Car Port

the_lunatic_is_in_my_head

Spring is hopefully coming and in addition to powering the shed we need to build a car port to help protect the cars from the elements. 

The rather crude diagram below shows the plan




So 9 posts (red) with the top chiselled out to have timber sit in the top (black) and then rafters (blue) go the other way (no doubt more than 7 and evenly spaced) with 2 diagonal braces on each post.

On top would go OBS board and then either felt or hopefully (if budget permits) corrugated metal roofing sheets. Flat roof (with a slope for water run off) rather than pitched. 

Questions are what size timber to use for posts, rafters and the timber at a right angle to the rafters.

Also am I missing anything? Budget is on the tighter side so I don't want to go overboard, simply keep the worst of the rain off the cars but equally don't want the thing to fall down after 5 minutes.

Thank you in advance for any advice.  

Brie

Lights for when you're parking at night?  Is there electric out there?  What if you get an EV? (assuming you don't have one already...)

How close is it to the house and an open window?  Corrugated metal can be pretty loud if there's a downpour.  

Drains for the rain?  A water butt might be a nice to add (some water companies give them away for free) but you'll still need something for the overflow.

Watch out for making any convenient nesting spots.  My car in it's car port is nicely protected from the rain (and a lot of the ice and frost too) but it's an easy target for some bird droppings.  Less than the neighbours who park under a huge tree but the bluetits have discovered it's a nice place for a nest.  fyi - my car also seems to get a lot of moss growing on the edge of the windows too - not enough air flowing through I think.

6 meters across gives you about 1/2 meter either side of each car - is that sufficient space to get in/out?  Obviously you could be parking in tight to the left on the lieft side and backing in tight to the right on the right side thus giving you over a meter in the middle if the post isn't in the way.  Don't know what width out car port is but it's fine for my tiddly VW up but impossible to park the OH's monster Volvo SUV.  

Ditzy_Mitzy

Can't comment on building methods as I don't know much about them, but a couple of thoughts:

1) You seem to be proposing a depth of 4 metres, or 13'1 and a bit".  That seems a bit shallow, given that a Ford Focus is 14'5" long.  Are you only proposing parking quite small cars?

2) Corrugated iron roofing panels on a wooden frame are likely to generate a significant amount of noise when it rains.  You might end up with a king sized steel drum if they aren't insulated or covered!  A felt roof wouldn't present such an issue, although I agree it wouldn't be as durable.  

Eldi_Dos

Take into account any flues.

FreeBear

The centre posts will get clobbered by car doors - You probably want to get rid of them.

If you use 150mm square posts and 150x50mm for the rafters, it should be plenty strong enough - Put some diagonal bracing between posts & rafters, and that will stiffen the structure up even further.

Oh, and use steel post supports rather than setting the posts in to the ground - Doing the latter, and the wood will rot away in 5-10 years.

Section62

the_lunatic_is_in_my_head said:

On top would go OBS board and then either felt or hopefully (if budget permits) corrugated metal roofing sheets. Flat roof (with a slope for water run off) rather than pitched. 

Condensation dripping off the underside of galvanised metal sheeting can do a lot of damage to car paintwork.  Make sure you either use a material not prone to condensation, or else specify/use anti-condensation felt. If you put metal sheets on top of OSB then use sheets of polythene (with taped joints and generous overlaps) under the steel, otherwise you may get drips coming through the gaps between OSB sheets.

The main issue with car ports is that being 'open' they need special attention to wind loads - a mono-pitch roof in particular is tricky to get right.  Most people concentrate on supporting the weight of the roof, but it is equally important to make sure it can't be lifted off (by negative pressure) with the wind passing over the top of it.  Use restraint straps liberally.

Also make sure you don't need planning consent.

Captainkirk54

Some areas will require planning permission for this - have you checked?

Rafters (Blue) 150mm x 50mm (145 x 45) C16 CLS recommended - these will come in 4.8 metre lengths and will not need mid span posts
or 175 x 50 graded saw carcassing timbers you may get 4.2 metre lengths to save waste
6m (Black) bearers  - again C16 150mm x 50mm CLS - will have to join at outer centre posts.

