Flat roof into a lean to roof

[Dapper Dan Man]

Hi all,

I live in a mid terraced house which has an extension with a flat roof which I would like to convert to a lean to roof with two roof windows approximately 150mm by 100mm.

To my knowledge the max span of a lean to roof is 2.4 metres but what I want would exceed that by a metre or so. approximate dimensions of pitch would be would be 3500mm width 938 mm height and a span of 3620 mm making it a 15 degree slope.

I've got 3 decent (i think) quotes so far but none have address the issue of the max span as i hear it can cause sagging. I was considering doing it myself until i found out about the 2.4 metre issue.

The questions i have are:

should I be concerned about the builders not addressing the 2.4 metre issue?

is 2.4 in fact the maximum span for a lean to roof? if not is it something I could do with a span of 3620 mm and would that be done in a traditional way?

If i could do it myself what would be the suggested rafter sizes?

Any info would be greatley appreciated.

 

[theprinceofdarkness]

There is no limit to a single slope roof's span. The rafters just larger and you might need a purlin (or two). One place to start would be to use Google, "sheds" could be a place to start as you are building half a shed roof. By the way I think you have mixed up "mm" and cm".
Frank

 

[Dapper Dan Man]

Hi and Thanks Frank.

Yeap i've definitely got the units of measure mixed. I've done as much research as I can on a lean to roof and i'm ok with the method without purling. guess i got some more work to do!

cheers!

 

[noseall]

Hi and Thanks Frank.

Yeap i've definitely got the units of measure mixed.

No kidding.....

I would like to convert to a lean to roof with two roof windows approximately 150mm by 100mm

 

[tony1851]

As above, there is no 'maximum span of a lean-to roof', you just enlarge the rafters for a larger span.
Something like 7"x2" rafters would do for that span without a messy purlin.

 

[geraldthehamster]

Centimetres are the bane of our lives. One of the most useful things anyone ever taught me was to forget about centimetres. So many times at the end of a day cutting something short, because every tape measure I've ever bought has a big number 1 after 10mm, 2 after 20mm, etc., rather than a more sensible layout like this:

The pictures above are Australian. Can you actually get a tape marked in mm in the UK?

Richard

 

[tony1851]

One problem is that when the UK started to go metric, we adopted the 'meter-kilogram' system, rather than the 'centimeter-gram' system ason the continent (many continental building products - such as Velux - are always detailed in cm).

So as the continental market is larger than ours, perhaps we have to have what the majority has, including tapes??

 

[geraldthehamster]

It's a bit bl00dy stupid though, doing an unconscious mental conversion from cm into mm every time. I wonder what volume of timber gets wasted across the country.