Standard practice is to specify PVCu windows 10mm smaller than the
opening, i.e. 5mm off each side. This seems to suggest that the window
will float 5mm off the bottom of the opening. You end up with a space
at the top that is rather too large for a bead of mastic. Surely it is
better to specify 5mm off the vertical direction. This will still be
more than enough for expansion of the plastic for most moderate sized
Or is there a flaw in my argument ?
Only thing I can think is to bed the sill in silicone, but I've never
seen anyone do this.
First use wedges to set the frame level.
It's then usual to fill the 5 mm gap with expanding foam - which as well
as filling the gap glues the frame in place. Remove the wedges after it's
set and fill those with foam too. Trim off the excess foam round the
outside and recess it by a few mm and use silicone to provide a neat
weatherproof seal. Don't leave any foam exposed as it deteriorates quickly
in sunlight. That's pretty well what the instructions were for the UPVC
windows I bought from Screwfix - and they've been fine.
If the brick opening is a bit untidy, shutter with wood strip and run a
cement fillet round it to improve the look.
*He who dies with the most toys is, nonetheless, dead.
Dave Plowman email@example.com London SW
Dave Plowman (News) coughed up some electrons that declared:
I concur having recently fitted two windows.
The 5mm gap usually ends up compensating for irregularities in the wall
anyway - and, even if using screws, filling 5-10mm with foam really firms
it up (I still believe in big hairy screws - but apparently some window
fitters just foam them in nowadays).
Of course, if one is absolutely sure of the opening and it is desireable to
reduce the gap, no reason why not - but do allow enough gap to get some
sealant in, even if not using foam.
I must admit that I used to measure these things as part of making a living,
and the teams that fitted those windows to be honest would be laughing at
the method used above.
Don't get me wrong here, foam and wedged have their uses, but not in the way
that has been described here - it's too slow when doing a lot of houses.
A 5mm gap all-round is about right, and very useful if the brickwork under
the sill is a bit askew - plastic packers would be very useful here!
Personally, I think that any gap under 5mm is next to useless when using
silicone mastic (particularly when used as a weather seal between frame and
wall) as the stuff is designed to be flexible when 'set' and the thinner the
bead, the less it will 'move' with the expansion and contraction of the
The only time very thin beads of mastic should be used is when they are used
as a decorative finish on the edges of any cover strips.
As for using expanding foam - in my situation, this was only ever used to
fill problematical gaps that would occur in stone work or No-Fine 
constructions or when the removal of box frames were removed and
difficulties occured in fixing the UPVc (but that was often overcome by
using patent fittings and add-on strips) - on normal brickwork, foam was
 No-Fine: A method of construction developed after WW2 where
external house walls were build using cement and Chipping's only which left
a large number of voids in the wall - *NO**SAND* was used.
This method of construction causes huge problems when renewing external
joinery because there is nothing really to drill because of the voids - a
damned nuisance to put it mildly.
The team that fitted mine must have been laughing a lot when they did
Gaps of 10mm or more each side of the frames, no filling, foam or
otherwise, they just stuck some plastic strip on with white silicon
sealant to make it look neat.
Come the first winter and a few gales the drafts were worse than ever.
On close inspection I found the 10mm gaps and totally naff workmanship.
Went out and bought some expanding foam. filled all the gaps. No more
drafts and the windows felt a lot firmer too. Had the house re-rendered
shortly afterwards and that tidied up the edges.
I do take a rather perverse delight in winding up their "sales team"
whenever I meet them at the likes of Focus when they have their displays
and are trying to drum up trade. I let them give me their pitch and
then I tell them my sorry tale and get their sympathy about dodgy
installers, until I tell them who it was. Normal reaction is "Why
didn't I go back to them?" What and let them bodge it again, no thanks.
Apart from the bay window I feel now that I would rather have done it
myself, next time eh?
I'd have to admit to finding one length very useful:
to cover up that monster gap created by the steeply sloping sill tiles.
Couldn't think of a better way - but I did get a reasonable finish on the
sides and top with foam then some frame sealant in brown - which is bloody
hard to apply neatly next to those wibbly bricks :)
Nice job Tim S, and don't be ashamed to say that you've used the stuff,
that's why they make it - you wouldn't think twice about sticking architrave
around a door to hide that joint twixt wall and frame. Would you?
