Convert flat roof to balcony

[Glasgow]

First and foremost I'd like to find out if a flat roof we have easy access to is suitable for bearing the weight of a few humans, table chairs etc. Can I establish that if I know the spec for the timbers supporting it - e.g. width & height of joists and gaps between them?

Secondly, if it is load bearing I'd like to cover the existing bitumen with something more attractive that's not too expensive. Any suggestions?

Thanks in advance

 

[Newboy]

1) It's very unlikely that the support structure for your flat roof will be able to carry anything more than a roof & some snow.

2) If I recall correctly, you will need planning permission

3) You can't plan on the basis of a "few humans and ...." Any design needs to assume that as many people could possibly fit on the balcony will be on the balcony, dancing, jumping up and down and carrying a backpack full of bricks

 

[freddiemercurystwin]

Planning Permission aside, it's highly unlikely that the roof structure would meet the requirements of a floor, also fitting a compliant handrail to the perimeter is very difficult, covering the existing roof finish with something suitable is the easy part. Whilst you may not agree, balconys have to be designed on the assumption that a bunch of drunk students will jump around on it and will rock the handrail. But what are the size of the joists and what centres are they at and what is the span?

 

[Glasgow]

Thanks for replies. Understood re students, bricks etc. I will check out the timbers and report back. The roof is already surrounded by railings.

 

[Glasgow]

The timbers are described as "50 x 50mm softwood cross battens at 400mm centres on 50 x 150mm flat roof joists over bathroom at 400mm centres and 50 x 200mm over bedroom at 400mm centres with two rows of herring bone strutting."

 

[OwainDIYer]

There are lots of variables. Basically you need to achieve a loading capacity of 1.5 kN per sq metre, the same as required for house floor joists

Achieveing this depends on the existing dead load on the roof, ie how it is built up. If it's a heavy construction then the dead load on the roof may be reaching the capacity of the existing joists. If it's a light construction of chipboard and felt then you may have adequate joist strength (but the chipboard might not take the point load).

Also depends on the strength of joists. If the joists are stamped with a BS timber strength class then that can be taken into account; otherwise it'll be assumed to be nominal C16 which is the lowest class.

Finally the span is not mentioned, for 150 x 50 joists typically the max span would be around 3 metres, so a 2 metre span and you'd probably be OK. A 4 metre span and you woudln't.

Really you need a dimensioned sketch plan of the walls and a cross section of the roof indicating materials and make up and get a structural engineer to cast an eye over it. If you have the original plans and calcs for the extension even better.

Typically it might cost £500 - 1000 for a Structural Engineer to produce calcs and verify for Building Control purposes.

 

[Glasgow]

Thanks very much for the further info. In case it helps at all...

The dead load is apparently made up of 225mm x 225mm asbestos tiles bedded in bitumen, 3 layers of felt to BS747 (constructed circa 1990) and 19mm exterior grade ply. There is also mention of 50 x 50mm softwood battens at 400mm centres on firrings. The bathroom (150mm x 50mm joists) is about 2m x 3m.

Unfortunately I don't know the joist timber strength. Presumably the best way to establish this would be to remove a chunk of ceiling from below which I don't fancy a lot!

I got all the info from what I believe to be the original plans.

Given the pre-existing railings and info I heard from neighbours, the previous owners who built the extension which the flat roof protects were clearly planning on using the roof as a balcony but I accept that proves nothing. The original plans, however, do show the railings but they also show a staircase from the original house which does not exist!

 

[OwainDIYer]

I got all the info from what I believe to be the original plans.
Given the pre-existing railings and info I heard from neighbours, the previous owners who built the extension which the flat roof protects were clearly planning on using the roof as a balcony

And if they advised the designer of that intention the roof should have been designed to meet that loading requirement.

The engineer I spoke to before replying says he'd have recommended to allow for domestic loading if he'd been designing the original job.

 

[Glasgow]

Thanks. I guess it's only a 7-8ft drop if the engineer was not as professional as the one you consulted. So we should survive the collapse

 

[OwainDIYer]

Thanks. I guess it's only a 7-8ft drop if the engineer was not as professional as the one you consulted. So we should survive the collapse

Whoever's on the balcony might survive.

Anyone in the kitchen at the time might not, however.

 

[Glasgow]

That's a fair point but it's only really guests that stay in the extension so it will save us cooking breakfast for them.