BAY WATCH - The Disaster Movie
Independent glazing consultant Philip Rougier examines the topical and controversial subject of replacing timber bay windows with PVCu ones.
Glass and Glazing Products, August 1996 - "Tech Talk"
Copyright © Philip Rougier 1996
There are several ways to wreck a house. Dynamite is noisy and creates a lot of dust. So does swinging a big steel ball at it. How about digging a nice cellar and taking out all that nasty concrete beneath the walls? Or - here's an idea - let's whip out the old 1930s timber bay windows that used to support it and replace them with lovely new plastic ones, letting the front elevation collapse slowly and quietly. In the hands of some fitters, plastics windows can cause as much damage as plastic explosives. I'm tired of hearing the standard set of excuses. Here are some examples:
"We did it exactly as the system supplier specifies"
"Our plastic windows are structurally load-bearing"
"It must be subsidence"
I haven't the space to be genteel here. Rubbish.
WHAT USED TO HAPPEN?
For better or worse, I'm a Chartered Builder (until the next change, but that's another yarn) and have studied past building techniques. In the 1930s, builders used windows with solid Hemlock mullions at 2 ft centres, supporting the structure above. The bay spandrel, upper floor loading, perhaps even some main roof load, all transmitted to the little bit of brickwork at the bottom of the bay. At corners, they usually relied on the paired solid timber mullions to act as posts; sometimes, for good measure, they installed extra posts at bay points. At the bottom, the sills were integral with the frame's bottom rail and in direct contact with the brickwork. At the top, the head rail was in tight, direct, contact with the spandrel frame. There was no need for a deep lintel to support loads from a 2 ft span, so just a 4 inch (width) by 2 inch (height) wall plate was installed as the lowest part of the spandrel frame studwork and the mullions were axially loaded. This 'structural frames' solution is no longer available to us and many other things have changed in those 60-odd years. Horribly, some glazing firms think we still remain in the 1930s.
WHAT HAPPENS NOW?
Once the job is sold and signed, the measurement surveyor gets the frame sizes, determines clearances - maybe checks for cracks and other signs of movement in the building - then schedules for fabrication. The fitters arrive, prop up (sometimes), remove the old frames and install nice new plastics ones. Three weeks later the job creaks, and cracks start to appear. Loud phone calls, denials, arguments, terse letters follow. Then the side frames distort, the casements stick shut. Louder phone calls, longer letters. The main frame bows downward in the middle. Lawyers engage glazing consultants who take off some trim to see what's beneath and they report: "Yes, the frames are reinforced - even the outer jambs. Yes, there are 40mm OD bay poles at the points. Oh, yes they're through-fixed to jamb reinforcement. And great, the heads of the bay poles are in direct contact with the wall plate under the spandrel." So, what's wrong? Why is it failing? (as it most certainly is). Well, the heads of the bay poles were not always in such good contact with the wall plate beneath the spandrel. They are now, because the upper structure has moved down slightly and closed the gap at the top. At the bottom, the poles are sitting on the cellular plastic sill, trying to punch their way through, and it's distorted, not a lot, but enough. And sadly there's more - much, much more. In a typical bay, removing the two intermediate mullions trebles the effective span of the spandrel forcing its 4 x 2 inch wall plate to behave as a real lintel. It can't and it never could. "But our frames are fully reinforced" they cry. Sure they are, but the reinforcement doesn't go right into the corners; if it did, the plastic welds would break. Also, it's only designed for horizontal wind loads. I know of no PVC-U frames that can handle vertical loads, either on the head rail or axially down the jambs. When loaded from above, the unit distorts. Then, because they're 'replacement' units, there's just a little matter of the clearance gaps all round the frames.
SO HOW DOW WE FIX IT?
Unfortunately, it's a tear-out. Here's the sequence of events:
- Call the BPF (Tel: 0171 457 5000) and invest £50 in the finest technical information available on the subject ; Code of Practice (COP)3-A (Survey) and COP 3-B (Installation), prepared jointly by the GGF and the BPF's Windows Group. Also call the GGF (Tel: 0171 403 7177) for a copy of the Data Sheet 6.2.
- Calculate the required inertia for bay poles and lintels, and talk to a local steel/aluminium supplier about stock sections, or maybe to Cego Engineering (Crittall) Tel: 01376-583241 (Alex McShaffery) about their adjustable 'jacking' bay poles and spigot plates.
- Support the floor, spandrel and other loads above.
- Take out the frames.
- Jack up the spandrel a smidgen.
- Install the lintel (probably a steel angle if that fits the case) to accept and distribute the former mullion loads.
- Install a spigoted steel plate at the bottom making full contact with the brickwork.
- Cut or drill the sill to allow the poles to pass through to the brickwork.
- Install some proper bay poles (expanding or shimmed).
- Back go the frames!
A very large number of firms know all this and more, and any glazing firm with building expertise can do it blindfold. But there are many who don't understand the basic problem, let alone the solution. A whole gang of proprietors know little more than how to buy and sell things and keep accounts; they engage untrained 'fitters' off the street, who commit the same simple technical boobs time and again.
Glass and Glazing Products, August 1996 - "Tech Talk"
Copyright © Philip Rougier 1996