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Foundation Lining
Preparation
Check Form-a-drain location - Adjust tensioning wires if necessary
Make sure that the top inner Form-a-drain has not been moved laterally by all the back filling and compacting. Potentially the cross wires between the Form-a-drain across the slab area can be tensioned a bit more if the Form-a-drain has moved slightly.
Check Form-a-drain height
It's also worth checking again that the Form-a-drain is at the right height relative to the batter board strings. If any slight raising of a corner is necessary then use a prying bar and slip a few slithers of crushed rock underneath. At this stage of the game we are looking for exact levelness. The absolute distance from the batter board strings is not that important as long as it is the same all the way round.
Remove wooden spacer lengths (if still present)
Using 37" lengths of 2x4 are an added precaution against movement of the Form-a-drain and are fairly easy to remove as you work your way round doing the lining.
Sweep the trenches clean
Clean up the job site. Get rid of rubbish and put away your tools. The chances are that some of the crushed rock has fallen into the trenches. Before putting down the membranes, make sure the soil in the trenches is free from sharp rocks and other debris.
Flush drainage pipes
It is possible that some dirt or even some rock has got into the drainage pipes. Use a hose (ideally a high pressure hose) down all the drainage pipes to flush everything out.
Foundation design
Throughout the process of adding the different layers you will want to constantly refer to the Foundation design drawing. (Click on image to see high resolution version)
Install Fab-Form in footing trench
Put geo-fabric at bottom of trench
The geo-fabric provides a slight cushion against any rough edges in the soil. Start by putting patches over the gravel used in the drainage ditches that go under the footing. Then line the bottom of the complete footing. The geo-fabric can fold over at the edges to use up the full 4 foot width.
Lay strip of Fab-Form along bottom of trench
In order to provide extra termite protection (given that there will be drainage slits in the bottom of the polyethylene termite membrane that is added later), lay a strip of Fab-Form along the bottom of the footing trench. It is important that this does not curl up the sides of the trench because that would form a water trap. We want water to be able to run off the Fab-Form and drain into the soil. Cut the Fab-Form to be the trench width, ie make it 37" wide. You can cut the Fab-Form while it is still on the roll using a chop saw. The roll of Fab-Form is 62" wide so you will have a 25" strip left over, which you may as well also lay along the bottom of the trench (ideally on the side of the trench you think is the lowest point as that is where the drainage slits will be).
Put double sided tape on inner and upper surface of outer upper Form-a-drain
Double sided tape is used to attach the Fab-form to the inner and upper edges of the outer upper Form-a-drain. Put the inside strip right at the top of the Form-a-drain. You can do this strip-by-strip as you add each piece of Fab-Form.
Line footing trench with Fab-Form
Fab-Form is a tough membrane specifically designed for constraining wet concrete, although in my case it's really just another protection membrane to keep out termites and other undesirables. The Fab-Form is 62" wide (assuming you buy this width). It would need to be about 10 foot wide to line the trench lengthways, so it needs to be used across the trench. Cut lengths of 129" inches (10'9"). You will overlap the joins by about 6 - 8", so each strip of Fab-Form will do about 54" (4'6") of trench length. Leave about an inch of slack each side in the Fab-form in the footing ditch to allow for any movement.
There is no need to use tape on the joins between the strips of Fab-Form and it would be undesirable to tape the seams as we want any water that collects during the construction of the foundations to be able to escape.
Start by attaching the Fab-Form to the inner upper Form-a-drain with a 4" overlap above the top of the Form-a-drain, and then line the trench and come up the other side of the trench to stick it to the double sided tape on the outer upper Form-a-drain. There should be about 15 inches left over on the outside beyond the outer upper Form-a-drain. On the inner Form-a-drain, the attaching is done by folding the 4" of spare over the top and down the other side of the Form-a-drain. Fold up the last 1" to make a double thickness at the edge. Secure it on the slab side of the Form-a-drain with self tapper screws every 6 inches. You might need to cut slit holes in the filter sock at the self tapper positions, but if you are lucky you may just be able to press hard with the power screwdriver to stop the cloth getting wrapped round the screw. At the corners you will probably need to start at the outer Form-a-drain first (with the 16" spare).
At the corners, the Fab-Form from the sides will overlap, which is all goodness as it forms a double layer.
The two strips of double sided tape on the upper outer Form-a-drain is sufficient to hold the Fab-Form . The extra Fab-Form is draped over the strengthening lumber.
