Carnation Construction
Contact Dave
T-03-01-11 Page loading indicator

Home Site Map - Techniques - Structure -

Carnation Walling - Window Bucking

Star In the window openings form the in cavity and projecting window bucking with wood surrounded by EPS.

Fit window bucking

The bucking design

There is "projecting" bucking that the window will eventually attach to and forms the window sills, and there is "cavity" bucking that stops the concrete leaking out of the cavity during the concrete pour.

There should be EPS properly cut all around the window opening before you push the projecting bucking horizontally into the hole from the inside of the building.  (The EPS is cut to go all the way to the projecting bucking (minus 1/8" clearance all the way round).)  Having the EPS all the way round first avoids the wall splaying out at the top of the window.  You may need to trim the EPS to make the bucking fit, but hopefully the EPS sheets will have been cut and fitted reasonably accurately.

There is no great difference between the bucking for windows and the bucking for doors and even openings in internal concrete walls.  For some doors and all the internal openings there is no cavity bucking at the bottom and the protruding bucking is direct onto the slab.  (It is screwed to the nailed down kickers on the slab to stop it moving around.)  For holes in internal walls, the projecting bucking is a lot thinner as EPS is not used on internal walls.


Here is the design for the 12" cavity external wall case...

Window bucking plan view

Window Bucking And Bracing Side

And here is the 8" case...

Window Bucking 8in Plan 

Window Bucking 8in Elevation 

Note that the cavity bucking gets cut away a bit after the concrete pour.

Make Squaring Bucking

This is a rectangle of 2x4s with the same dimensions as the projecting bucking, ie on the outside it is 3" wider and taller than the window rough opening.  It is only used during the concrete pour.  It can be reused on windows of the same size on other levels of the house.  It goes into the cavity bucking on the inside of the wall and is used to hold everything square and to help take the weight of the wet concrete above.  It has a 45 degree triangulation piece of 2x4 as well as a vertical 2x4 in the center.  The vertical 2x4 is nearest the center of the wall.  A horizontal length of 2x4 is used to hold the sides out at the right dimension.

Squaring Bucking

The bottom piece of 2x4 needs to have notches corresponding to where the concrete fill holes are.  Start by finding the center.  Measure 5" either side of the center and that is the start of the rectangular holes on each side.  The notches are 10" wide and 1.5" deep.  In the center, 10" of wood is used for the internal bracing to push on.  The rectangular holes then repeat on 1'1" centers.

Squaring Bucking Plan View 

First put together the rectangle (adding PL-Premium glue to all joints), making it approximately square using an L shaped square or T square.

Before the glue dries, to get the rectangle accurately square, first determine the longest diagonal.  Put wood screws at the longest corners and attach some garden wire between them across the longest diagonal.  Put the screws on the external edge.  Slightly offset the screws from the corners so you can still measure the diagonal.  Twist the wire to pull in the longest corners until the length of both diagonals is the same.  When everything is square, add (and glue) the triangulation diagonal piece.

The squaring bucking is the same for both the 12" and 8" cavity cases.

Make Projecting Bucking

For the 12" cavity case, using 10-1/2" wide lumber (buy 2x12) for the top and bottom and 9" (buy 2x10) for the sides, construct a rectangle that forms the rough opening of the window.  The outside edge is flush (so it can take the window nail fin all the way round).  The top and bottom pieces overlap the side pieces.  If the window is an 8'x5' rough opening then the inside dimensions of the rectangle should be 8'x5'.  There is no need or desire for any diagonal bracing within the rectangle.

For the 8" cavity case, use 6-1/2" wide lumber (buy 2x8) for the top, bottom, and sides.

Try to use good straight lumber, but if the lumber is slightly warped then you will want to arrange for the bow to be inwards.  The padded 2x4 cross that will be added later will push out any bow.  If there is a slight bend in the other dimension then make the window bucking bow to the outside of the house (this is just for consistency).

This lumber rectangle will be permanently visible and will form part of the window sill inside the house so you want it to be unblemished.  It will be sanded later to clean off any concrete residue and any dried glue.

Glue (with PL-Premium) and screw.

You can use the squaring bucking as a guide to get it square while the glue is drying.

