Home Site Map - Steps - Basement Walls and Ceiling -
ICF, Rebar, Bucking, Bracing
Order and get ICF blocks (Basement walls including internal)
See the page here that describes suitable ICF blocks to buy.
Buy enough for the basement walls including the internal walls and the portico bases.
The first ICF course
Loosely fit first two courses of ICF bricks
The reason to temporarily build up two layers of ICF blocks is that they interlock together to form a solid integral rectangle. This allows you to check the diagonal dimensions of the ICF wall rectangle are the same, ie it has accurate right angles. It also lets you find out if you need to do any shimming of the bottom of the first course of bricks to get everything level.
Pick a corner as a starting point. Note that there are left handed and right handed corners. You need to use an equal number of left handed and right handed corners on each course. For Greenblock ICF, the tail ends of the corner blocks are 1'4" and 2'4" on the outside dimensions.
Always cut on a cut line. The cut lines are 1" apart. This ensures that the Lego turrets will line up. When planning which blocks to cut, try to avoid having short lengths of block. At a minimum you should have 2 webs in a block, but it is better to have more.
At the join you will need to cut the ridge off on the uncut block that butts up to the cut block.
The blocks on the course above should be offset by at least 2 webs (1 foot). This is typically achieved by using the opposite corner type on the course above.
The cut blocks should be half way along the wall. It will cause a discontinuity in the webbing so the baton position and the drywall fixing will have a pattern discontinuity in the middle of a wall section. Try to keep the discontinuity to a minimum. You will need extra bracing around the cut area.
For the moment don't worry about any door openings. Even if you don't need a second course of ICF in the door area, add a second course anyway in order to hold the wall rectangle together. Knowing where a door is may however influence how you choose which of the corner types is used in that area.
Check wall rectangle squareness
Once you have a solid ICF rectangle you can check the walls are straight and the corners are accurately 90 degrees.
The outer face of the ICF is the reference and this needs to be as per your house CAD drawings. In the case of my evaluation building it is a simple rectangle of 15' x 11', but the house has bump outs for the porticos.
Check the straightness using a laser along the wall. Use a tape measure to check that the laser line is the same distance from the face of the wall all the way along.
Check the right angles by measuring the diagonals across the rectangle. The diagonals need to be the same.
Hopefully the wall rectangle (center of the ICF cavity) will correspond with where the key notch has been formed in the slab.
Once everything is adjusted to be exactly correct, use a marker pen on the concrete slab to mark the outside face of the ICF. This is an important reference line. It is also helpful to mark it on the inside too, even though this is not the official reference.
Laser level top of first course
Setup a self leveling laser on the center of the slab on a small platform to produce a horizontal beam that is 1 foot (the height of an ICF block) above the slab height.
Find the ICF block which is highest (greatest distance from the laser line to the top of the block). This block will be the reference. Use padding shims (polystyrene, plastic and/or paper) to raise the laser so that it shines on the very top of the reference block (the base of the turrets). Other ICF blocks will be shimmed up under them to get them to that same height.
The shims are made from slithers of EPS cut from ICF block offcuts.
First just put a shim at both ends of any block that needs it to bring its top up to the laser line. Do this all the way round on the inside of the building until all the blocks are up to the laser line.
Then use a spirit level on the outside face of each block to determine if it is exactly vertical. If it is not then add whatever shims are necessary under the outside EPS of the block at the block ends to get it vertical. Do this process all around the outside of the building.
Check that on the inside the top of the blocks are still exactly on the laser line.
Once you are happy, go round filling in the gaps between the concrete and the bottom of the blocks using the appropriate thickness shims in the same alignment as the wall EPS. Do this on the outside and on the inside. The net result is as if the ICF block has EPS walls that were slightly longer than 1 foot to have the EPS reach all the way down to the concrete. There should be no gaps in the extended EPS walls of the ICF block.
Gluing ICF EPS
Throughout subsequent steps it is necessary to glue ICF EPS and other bits of polystyrene. The best gluing method is to use foam from a foam gun.