Cover slab area
Add a layer of geo-fabric over slab area
The crushed rock should be at a height of 1" below the top of the inner Form-a-drain and should be well compacted and level. The geo-fabric over the under-slab area is to help prevent the crushed rock from unduly puncturing the first 6 mill polyethylene sheet. Geo-fabric is fairly inexpensive so you may as well use it.
Install 6 mil polyethylene sheet over slab area
The primary job of this 6 mil polyethylene sheet is to add to the protection of the protection sheet of polystyrene from termites. It goes on top of the geo-fabric that is over the crushed rock.
The polyethylene sheet does not need to be sealed and may well get punctured by sharp pieces of rock. Just fold over the edges and cut around the pipes that come up through the slab. Because it does not need to be sealed, there is no need to use a continuous big sheet as joins don't matter. Just overlap the sheets by 6" and there is no need to tape the joins.
Put 1" thick polystyrene sheet over slab area
The Expanded Polystyrene Sheet is a slightly compliant layer to protect the 6 mil polyethylene sheet above (the next layer). Take note of what PSI version of EPS sheet to use as described here in the section called "Distributing the load".
The top of the polystyrene sheet should be level with the top of the inner top Form-a-drain. Obviously you need to cut it around the pipes that come up through the slab.
Install first polyethylene sheet in footing trench
Design and function
This in the first of the membranes that lines the entire underground area. It is to catch any radon gas and it also stops termites getting to the main layer of polystyrene.
It is implemented such that any radon gas caught by it is prevented from seeping upwards (except via the vent pipe) and yet it can let water out at the bottom, while at the same time stopping termites from digging up through the bottom. This may sound like an impossible task, and to some extent it is, but the implementation described constitutes the best way I can think of.
The membrane needs to be something tough enough to stop termites, and 6 mil polyethylene sheet is better than say landscape fabric. To catch radon gas, the polyethylene needs to be tightly sealed at all the high points (such as over the slab area). To allow out any rain water, that is bound to accumulate while implementing the foundations, it should not be sealed at the low points (such as the bottom of the footing trench). Even though not a waterproof seal at the bottom of the footing trench, joins and cuts in the polyethylene need to be covered with an additional strip of polyethylene sheet to avoid a straight upward path for termites. This covered unsealed join will allow any collected water to find its way out (especially when pressurized by the weight of lots of wet concrete on top) and yet will form a termite resistant join by having that weight of concrete and the rest of the house pressing the polyethylene sheets together.
Obtain polyethylene sheeting
Even though not as critical as the official vapor/water membrane that gets added later, it is still good to avoid joins. You will be making cuts in the sheet at the bottom of the footing trench, but in the higher area over the slab you want to try to get a good gas-tight seal. See here for purchasing details of large polyethylene sheets.
Note that you will need to purchase two of these sheets as there are two polyethylene membranes used in the foundations.
If you are using 20'x100' polyethylene sheet, it's best to take advantage of the 100 feet long dimension and run the sheets along the longest dimension of the building.
Measure out polyethylene sheet (if necessary)
The sheet needs to be cut to be long enough to go from 4 feet above grade, then 2 feet to the Form-a-drain, then down the inside of the outer Form-a-drain, then along the bottom of the footing trench, then up the inner edge of the inner Form-a-drain, then across the slab area, then the same on the other side up to the 4 feet above grade. It's worth also adding a foot or so extra to be safe. With the roll still in its folded state, unroll it and measure the required length and cut it If you had the sheet custom made to the size you need then there will be no need to cut it.
Mark center positions
It's worth marking the center positions (length ways and width ways) for the sheet as this will make it easier to position the sheet correctly. With a large polyethylene sheet this process can be hard, so you may just have to just judge the center positions when the sheet is approximately in place.
Add another 2 strips of double sided tape at upper Form-a-drain locations
This double sided tape will be used to hold the 6 mil polyethylene sheet in place. It is stuck on the Fab-form directly above the tape that was used to hold the Fab-Form. Do this on both the inner and outer upper Form-a-drain (on the trench facing sides), but in the case of the inner Form-a-drain it is not needed on the actual top edge). Keep the backing tape on the double sided tape until you actually need to stick it to the polyethylene sheet.
Put polyethylene sheet in footing and over slab area
The sheet starts with enough spare to go from 4 feet above grade, then down 2 feet to the Form-a-drain, then down the inside of the outer Form-a-drain, then along the bottom of the footing trench, then up the inner edge of the inner Form-a-drain, then across the slab area, then the same on the other side up to the 4 feet above grade. It is held in place using the double sided tape .