Apply several coats of clear wood protection to all sides of the wood rectangle to protect it from moisture.

Apply double sided tape where the nail fin will go and wrap the outer open face and the sides with 6mil polyethylene sheet .  This forms a temporary window to keep the rain out.  Keep the polyethylene sheets 6 inches longer than the concrete cavity width (1'6" in the case of a 12" cavity).  Do not tape the polyethylene sheet to the sides of the bucking.  Although not in practice necessary, the polyethylene sheet also separates the wood from the concrete which will keep the building inspector happy.

Double sided tape polyethylene onto bucking

To temporarily support the inner edge of the Projecting Bucking so it does not drop out of the EPS hole, screw on a piece of 2x4 to each side.  The 2x4 temporary legs need to be long enough to go down to the concrete slab.  The 2x4 temporary legs need to be positioned 3-1/4" from the nail fin outer edge.  This allows the baton to go flush against the EPS to set the correct insertion depth and also means there is still plenty of space to get the Cavity Bucking enough on that it can take over the job of supporting the bucking.

Fit Projecting Bucking

The Projecting Bucking is first fitted to the hole in the EPS.  Using two people, walk the bucking towards the EPS hole (helped by the temporary legs).  Shim the 2x4 temporary legs to adjust the height.

For the 12" cavity case, the Projecting Bucking is 10-1/2" wide at the top and bottom and 6-1/2" wide for the 8in cavity case.  For the 12" cavity case, 3-1/4" of it goes into the EPS and that leaves 7-1/4" (at the top and bottom) protruding into the concrete wall cavity, and at the sides it protrudes 5-3/4".  For the 8" cavity case it protrudes 3-1/4" top, bottom, and sides.

The polyethylene sheet should be flapping about in the concrete cavity.

Make Cavity Bucking

For the 12" cavity case, make a rectangle of Cavity Bucking using 10-1/2" wide lumber (buy 2x12) with 1-1/2" glued on to make it 12" wide.  The 1.5" extra bit is on the outside of the cavity (against the EPS).  If the cavity is only 8" then the Cavity Bucking is 8" wide (buy 2x10).

The Cavity Bucking is deliberately in both the width and the height made 1/8" larger than the rough opening+3" size so there is 1/16" tolerance clearance all the way round.

Try to use good straight lumber, but if the lumber is slightly warped then you want to arrange for the bow to be inwards.  It will be pushed outwards by the Squaring Bucking and Protruding Bucking.  If there is a slight bend in the other dimension then make the window bucking bow to the outside of the house (this is just for consistency).

Get the rectangle approximately square using an L shaped square or T square.

Glue (with PL-Premium) and screw.

Cut holes in the bottom of the Cavity Bucking.  The holes are 3.5" tall and 10" wide.  Measure 5" either side of the center and that is the start of the rectangular holes on each side.  In the center, 10" of wood is used for the internal bracing to push on.  The rectangular holes then repeat on 1'1" centers.  The holes start 1-1/4" from the edge nearest the center of the house (This needs to be right to match up with the edge of the Protruding Bucking above).  T shaped 2-by plugs will be used with props from the top to seal the holes once the concrete rises up the wall.

It is necessary to make it possible (after the concrete pour) to cut at 45 degrees the side edges of the windows on the inside.  This is done by using EPS to avoid concrete in the area that will be cut.  For the 12" cavity case, onto the sides glue with foam adhesive nearest to the inside of the house one 2" thick piece of EPS that is 6" wide.  On the outside of that, again nearest the inside of the house, glue on to the sides a piece of 1.5" wide 1.5" EPS.  For the 8" cavity case, there is just one piece of 2" thick EPS that is 3-3/4" wide.

Here is the 12" cavity case (for a 5' wide window)...

Bottom of Cavity Bucking 

And the 8" cavity case...

