A DIY & home improvement forum. DIYbanter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » DIYbanter forum » Do - it - Yourself » UK diy
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions.

How much harder is it to make a flat roof with a small slope?



 
 
LinkBack Thread Tools Display Modes
  #1  
Old July 20th 11, 07:49 PM posted to uk.d-i-y
GB
external usenet poster
 
Posts: 1,230
Default How much harder is it to make a flat roof with a small slope?

I see them all the time - flat roofs that are completely level, and water
pools on the roof just waiting to get into the building. Why don't they get
built with a slight slope, maybe 1-2cms per metre? If that were done, the
worst of the water would run off rather than pooling and the roof would dry
out quickly once the rain stops.

Of course, that's harder to build, but how much harder? Is the main issue
packing out the joists so the roof slopes, or is there more to it than that?



--
Murphy's ultimate law is that if something that could go wrong doesn't, it
turns out that it would have been better if it had gone wrong.


Ads
  #2  
Old July 20th 11, 07:51 PM posted to uk.d-i-y
external usenet poster
 
Posts: 138
Default How much harder is it to make a flat roof with a small slope?

GB wrote:
I see them all the time - flat roofs that are completely level, and
water pools on the roof just waiting to get into the building. Why
don't they get built with a slight slope, maybe 1-2cms per metre? If
that were done, the worst of the water would run off rather than
pooling and the roof would dry out quickly once the rain stops.

Of course, that's harder to build, but how much harder? Is the main
issue packing out the joists so the roof slopes, or is there more to
it than that?


Hmmm.

Is this a wind up?

'flat' rooves are not flat, they already have this slope built into them,
and no, it's not hard to do, it's a taper - timber merchants will cut any
sized taper you want, up to 6m in length - these are placed one upon each
joist to achieve the fall.


  #3  
Old July 20th 11, 07:54 PM posted to uk.d-i-y
external usenet poster
 
Posts: 17,439
Default How much harder is it to make a flat roof with a small slope?

GB wrote:
I see them all the time - flat roofs that are completely level, and water
pools on the roof just waiting to get into the building. Why don't they get
built with a slight slope, maybe 1-2cms per metre? If that were done, the
worst of the water would run off rather than pooling and the roof would dry
out quickly once the rain stops.

Of course, that's harder to build, but how much harder? Is the main issue
packing out the joists so the roof slopes, or is there more to it than that?



I suspect because like my carpenters, the principles of trigonometry
escape them. They insisted on laying out the rafters for the 57 degree
pitched roof on the ground, and not trusting to sins coses and tans...

Bill: ere, how come this sheet of ply don't fit?
Alf: I told you that it doesn't work if it ain't flat.
Bill: so why dont we make it a bit bigger?
Alf: oh YES, but how MUCH bigger eh? That's rocket science, that is.
Trigernomettry. Not carpentry.
  #4  
Old July 20th 11, 08:10 PM posted to uk.d-i-y
external usenet poster
 
Posts: 2,694
Default How much harder is it to make a flat roof with a small slope?

GB wrote:

I see them all the time - flat roofs that are completely level, and water
pools on the roof just waiting to get into the building. Why don't they get
built with a slight slope, maybe 1-2cms per metre? If that were done, the
worst of the water would run off rather than pooling and the roof would dry
out quickly once the rain stops.


Err they do ...

Of course, that's harder to build, but how much harder? Is the main issue
packing out the joists so the roof slopes, or is there more to it than that?


Search for "firring strips"

  #5  
Old July 21st 11, 09:48 AM posted to uk.d-i-y
external usenet poster
 
Posts: 706
Default How much harder is it to make a flat roof with a small slope?


"GB" wrote in message
...
I see them all the time - flat roofs that are completely level, and water
pools on the roof just waiting to get into the building. Why don't they get
built with a slight slope, maybe 1-2cms per metre? If that were done, the
worst of the water would run off rather than pooling and the roof would dry
out quickly once the rain stops.

Of course, that's harder to build, but how much harder? Is the main issue
packing out the joists so the roof slopes, or is there more to it than
that?



--
Murphy's ultimate law is that if something that could go wrong doesn't,
it turns out that it would have been better if it had gone wrong.



They usually are sloped. "Firring pieces" are attached to the roof joists to
achieve this. The roof drain may be blocked.
But flat roofs are bad news, you're right.


  #6  
Old July 21st 11, 09:54 AM posted to uk.d-i-y
external usenet poster
 
Posts: 650
Default How much harder is it to make a flat roof with a small slope?

The Natural Philosopher wrote:

I suspect because like my carpenters, the principles of trigonometry
escape them. They insisted on laying out the rafters for the 57 degree
pitched roof on the ground, and not trusting to sins coses and tans...