It is worth having a few people to help you as a big polyethylene sheet is fairly tricky to unfold and position.
Leave a bit of slack (say 1" either side of the footing) in the 6 mil polyethylene sheet to allow for some movement. If you have any joins (best avoided), you should make the overlap about 2 feet. Some creative folding is needed at the corners.
We are not looking for neatness in the folding of the sheet. The important thing is that it forms a continuous gas tight membrane (in the high areas). We do NOT want to cut the sheet to achieve neat folds. It's much better to have crumpled up spare flaps rather than cutting to achieve neatness.
The 4 feet above grade (and 2 feet to Form-a-drain) spare round the edges can be laid over the sides of the outer upper Form-a-drain and tucked out of the way.
If you need to join polyethylene sheets (best avoided), use double sided tape, then single sided 2 inch tape, then single sided 4 inch tape on top of that. This process is repeated on the other side of the sheet join. Using the belt and suspender approach helps keep the join gas tight even if one of the tapes fails with age. Note that with the 2 foot overlap of the sheets, the double sided tape will be offset 1 foot one way and the single sided tape will be offset 1 foot the other way, then the opposite on the other side of the sheet. At the 2 foot overlap, where it goes past the upper outer Form-a-drain, add an additional two strips of 2 foot pieces of double sided tape. This is to make a well secured base for sticking successive layers.
If you have pipes coming through the slab (eg sewer or a radon vent) then you need to cut openings to allow the pipes to come through. Cut a cross in the 6 mil polyethylene sheet at the carefully figured out place, but keep the cutting to an absolute minimum as we are aiming for a sealed membrane. Gently forcing an under-sized cross over the pipes will nicely stretch the 6 mil polyethylene sheet to help form a good seal. The flaps of the cut cross should face upwards around the pipe so they can be taped. Leave a bit of extra slack in the polyethylene sheet between the pipes.
I have decided that it is best for the sewer pipe to come through the wall just above slab height rather than come through the slab. I have also decided to route the radon vent pipe under the footing rather than have it come through the slab.
Seal around pipes with tape
We are aiming to have this sheet fully sealed in the under-slab area so that no radon gas escapes upwards. Tape it well all around the perforations due to the pipes that come up through the slab. Use a couple of rings of double sided tape round the pipe and then single sided 2 inch tape on top, then single sided 4 inch tape on top of that.
Cut water exit cuts at the bottom of footing trench
It is necessary to provide a path out for any water that collects in the footing trench during the construction of the foundations. Obviously water will tend to collect at the lowest point, so this is where you need to make cuts in the polyethylene sheet. Make the cuts something like 1 foot long. Put a cut about every 4 feet, but particularly concentrate them in any place that you observe rain water build up as that will be a low point.
Don't cut through the Fab-Form as that already is not joined so already has a water exit path (and you want this path to be offset relative to the cuts in the polyethylene sheet to avoid a direct upward path for termites.
Lay and secure strips of polyethylene sheet over the cuts
Cut pieces of polyethylene sheet such that they overlay the cut hole by at least a foot all the way round the hole. That equates to more than 2 feet wide and the length of the cut plus 2 feet. As needed, use single sided tape on the corners to hold it in place so that it does not move around (but do not make a taped seal as you want water to be able to get out).
Cover slab area with a layer of 2" EPS
Get the right mix of 40psi and 15psi EPS
As described in Techniques - Foundation and Wall design in the section called "Distributing the load", it is important to use the right mix of psi rated polystyrene sheets in order to give the right distribution of load on the slab.
The first layer (the layer being described here) is done using 4' wide sheets of 40psi EPS adjacent to the footings and then 15psi sheets for the rest of the slab to the center.
Lay EPS sheets to overlap edge of Form-a-drain
The sheets go over (ie overlap) the upper inner Form-a-drain. The edge of the polystyrene sheet is flush with the trench side edge of the Form-a-drain. Use foam adhesive on the sheet edges and duct tape to join the sheets together at all the seams so they don't move around. You will of course have to cut holes for the stack pipes that come through the slab.
The reason the first layer over the slab area is added at this stage is to provide something to tape and glue to for the EPS on the inside of the footing trench.
