In uk.d-i-y, David Hearn wrote:
The guttering of our house is about 5.5m above the ground. I want a ladder
which will reach that height safely. I've no plans on doing any serious
work up there

[ ... ]

What is the recommended angle for ladders? If I need to reach 5.5m vertical
and the ladder is at the recommended angle - what length (extended) ladder
do I require? I've been looking at 7m+ ones - though I'm wondering whether
6.5m will be sufficient.

I'm pretty sure the suggested ratio is 1:4; that's what a moment's Google
with the words "ladder angle safe" produces, too. The less helpful sites
say "75 degrees"; the publications by total morons describe the "correct
angle" as 75.5 degrees as a result of computing acos(0.25), unbelievably!
As if safety is helped by working to tenths of a degree, and as if the
guideline of 1:4 isn't itself a generally-sensible-rule-of-thumb rather
than holy writ. Egads... what kind of semi-numerate non-thinking moron
is it who gets a job writing Safety Leaflets and Safe Working Specifications?

Thus, for a vertical reach of 5.5m, at the 1:4 ratio, Mr Pythagoras says
you'll want a hypotenuse of at least sqrt( 5.5^2 + 1.4^2 ) = 5.7m
(minimally longer than the long side, since at these small angles,
sin(x) is only just a little less than x).

However, the ladder specs may be a little optimistic about the full
extension: to stop it bouncing alarmingly when you reach the middle in
your tapdance up the rungs, you may want a bit more overlap than the
2-3 rungs which full extension is quoted at. So a 6.5m ladder would have
just 0.8m of overlap if two-section; you might prefer the greater rigidity
of the 7m flavour, especially if your physique is closer to "cuddly" than
"boney" ;-)

Incidentally, any good places to purchase a standoff unit for the ladder?

I'm pretty sure the one I have is an Abru (but it's peeing down outside
and I'm certainly not going to go out to the garage in *that*) on a non-Abru
ladder; seem to remember paying about 25quid for it at Focus. *Well* worth
having, IMHO: makes the process of clearing out the gutters or reattaching
downpipes need a lot less contortion, and therefore genuinely usefully
safer (rather than obsessing about the last half a degree of the ladder
angle ;-) I'd also encourage you to stabilise the bottom of the ladder:
sandbags, stakes in softer ground, a rope secured between the base and
something firm at or around the base of the wall, or a charming and
talented assistant to prop his or her size 9s at the foot of the ladder
all reduce the scope for an embarassing and injurious topple. (Time for
the other bit of 0-level maths: how fast will you hit the ground if you
do fall off? Let's say your CofG is 5m above the ground, and let g be
10m per sec per sec; v-squared = 2gs = 100, so you'll be falling at
sqrt(100) = 10m/s which is near as dammit 20mph. Quite fast enough to hurt
and break bones, worse if you're unlucky. So don't fall!)

HTH - Stefek