And I've seen even worse "bodges" by some of the DIY brigade Dave - but
there again, bodges are practised by all trades and professions,
Not interested Dave, as far as I am concerned I merely expressed an opinion
from experience (as you do when you post to various groups) - but as for
your "Where the idea is to do a good job...) I have seen some almighty
cock-ups when the unintelligent DIYer gets into full swing (and I don't
confine that to DIY plastic window installations).
By the way, I have never used Screwfix for buying in plastic windows, during
my involvement with the bloody things, each window was bespoke for its
opening - and we are talking about streets of houses rather than the odd one
or two here and there in a single house.
Not necessarily - if the window opening is correctly examined and measured,
then that "dodgy sill" will be taken into consideration and the window made
and fitted accordingly - the window opening should be cut only as a last
resort with minor adjustments being 'cut' off the flanges of the window
frame (for which the UPVc extrusion is actually designed for. And if you
don't believe that, ring up VEKA and ask their technical people the
Then I would suggest that your "real" experience in general building is
rather limited - especially when working in an environment where damage
limitation to internal decoration is a priority.
But you were effectively saying the Screwfix way was poor and only your
'pro' method worth considering.
Screwfix only supply certain common sizes. If you need bespoke UPC ones
made it's likely they're not the right choice for the job. I absolutely
hate them on Victorian etc properties. The two I've fitted are at the back
of the house where I wanted much larger windows than original. I certainly
wouldn't replace the front sash windows with them - and there are plenty
of examples in this street which show why.
Not really interested in what one maker says. Truing up the opening before
fitting makes for a far neater job. Of course not cost effective for pro
But it's not 'damage limitation'. It's covering up damage which has
already been done. And often totally inappropriate to the room.
Dave Plowman firstname.lastname@example.org London SW
To e-mail, change noise into sound.
Nah! Just plain ordinary oil based undercoat - but the bloody rain held off
long enough for me to at least give the first coat - and that's enough to
make me happy today! Only three more coats to go!
sm_jamieson coughed up some electrons that declared:
Not intending to teach granny re egg suckage - but *if* you've not done uPVC
windows before, I found this most helpful - you may too:
Especially with respect to setting the glazing in correctly...
I've only done pre-glazed ones where fixing brackets are used rather
the frame. Also, the brickwork was very level.
That link is interesting, mentioned cutting the sill for horns. I've
noticed a lot of windows
fitted where the horns are not bothered with. I guess with cavity
walls, the slight extra
waterproofing they give are not really worth it and it takes longer.
The end caps for
PVCu cover the whole end of the profile, although of course they can
Don't horns originate from when the window sill was built into the
In that case, make sure they give you lots of "bridge packers" (I use
typically 4 per pane - methods differ, you may need more per pane for
larger panes) and *loads* of glazing packers in all sizes from 1mm to as
thick as possible (1mm increments).
Both of those are a bastard to buy in small quantities (often in boxes of
1000) and differ a lot from the more common "frame packers" which you can
get more easily (these are of course also useful for setting the frame).
Must admit I hadn't paid much attention - I have tile sills and consequently
bought windows without sills.
Not sure, beyond that link I posted.
Frame packers - use as you'd imagine, around or next to the screws to level
the frame and allow the fixing to be pulled tight without loading the frame
Bridge packers: You want the width that suits the DG panel. Length is about
5-7cm. Thickness - usually one thickness, about 6-8mm IIRC. The important
thing with these is that they are a plastic strip with 2 ridges along the
length. These should be used, one per packing position between the glass
and the inside of the frame. The bridge packer should be against the frame
with the ridges to the frame and the flat side to the glass. This allows
free drainage of any water and condensation.
Glazing packers: Simple plastic shims from 1mm to many mm in 1mm increments.
Shims are similar in length and width to the bridge packers and you use as
many and of whatever thickness between the bridge packer and the glass to
get the glass where you want it.
The glass should be wedged in firmly before you bang the beads in and
consequently should not move (much) when beading.
The packers also allow the glass to add rigidity to the frame, helping to
keep it square (assuming you pack the top and sides too). I didn't have
this issue - my windows were small and contained *much* steel - but I
suspect it would be more important on larger windows or ones of cheaper
quality with less steel.
Sadly I don;t have any pictures from when I did mine...
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