OSB board outside is not recommended even with felt on top, it will draw dampness on underside - again felt is not desirable as has limited life particularly with exposed OSB - if used it will need to be 18mm for 600 rafter spacings or 50 x 38 cross battens will be required for 13mm OSB

Corrugated steel sheets are the best maintenance free solution - again subject to planning - despite the possible noise issue during very heavy rain or hailstones - if sheets are running parallel to rafters 50mm x 38mm cross battens will be required to secure roofing sheets. 4 metre sheets very hard to get and transport. Best option is 50% 2m sheets and 50 % 2.5m sheets to provide the overlap. Must have cross battens under overlap join for fixing sheets 5 - cross battens minimum. You will get condensation on the underside of the steel sheets in cold weather unless a breather membrane is laid between the rafters and cross battens.

Rainwater run off cannot be run onto neighbouring land if at boundary. This may also be a planning issue and may have to be directed to a RWC drain or a soakaway on your own land if not at a boundary 

born_again

Look at something like

Onduline Easyline Intense Black Bitumen Corrugated Roof Sheet

For the roof. Good stuff. Have it on the garage roof.

the_lunatic_is_in_my_head

Thanks for the comments, this won't be attached to the house and the walls are very thick, we rarely hear any noise inside but an over the road neighbour has something similar with a metal roof I'll ask them about the noise. 

Plenty of birds around, if they nest, they nest, but will do the best to deter them and have some bird boxes to go up this year which will hopefully will make a more attractive spot for them. 

4m is just a rough plan, timber easily comes in 4.8m lengths so will park the two cars there and decide on an exact size. No wiggle room on the width unfortunately, we'll just have to make do. 

FreeBear said:

Oh, and use steel post supports rather than setting the posts in to the ground - Doing the latter, and the wood will rot away in 5-10 years.

Yeap will be mounted on post shoes  

FreeBear said:

If you use 150mm square posts and 150x50mm for the rafters, it should be plenty strong enough 

Captainkirk54 said:

Some areas will require planning permission for this - have you checked?

Rafters (Blue) 150mm x 50mm (145 x 45) C16 CLS recommended - these will come in 4.8 metre lengths and will not need mid span posts

Those 150mm x 150mm lengths are blooming expensive, seems over kill for rafters?

We have a 7x3m wood store with a slate roof (very old slates that are rather thick and heavy) sitting on 50*150mm timbers as the rafters, with the weight of roof on this being much lighter is 50x75mm not sufficient?  

I assume the larger the timber the less chance of it bowing over time, I did wonder about the centre posts but 4m seems a large span without anything supporting it in the middle? 

Captainkirk54 said:

6m (Black) bearers  - again C16 150mm x 50mm CLS - will have to join at outer centre posts.

OSB board outside is not recommended even with felt on top, it will draw dampness on underside - again felt is not desirable as has limited life particularly with exposed OSB - if used it will need to be 18mm for 600 rafter spacings or 50 x 38 cross battens will be required for 13mm OSB

Thanks for this, if you don't use OBS but went with felt what would you put under the felt instead please? I'd prefer not to use felt but best to know in advance depending upon funds. 

18mm would be fine but would the OBS (or it's alternative) be required if using corrugated sheets?

Captainkirk54 said:

 You will get condensation on the underside of the steel sheets in cold weather unless a breather membrane is laid between the rafters and cross battens.

Section62 said:

or else specify/use anti-condensation felt. 

Breather membrane is simple enough, the same thing you'd put on the roof of your house? Just stapled to the rafters with the cross battens and then corrugated metal straight on top? 

This site:

https://www.roofingmegastore.co.uk/roofing-categories/corrugated-sheets/metal/sheets.html

Seems to sell cut to length although I haven't looked at prices elsewhere to see how much extra is being paid for this option.

There also seems to be anti-condensation roofing, does it work? Although it seems the membrane would be cheaper. 

Section62 said: Most people concentrate on supporting the weight of the roof, but it is equally important to make sure it can't be lifted off (by negative pressure) with the wind passing over the top of it.  Use restraint straps liberally.

As this isn't next to the house does that alter the wind issue and is there anything as an alternative or am I misunderstanding where the restraint straps go?

Generally I don't think planning is an issue, less than 30sq, we're not covering more than half the land (other out buildings accounted for), single storey with eaves less than 2.5m but I will check with the council before investing in all the materials. 