Details are here .
Use cleaner on the foam gun after you finish for the day.
(Note that the pictures below show a different "light blue" foam adhesive, but using the foam adhesive is a much better choice.)
Glue down the shims
A few blocks at a time, remove the ICF blocks that were temporarily put in place. Be careful not to disturb the shim pieces of polystyrene. Make sure it is not a windy day. Put a dot on each shim with a corresponding dot on the slab concrete as this will help get it the right way up and in the right place. Glue each shim down to the concrete slab using foam adhesive .
Glue down first course of ICF bricks
The first course of ICF blocks are glued down to the concrete and the shims (using foam adhesive) so they cannot move. The blocks will have previously been cut as needed. Accurately positioning them using the marker pen lines on the concrete slab. It is worth using the laser level again as you place each block to make sure everything is still accurately level.
As well as gluing each ICF block to the floor, also use foam adhesive between the blocks to glue them to each other. At the cut joins use plenty of foam adhesive. To hold the blocks together while the glue to the floor is drying, temporarily add the second course of ICF blocks.
Nail 2x4s to concrete slab on inside edge
On the concrete it is necessary to nail some 2x4s flat all around the inside of the ICF rectangle. It is actually positioned with a 1.5" space to the outside edge of the ICF. The main reason for the gap is so that there is something to hook the vertical bracing into. Spacers of 2x4 are used to fill that gap where there is no vertical bracing. They will also help you get the nailed down long lengths of 2x4 in the right place. Typically the spacer bits of 2x4 are 1'8-7/16" long (but short offcuts will work fine for the moment). The spacer pieces will fit between the vertical bracing pieces that are every 24". Near the window openings, different length spacer pieces will be needed to place vertical bracing either side of the window opening. The spacer pieces do not need to be nailed down. The spacers will extend 3.5" up the wall, ie will stand on edge.
As the ICF blocks are already glued in place, use the spacers to position the long lengths of 2x4. Attach the long bits of 2x4 (not the short spacer pieces) using a concrete nailer with 2.5" nails. You will need the most powerful caps in the gun to penetrate the concrete.
It is not necessary to nail 2x4s around the outside face of the ICF
rectangle because the majority of the bracing is on the inside of
the wall and the outside bracing verticals are screwed to the ICF
web that is held by the inside bracing.
Install rebar in the first course
Put 1" lengths of 1.5" PVC pipe over the vertical rebar from the slab that will be used to constrain the long vertical rebar. Glue them to the concrete using PL Premium adhesive and make them all face outwards from the building so you know where to aim for when fitting the vertical rebar later.
Install the horizontal rebar in the first course of blocks. Use two runs of rebar at the bottom of the block (side by side, one on the inside, and one on the outside) on the first course. Feed the rebar though the center of the webbing and cable tie it to the top of the bottom web. One of the functions of the rebar either side is to constrain the ends of the long vertical rebar that will later be added to the walls (just in case it fails to end up constrained by the pipe collars).
Also in the first course of ICF blocks, cradled by the top web (and cable tied to it), put two more runs or horizontal rebar, one in the center and one on the inside.
Cable tie the vertical rebar from the footing to the center horizontal rebar to help resist upward movement of the blocks.
At the corners you need 90 degree bent pieces of rebar. Put these in the center top cradle (cable tied in place).
Install other ICF bricks with rebar and bucking
Install second course of ICF bricks
Add the second course. The blocks will have previously been cut as needed. Use foam adhesive between each brick, both vertically and horizontally. Use a small amount of foam adhesive in lots of places (even though this is a bit time consuming to do). The reason to glue is to avoid the danger of high winds blowing down the wall. Use plenty of foam adhesive at the cut joins.
Add two runs of horizontal rebar, one in the center and one on the inside, because the second course will also be below grade.
Cable tie the horizontal rebar (in the top webbing cradle) to the vertical rebar as this will help stop the second course from moving upwards off the first course.
