Hi All

Currently in the process of fitting aluminium windows to a timberframe self-build. I'm using various butyl-type sealing tapes to waterproof the cill plate and surrounds (as per manufactures details - pro clima and ampacol). However, I can't find any information as to whether to fix the cill to the cill plate. For the smaller windows, jamb fixings have been more than sufficient, however I'm wondering whether simply resting the cill on packers and cill plate is sufficient for larger (longer than 1600mm) windows as recommended by manufacturers. Although the butyl tape should in theory self seal around any screw penetration, it seems a shame to puncture an otherwise waterproof layer.

If anybody has any experience, it would be appreciated, thanks.

What windows did you use? Window sills are generally fitted to the bottom of the window. They'd either have a precast sill that the windows sit on top of, or a section underneath recessed, where the window sill is fixed to. Every system is different in how they do things but, usually that is the standard method. Do you have a picture to show what it is you are trying to do?

Edited 22 hours ago22 hr by craig

Hi Craig. Thanks for your reply. We chose a product from a local supplier which I believe is manufactured by Smart Systems/Architectural Aluminium. I've fixed the supplied 225mm cills to the frames and have installed the first window sitting on top of the cill plate (which has been waterproofed using tape as per the Back to Earth guidance - image attached).

I'm happy that the smaller windows will be fine simply sitting on the wooden cill plate, but was curious as to whether bigger windows need to have the cill mechanical fixed to whatever substrate it is sitting on.

For the bigger openings I'd give the cill some mechanical restraint, but you don't need to go through the tape on the flat to do it.

The packers do the real job of carrying the weight, so make sure there's a solid packer under each mullion and at each end, roughly 150mm in from the jambs, and the frame's own jamb and head fixings stop it moving. The problem on a 1600mm plus cill is more the front edge flexing or getting knocked, which can break the seal at the back upstand and the end caps over time.

What I've seen work well is small aluminium or stainless angle brackets (or the manufacturer's cill clips if they do them) fixed to the face of the cill plate or the frame below the tape line, with the cill sitting on or clipped to them. That keeps every fixing out of the horizontal drainage plane. If you must screw down through the plate, keep the fixings to the back third, under the window frame rather than out at the drip, and put a dab of butyl or a sealing washer on each one even though the tape should self seal.

Also worth checking the cill has a slight fall outwards once packed and the end caps are sealed to the reveal tape, as that's where most leaks start.

2 hours ago, JpManning said:

For the bigger openings I'd give the cill some mechanical restraint, but you don't need to go through the tape on the flat to do it.

The packers do the real job of carrying the weight, so make sure there's a solid packer under each mullion and at each end, roughly 150mm in from the jambs, and the frame's own jamb and head fixings stop it moving. The problem on a 1600mm plus cill is more the front edge flexing or getting knocked, which can break the seal at the back upstand and the end caps over time.

What I've seen work well is small aluminium or stainless angle brackets (or the manufacturer's cill clips if they do them) fixed to the face of the cill plate or the frame below the tape line, with the cill sitting on or clipped to them. That keeps every fixing out of the horizontal drainage plane. If you must screw down through the plate, keep the fixings to the back third, under the window frame rather than out at the drip, and put a dab of butyl or a sealing washer on each one even though the tape should self seal.

Also worth checking the cill has a slight fall outwards once packed and the end caps are sealed to the reveal tape, as that's where most leaks start.

That's really useful to know - thanks for taking the time to explain the detail. I can use this for the larger, 2400mm, windows with some peace of mind.

No problem at all, glad it helped. On a 2400mm cill I'd add an extra packer and a cill clip or bracket roughly every 600mm along the length rather than just at the ends and mullions, as the long ones can bow slightly in the middle once a bit of weight goes on. Also worth leaving a small gap at each end for expansion before the end caps go on, as aluminium moves more than you'd think over that length in summer sun. Good luck with the install.