Rain water drainage isn't a problem, there is already a vast amount of rain water being diverted to somewhere it can drain away and can easily tap into that set up. 

No EV (unless the government start giving them away ), not sure about lights yet but it's already pitch black at night so can't be any worse. 

born_again said:

Look at something like

Onduline Easyline Intense Black Bitumen Corrugated Roof Sheet

For the roof. Good stuff. Have it on the garage roof.

Thanks born_again, will take a look. 

Section62

the_lunatic_is_in_my_head said:

Breather membrane is simple enough, the same thing you'd put on the roof of your house? Just stapled to the rafters with the cross battens and then corrugated metal straight on top? 

This site:

https://www.roofingmegastore.co.uk/roofing-categories/corrugated-sheets/metal/sheets.html

Seems to sell cut to length although I haven't looked at prices elsewhere to see how much extra is being paid for this option.

There also seems to be anti-condensation roofing, does it work? Although it seems the membrane would be cheaper.

Membrane is not the right material.  Unless you thermally insulate the underside of metal sheeting you will get condensation forming on the underside.  The amount of airflow and/or breathability makes no difference.  Anti-condensation felt is designed to absorb the condensation which inevitably forms so it doesn't drip, then allows it to evaporate during the day.

I was surprised to read 4m sheets of steel roofing were very hard to get.  The suppliers (e.g. as per your link) will cut sheets to the length you specify, limited only by the size of lorry used for delivery and the maximum roll length.  Longer sheets are more difficult to handle, but much preferred to using shorter sheets with a lap.  Shop around and you'll find suppliers who will have no difficulty selling you 4m+ (you'll want some overhang top and bottom) lengths delivered to site.

Likewise, I've not heard of people having problems with OSB3 absorbing moisture from the air.  It needs to be protected from rain and standing water, but environmental water vapour (at atmospheric temperature and pressure) isn't an issue as far as I know.

the_lunatic_is_in_my_head said:

As this isn't next to the house does that alter the wind issue and is there anything as an alternative or am I misunderstanding where the restraint straps go?

You'd use restraint straps between the roof structure and the support posts - to tie the roof down onto the posts (which in turn would be secured to the ground).  Nothing to do with whether or not the car port is attached to the house.

The complexity of wind loading means whether freestanding or attached you could get increased or decreased loading depending on the exact situation.  In principle an attached car port may gain from being sheltered by the house with the wind blowing in one direction, but exposed to increased loading when the wind is blowing another way (e.g. wind funneling).  It is so hard to predict wind loading that for practical purposes it is simpler to just go overboard on strapping the roof structure to the walls (or support posts).

the_lunatic_is_in_my_head

Section62 said:

You'd use restraint straps between the roof structure and the support posts - to tie the roof down onto the posts (which in turn would be secured to the ground).  Nothing to do with whether or not the car port is attached to the house.

The complexity of wind loading means whether freestanding or attached you could get increased or decreased loading depending on the exact situation.  In principle an attached car port may gain from being sheltered by the house with the wind blowing in one direction, but exposed to increased loading when the wind is blowing another way (e.g. wind funneling).  It is so hard to predict wind loading that for practical purposes it is simpler to just go overboard on strapping the roof structure to the walls (or support posts).

Thank you very much @Section62

With you now, makes perfect sense

Section62 said:

the_lunatic_is_in_my_head said:

Breather membrane is simple enough, the same thing you'd put on the roof of your house? Just stapled to the rafters with the cross battens and then corrugated metal straight on top? 

This site:

https://www.roofingmegastore.co.uk/roofing-categories/corrugated-sheets/metal/sheets.html

Seems to sell cut to length although I haven't looked at prices elsewhere to see how much extra is being paid for this option.

There also seems to be anti-condensation roofing, does it work? Although it seems the membrane would be cheaper.

Membrane is not the right material.  Unless you thermally insulate the underside of metal sheeting you will get condensation forming on the underside.  The amount of airflow and/or breathability makes no difference.  Anti-condensation felt is designed to absorb the condensation which inevitably forms so it doesn't drip, then allows it to evaporate during the day.

Do you have a link to an example by any chance please?

Google is throwing up lot of standard roofing membrane and a lot of results for the metal roofing with this already applied but not having much luck finding the right thing.