As well as the exterior walls, do not forget the internal ICF walls and the portico bases with their internal walls. It's necessary to improvise T joins in the ICF by cutting out a bit of the ICF polystyrene and then gluing with foam adhesive .
Mark window and door positions
This is best done with a marker pen on the inside face of the ICF EPS. The marks should be in the 1" spaced cut lines on the ICF. The marks will be a guide when placing the upper layers of ICF bricks.
Installing remaining ICF blocks
This step needs to be done in parallel with the adding of the rebar (described in the step after) and with adding the window and door bucking.
As well as the basement walls, this includes the internal ICF walls and portico bases.
Where there are window and door openings, cut the right horizontal length to make both sides of the opening (and don't brick up the opening).
Use cable ties horizontally between the ICF blocks (attached to the webbing) to hold blocks together under window openings.
Put the door and window bucks in place at the earliest opportunity, which in practice means as soon as you get to sill height (see the description of the bucking). Typically you will need to cut the blocks under the window. Often this will mean cutting 12" blocks down to 9" in height, but other heights are required under doors and where there is more than one window in a bucking assembly. Note that it is the top of the block (with the turrets) that is removed. Given that there is plastic webbing, you will need to use a reciprocating saw to do the cutting. Typically (eg for a 9" high block), the top webbing will also be removed (using the reciprocating saw). That also nicely on a 9" block avoids having to cut the EPS surround on the window bucking as there is more than 3.5" of clearance.
Glue the blocks together using foam adhesive . This will prevent the polystyrene walls from being blown apart in high winds during construction, and it also makes them more able to resist the wet concrete force. Use small blobs of foam adhesive in lots of places (even though it is time consuming).
Cutting of the length of bricks is necessary to produce the staggered brick pattern although the alternating corner pieces produce a useful offset. As long as the brick offset is 1 foot or more then that's fine. Cutting is of course required in the more complex parts. Around window openings you may need to violate this brick overlap guidance.
When gluing blocks in a particular course it is typically useful to temporarily add bricks on the layer above in order to hold the bricks in the right position while the foam adhesive dries.
For short lengths of ICF wall between windows it is useful to use a variant of ICF block that has the webbing separate from the polystyrene. This avoids having to work the blocks down from the top of the window (the top of the window may well have a tarpaulin over it).
Use any damaged blocks at the top of the wall where there is less concrete outward force.
Note that the portico bases (safe room and under the south portico) are included in the basement phase.
Install Rebar in ICF
This needs to happen in parallel with building up the ICF blocks.
Install the rebar as per the structural engineering drawings, ie as per what the structural engineer has designed (after you gave them the house drawings). The structural engineering drawings show the minimum rebar that is required, but while building the walls it is good to look for opportunities to add additional rebar.
The HUD prescriptive guidelines call for a vertical piece of rebar within 6" of each side of openings. It also requires vertical rebar on all the corners. HUD requires 24" centers for vertical rebar, but that's for steel and that's not for a D1 seismic area. Horizontally HUD allows a wide spacing, ie top, bottom and then at third wall heights, but again this is not for D1 seismic areas. HUD requires 90 degree bent rebar (4' bent in the center separate rebar pieces is ok) at the top of the vertical rebar at the top of the top story in the plane of the wall. The bent over pieces need to be within 4" of the top of the wall. The bend pieces seem a bit unnecessary, but that's what HUD says. Per HUD in D1 areas, either side of a window or door opening, you need to provide two #4 vertical rebars within 12" of each side of the opening. It would be good to put one say 2 inches away and another say 9 inches away. The fact that we're using polypropylene strands in the concrete should theoretically slightly reduce the amount of rebar needed, but in practice it is still best to overkill.
The detail of the rebar placement needs to be worked out by a structural engineer that is licensed in your area. Here's what I expect a structural engineer to specify: Use 8mm rock rebar (or half inch fiberglass rebar) on 12" center both horizontally and vertically on the walls, slab, and ceiling. Around window and door openings also add 4 foot 45 degree pieces at the corners. Also an extra two pieces of horizontal above window openings. Where the wall is below grade then put a second run of horizontal rebar on the inside edge of the cavity.