IMO, a physical check is so much easier than trying to get the maths
right. If I had a pound for every calculation where I try and be clever
with the maths, and fail, I could probably afford to emply an emmeritus
professor of trig!

--
Scott

Where are we going and why am I in this handbasket?
  #7  
Old July 21st 11, 10:08 AM posted to uk.d-i-y
jim
external usenet poster
 
Posts: 210
Default How much harder is it to make a flat roof with a small slope?

On Jul 20, 9:15*pm, John Rumm wrote:
On 20/07/2011 19:49, GB wrote:

I see them all the time - flat roofs that are completely level, and water
pools on the roof just waiting to get into the building. Why don't they get
built with a slight slope, maybe 1-2cms per metre? If that were done, the
worst of the water would run off rather than pooling and the roof would dry
out quickly once the rain stops.


There's 2 sorts of 'flat roof':

1. Completely flat

The roof sides, and the deck itself, are tanked ie completely
waterproofed. The resulting ponding is deliberate part of the design
used to control temperature of the roofing. There may even be an
arrangement to flood the roof in drought conditions.

There some like this in SE London which you can look down on from the
west side of the line into Charing Cross from Tonbridge.


Of course, that's harder to build, but how much harder? Is the main issue
packing out the joists so the roof slopes, or is there more to it than that?


The term "flat roof" really should me a misnomer in the sense that while
they should be flat, they should not be level!

The minimum fall on a roof should be about 1 in 80. Typically achieved
by placing firings (i.e. tapered timbers) on top of the roof joists.



2. Standard 'flat'

These have a slope of at least 1 in 80, but IIRC Ruberoid recommend
(or certainly did 20 years back!) at least 1 in 40.

Apart from having to resist snow, rain & tempest, a flat roof can
become extremely hot in sunny weather with consequent risk of
warping. Another factor is that a really heavy fall of snow really is
*heavy*! My guess is that 1 in 80 is the minimum required when the
roof is new to allow for resulting distortions and avoid ponding after
a few years of the worst our weather can throw at it.

A 1 in 40 (about 2.5deg) deck doesn't sound much, but shows a
pronounced tilt when viewed sideways on. That can be difficult to
make look 'level' if required for aesthetic reasons, for instance on a
double garage roof sloping to the side rather than front to back, when
viewed from the road.

HTH
  #8  
Old July 21st 11, 10:34 AM posted to uk.d-i-y
external usenet poster
 
Posts: 18,506
Default How much harder is it to make a flat roof with a small slope?

In article ,
harryagain wrote:
But flat roofs are bad news, you're right.


They are, of course, sometimes unavoidable. Properly done, they are not
such bad news as most say.

--
*Speak softly and carry a cellular phone *

Dave Plowman London SW
To e-mail, change noise into sound.
  #9  
Old July 21st 11, 10:45 AM posted to uk.d-i-y
external usenet poster
 
Posts: 9,254
Default How much harder is it to make a flat roof with a small slope?



"Dave Plowman (News)" wrote in message
...
In article ,
harryagain wrote:
But flat roofs are bad news, you're right.


They are, of course, sometimes unavoidable. Properly done, they are not
such bad news as most say.


They work OK when maintained.
They just don't last forever like tiles/slates do.
When they start to leak its often too late.
And they always start to leak when the weather makes repair difficult.

  #10  
Old July 21st 11, 10:50 AM posted to uk.d-i-y
GB
external usenet poster
 
Posts: 1,230
Default How much harder is it to make a flat roof with a small slope?

Dave Plowman (News) wrote:
In article ,
harryagain wrote:
But flat roofs are bad news, you're right.


They are, of course, sometimes unavoidable. Properly done, they are
not such bad news as most say.


Thanks all. The two I am thinking about are an extension roof done around 60
years ago and a garage roof done around 50 years ago. Maybe things were
different then, or maybe they were just done badly, but they both pond.



--
Murphy's ultimate law is that if something that could go wrong doesn't,
it turns out that it would have been better if it had gone wrong.


 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Best way to get a shallow slope on a flat roof David WE Roberts[_2_] UK diy 7 March 12th 10 11:42 PM
small flat roof leedsbob UK diy 3 November 14th 07 02:23 PM
Small flat roof Duncan Tuna Home Repair 5 December 1st 05 04:08 AM
Best stuff for sealing a small flat asphalt roof? Don Rojo UK diy 2 October 15th 04 09:19 PM
flat roof - slope for run off duncan UK diy 3 July 13th 03 02:55 PM


All times are GMT +1. The time now is 05:13 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.SEO by vBSEO ©2010, Crawlability, Inc.
Copyright 2004-2014 DIYbanter.
The comments are property of their posters.