Bottom Cavity Bucking 8in 

Drill holes in the lumber rectangle for the concrete anchoring studding.  For the 12" cavity case, these holes are 3-1/2" in from the external edge.  For the 8" cavity case, they are 2-1/2" in from the external edge.  On the sides the studs are designed to be at the X'6" points (to avoid the wall ties).  That means they can be at 6", 1'6", 2'6" etc measured from the inside edge of the TOP lumber that forms the Cavity Bucking.  Make sure you measure from the top.  Put holes on 1' centers.  On the top and bottom put the holes 6" and 9" from the outside edge of the frame and then the double holes on 1' centers after that.  The actual holes chosen (6" or 9") will depend on the relative location of the wall ties that need to be avoided.

In the 1/2" holes will go 8" long threaded studs that protrude out into where the concrete will be.  They have a washer held by two nuts near the end which will provide the anchor in the concrete.  They will eventually be glued in place with PL-Premium adhesive after the bucking assembly is in place.  For the moment we just want the holes.  Put single sided tape over the holes to stop concrete runoff water seeping through.

Also 3-1/2" in from the external edge (2-1/2" for the 8" cavity case), drill 3/16" screw holes at the X' points (measured from the inside edge of the TOP lumber), ie half way between the stud holes.  These are used to screw from the Cavity Bucking into the inner Protruding Bucking.

Cavity bucking glued studding 

Have the vertical 2x4 batons ready with a marker to show the exact screw-on location.

Make a couple of lengths of 2x4s to temporarily prop support the Cavity bucking when it is being fitted over the Protruding Bucking.  You cannot attach the internal vertical 2x4 batons yet because you will not be able to get them over the nailed down kicker boards.

Fit Cavity Bucking

The Projecting Bucking is in its final location so it is ok to be screwed from the cavity bucking side.

Fit the Cavity Bucking rectangle over the in-place Projecting Bucking.  You will need a couple of people hold it and another person who is positioning the temporary props and is ready to screw on the vertical 2x4 batons.

An advantage of Carnation Walling over using ICF is that there is no need to lift the bucking assembly up to clear the inner ICF EPS.

It goes under the polyethylene sheet, ie the polyethylene goes on the edge of the concrete.  Even with the 1/8" clearance you may need to gradually work the sliding bucking into the cavity bucking (but make sure you do not damage the wall EPS).

Screws from the cavity space are used to hold it.  There will be no screw heads visible in the window surround (which is another advantage with Carnation Walling).

Use some double sided tape to fix the polyethylene sheet to the Cavity bucking so it does not flap around and get in the way.

Get the vertical 2x4 batons screwed on at the earliest opportunity.  When fitting you need a 3/4" spacer to take the place of the thin plywood and the thick plywood.  This is to hold the vertical 2x4 baton the right distance (3/4") from the edge.

Make and fit padded cross into Protruding Bucking

This is a vertical and horizontal cross of 2x4 with a thin covering of cloth at the ends that is used to take the weight of the wet concrete.  It stops the top sagging and prevents the sides from bowing in, both of which would be a problem when it later comes to fitting the window.  The overall size is the window rough opening vertical dimension.  It needs to be added from the inside to the Protruding Bucking because there is plastic on the outside of the Protruding Bucking.  It is just held in place by friction.  The cross can be reused for windows of the same size on other floors.

Fit Squaring Bucking

This just slides in from the inside.  Use 1/16" wood shims if needed to pad between it and the Cavity Bucking.

Screw on an edge for the plywood

All around the outside (horizontally and vertically) screw on pieces of on-the-flat 2x4 between where the vertical 2x4 batons are.  This is to prevent the plywood bowing out and spilling concrete.

True up everything

The bucking assembly needs to be exactly vertical in all dimensions.  Use a self leveling laser and/or a spirit level.  Fit appropriate wood spacers under the vertical 2x4 baton legs to get everything right. 

Make the wood spacers from some spare 2-by.  If needed, use a crowbar under the vertical batons to fit the shims.

Shims for bucking levelling 

Fill round bucking with foam

There will be a bit of a gap between the protruding bucking and the wall EPS.  Once the bucking is exactly square and vertical, fill the gap with foam gap filler.

Make hole plugs and props

The holes used to fill under the bucking with concrete need to have wooden plugs so concrete does not escape when the walls are filled higher.  The T shaped plugs (with bits that fit into the notches) are held in place by 2x4 props that go to the top of the Squaring Bucking.