Tie rebar to ICF webs and each other using
cable ties. Note that
the cable ties are special high strength cable ties.
Use horizontal rebar straights and horizontal bends on the wall
corners, ensuring a
20" overlap on the joins between the two types.
Use 45 degree 4 foot rebar straights at the corners of windows.
Above windows and door openings
(particularly large openings) it is necessary to use more rebar.
It is worth using 6 horizontal runs of rebar in the blocks above
openings (3 on the top webbing and 3 on the lower webbing).
Extend this 3 feet either side of the opening.
The picture below shows the rebar under the window opening (less needed under the window compared with above the window).
Half way up the wall (and at other heights you deem useful) it is good to put horizontal rebar on both the outside upper cradle position as well as the inside. This is to constrain the vertical rebar (that will be added later) to the center of the ICF blocks.
When you are about 3 or 4 feet from the top of the wall (which will typically be at floor/ceiling level, then you need to add the vertical rebar. The rebar will be the full height of the wall minus about an inch. You may want to wait until you have the floor in place so you have somewhere to stand. You need to thread the rebar down inside the wall and aim to get the end into the PVC pipe ring at the bottom. You will need to shine a strong flash light down into the wall cavity. It's a bit of a trial and error fishing task. It will also be constrained into the right position by the inner and outer horizontal rebar.
Where there is one window above another window, don't forget to add vertical rebar in the concrete wall between the windows.
Throughout the building of the walls, look for opportunities to add more rebar anywhere you think it will help. Remember that the rebar placement spec from the structural engineer is just the bare minimum.
Above window and door openings it is necessary to use lots of rebar. I use 6 lengths of rebar in the ICF block that is immediately above the window opening and then 3 lengths in the layer of blocks above that. Also remember to put in the threaded stud anchors in the top of the window bucking.
Window and door Bucking
Details regarding the window bucking are here . As mentioned, the bucking needs to be done in parallel with the wall being built up and the rebar being installed.
Large (and even medium sized) bucking assemblies need legs that are formed from the ICF bracing assemblies (see bucking details). The large bucking assemblies are pivoted into place using ropes, leveraging off the 2x4s nailed to the concrete round the wall on the inside.
Initially get the sliding bucking positioned in approximately the right place on the site (with the legs on the underside, ie the inner face).
Then fit the cavity bucking over the top and use wood screws to set it in the right place (which is 1-1/8" from the inner edge of the sliding bucking). You can use some scrap sections of ICF block wall that is 2-5/8" to set the position.
Use double sided tape to fix a 6mil polyethylene sheet over the external face of the bucking assembly. Leave at least 2 feet of sheet on all sides. The tape goes where the window nail fin will eventually attach.
When the cut ICF block under the window does not provide 3.5" of clearance before hitting the webbing it will be necessary to cut corresponding slots in the bucking's EPS bottom. (If there is not at least 1.5" to the webbing then you will need to cut out the webbing.)
As well as cutting the EPS it is also necessary to cut a slit in the polyethylene sheet in the center of the cut EPS slots.
You will ideally have already cut the EPS on the sides to provide clearance for the ICF webbing for the side blocks. Sometimes it is only necessary to cut the EPS slightly, ie just enough to clear the webbing.
For a large bucking assembly that has legs, pull it into a vertical position using ropes. The feet are positioned against the nailed down 2x4s. Initially the force can be taken by the lower stronger 2x4, but after the bucking gets more vertical you will need to lift the legs onto the stack of three 2x4s (temporarily screwed together) so that it is high enough to clear the top of the ICF blocks.
You need a rope on both sides so it does not topple over. It will need at least 3 people. Two people will lift up the top, initially with their arms and then pushing further using 2x4 sticks, and one or two people will pull on the rope. When near to vertical, make sure the feet are properly on the stack of three 2x4 spacers to raise it high enough to clear the ICF blocks.
