mal wrote:
A ground floor window frame of my house has a row of bricks across the top of it
as a kind of lintel. The bricks are arranged vertically oriented and placed side
by side across the top of the frame. On top of the bricks is a row of tiles that
stick out a bit to form a narrow shelf. The house was built in the 50's.

Afai can see, those bricks *are* the lintel. If that is the case, is the
(wooden) window frame taking all the load of the wall above it? I'm thinking of
replacing the frame with uPvc double glazing and am concerned about plastic
being able to take that kind of load.

I'd say it's likely that the wooden frame is load bearing. But assuming
you have a cavity wall, the 1st floor joists are likely to be bearing on
the inner leaf of the wall, and there may well actually be a lintel on
the inner leaf. That would limit the load on the window frame, and if
you were to remove it unsupported, the risk is that an area of bricks
forming an isoceles triangle from the corners above the window opening
(IYSWIM) could fall; but not the entire house. NB - lots of assumptions
here; I'm not a structural engineer; just my personal experience.

If the above is correct, you'd need to support the external wall with
props while a lintel is fitted (probably an L-section steel one) to the
external wall, then you can fit the upvc window. Old timber windows
were made pretty strong, a lot more so than modern upvc equivalents.

I believe there are uPVC windows with built-in steels which may help?
Don't know any more than that.

Is there perhaps a real lintel behind those bricks? Thanks for some advice on
this as I'm not too keen on the wall of my house falling down if I get it wrong!

Might be, might well not be. Can be very hard to tell. Is there any
evidence of a thin steel plate between the top of the window and the
first bricks? (Might not be visible even if it's present).

Have any of your neighbours houses of the same age and design had their
windows replaced? Might be a source of info?

You can be sure that if you get a DG company in, they will tell you
whatever you have now is fine; they aren't geared up to fitting lintels
and will slap in a new window and run. Later on you'll likely get
sagging brickwork, diagonal cracks in the mortar above the windows,
windows which warp and jam shut.

hth
David