestion out of hand. Having thoug= ht a bit more about it I remembered that a friend had used Kingspan on his = garage project. I had thought it inappropriate for my purposes because of = the open nature of the carport. But that was because I was thinking of it= s insulation properties rather than its structural properties.

My comment about joist spacing was prompted by not knowing the support requ= irements of Kingspan.

Sorry again.

Richard

[RJS]

Hi Kevin

Didn't mean to appear to have dismissed your suggestion out of hand. A fri= end used Kingspan insulated steel panels for his garage project - which is = what prompted my link. But I thought it would be too expensive for . I di= scounted it for my carport because I considered that the open-side nature o= f the carport would negate any benefits from the insulation. I hadn't cons= idered the structural benefits.

Sorry

Richard

[RJS]

My comments about joist and purlin spacing are purely driven by not knowing what support is required by Kingspan.

R