After the bucking assembly is vertical it can be worked into the required position along the wall. Initially leave the 4.5" of spacers (stack of three 2x4s) under the legs to clear the top of the ICF blocks.
The studs under the window bucking need gluing in place before the bucking is dropped into its final position. Glue the studs using PL-Premium adhesive. These are used to anchor the bucking in the concrete. Do not forget to fit these studs.
The following pictures show some large bucking in the vertical position.
Once the rebar has been fitted and any necessary trimming of the bucking polystyrene and ICF webbing has been done, the bucking assembly can be moved outwards and lowered into the cavity of the ICF blocks by gradually removing the 2x4 spacers (rock the bucking from side to side to pull out the spacer pieces of 2x4).
You will need a door to get into and out of the building when you build up the ICF blocks so you will need to cut and tape a triangle in the door bucking.
To get the bucking set to the exact height and exactly side to side level, you need to add shims under the bucking feet. Cut them from some spare 2-by. Use a crowbar to fit the shims and use a laser to get everything vertical.
Don't forget to glue in the side studs that are used to anchor the bucking into the concrete. You need to do this only after the appropriate ICF block has been fitted (or else you will typically not be able to get the block in place).
Internal wall bucking
Bucking is also required for the openings in the internal walls. Some of the openings in the internal walls are very large. In the case of the internal openings there is already a concrete floor slab at the bottom of the opening so there is no need to use any wooden bucking on the bottom edge.
ICF Bracing
The detail on the bracing system can be found here . To be super safe and avoid blow-outs t is good to brace the ICF walls with vertical bracing about every 2 feet. It is only necessary to put the 45 degree adjustable pieces on alternate vertical bracing pieces.
On the inside of the building the verticals consist of two bits of 2x4. On the outside of the building, only a single 2x4 is used.
The verticals are screwed to the ICF webbing using 5" long wood screws. Before screwing, choose a spacer shim for under the leg that gets it to the right height (eg for supporting a concrete lip). Having the right spacer is important to allow the bracing to take the weight of the floor above.
Make the verticals the exact right length necessary to support the wood of the floor above. Typically 2x4s are only 8 foot long, so you may need to join more than one 2x4 to get the required length.
Finishing details
The turrets on the top of the ICF needs to be protected so they don't get covered in concrete. Rather than the pieces of spare ICF bottoms that are shown in the photos below, it is best to just use duct tape.
Roof details
Triangular ICF pieces
Make horizontal beams
At this stage you only need 4 of these.
The horizontal beams on the gables have extra holes on 2' centers for additional studs through the concrete of the gable walls (see AutoCAD drawings).
Fit gable horizontal beams
The horizontal roof beams attach on both sides of the ICF of the gable walls using 1/2" studs through the ICF blocks..
One of the studs used to hold on the 45 degree roof beams touches on the concrete so needs to be fitted before the concrete is poured. The stud length is 21". It sticks out 2.5" equally on each side.
Studs for metal straps
It is very important to ensure the roof is well held down so it does not lift off if a hurricane comes past. Vertical meta straps need to go from studding through the concrete to the roof beams. The roof beams are 2 foot on center and so the studs should be 2 foot on center. The first stud is 4" from the inner edge of the ICF corner block to correspond with the first inner roof beam, and then every 2 foot after that. In the center of the north/south walls the spacing will be something other than 2 foot. The studs are 1/2" threaded and have a length of 18". 2-by bracing is needed on both sides of the ICF wall. On the outside the studs only need to come through the 2-by half an inch. On the inside the stud comes through an inch and a half.
The roof beams (other than on the gables) are put in place after the concrete has set in the walls. A bird's mouth cut in the beam will sit on the edge of the concrete after the ICF EPS is cut away.
In the top of the gable wall there will be two layers of 2" EPS, therefore the concrete needs to stop 4" from the top of the gable wall ICF blocks. So that you know the height when pouring concrete, make a 4" wooden height template, come concrete scraper, that fits in the 8" cavity.
Finally, make sure all the ICF walls are well braced.