Line footing trench with 2" polystyrene
Line outside of footing trench
Cut some 2" EPS sheet (15psi) to a width of 41.5". Attach it at the top (using single sided 2 inch tape) to the polyethylene sheet that is over the upper outer Form-a-drain. Attaching only at the top is ok because the horizontal EPS at the bottom of the trench will hold the bottom of the vertical EPS.
There is no real need to glue the sheet edges and no real need to tape the joins.
Line inside of footing trench
Cut some 2" EPS sheet (15psi) to a width of 30.5". Attach it at the top using foam adhesive on the sheet edges and duct tape to the EPS that's over the slab area. It should stick up 2" above the top of the upper inner Form-a-drain which means it is level with the EPS that's already over the slab area. There is no need to tape the bottom of the EPS sheets because the EPS at the bottom of the footing will hold it in place, but it is good to tape the sheets together at the corners while the glue is drying.
Add extra outside polystyrene
Cut some 2" EPS sheet (15psi) to a width of 30". Use PL-300 adhesive to glue it at the top of the existing sheet of EPS that is on the outside of the footing trench. A small blob of foam adhesive on 6" centers is fine. Hold the EPS sheets in place with duct tape top and bottom. The best approach is to glue full 8 foot lengths in place, away from the corners, and then after the glue has dried, force in slightly oversized pieces at the corners. The oversized corner pieces help compress the polyethylene sheet that is under the existing EPS.
Add additional extra outside polystyrene
This is two sheets of 2" EPS (15psi) that goes round the upper edge of the outer footing, ie inside from the upper outer Form-a-drain. Glue together 18" wide and 16" wide 2" thickness EPS sheets with them inline at one end and staggered at the other end. Use small blobs of foam adhesive.
After the glue has dried, cut it off at 45 degrees at the staggered end. After cutting, the long edge should be 18" and the short edge should be 14". Glue this to the EPS that's already in place. Hold in place while the adhesive dries with duct tape top and bottom.
Line bottom of footing trench
Under the footing there will be only one layer of 2" thick EPS (40psi). This contrasts with the slab area that has two layers.
Cut some 2" EPS (40psi) to a width of 33". Fit it between the vertical side polystyrene at the bottom of the footing trench. This will nicely hold the vertical pieces of EPS at the bottom. It is worth measuring the actual width required so as to achieve a very tight fit. It should by design need plenty of force to work the EPS sheet into position at the bottom of the footing. Having this tight is part of the strategy for holding back the force of the crushed rock.
After putting the EPS at the bottom of the footing trench you will need to be very careful walking on it with big boots. Ideally put down some scrap pieces of EPS to act as stepping stones. Alternatively if you are fairly light you can wear smooth flat soled shoes.
Add extra inside polystyrene
Note that once the extra inside polystyrene has been added it will not be possible to remove the polystyrene at the bottom of the footing trench.
Cut some 2" EPS sheet (15psi) to a width of 17". It is installed 2 inches lower than the first EPS sheet, ie 2 inches below the polystyrene that is currently over the slab area. Use foam adhesive to glue it to the existing sheet of EPS. Hold it in place while the adhesive dries with duct tape top and bottom.
Cover slab area with second layer of 2" EPS
Get the right mix of 40psi and 15psi EPS
As described in Techniques - Foundation and Wall design in the section called "Distributing the load", it is important to use the right mix of psi rated polystyrene sheets in order to give the right distribution of load on the slab.
The second layer (the layer being described here) is done using 2' wide sheets of 40psi adjacent to the footings and then 15psi sheets for the rest of the slab to the center.
Lay EPS sheets in a staggered joint pattern
This layer of EPS is cut to be the same size as the first layer of sheeting. The staggering is achieved because 40psi EPS on this layer is cut to be only 2 foot wide. Use duct tape to join the sheets together at all the seams so they don't move around. Use some blobs of foam adhesive between the two EPS sheet layers. You will of course have to cut holes for the stack pipes that come through the slab.
Cut inner EPS at a 45 degree angle
As will be seen by referring to the foundation drawing, it is necessary to cut the top of the inner EPS and slab EPS to form a 45 degree angle. This is done so that the concrete is formed at 45 degrees between the slab and the footing to increase the strength in that important area. The bottom one of the two sheets does not get cut, but the upper one is cut at 45 degrees.
It is useful to temporarily lay some OSB sheets on the slab area to provide a solid base on which to work. This is going to be important for the next stage as that involves carrying heavy buckets of crushed rock.
Fill round the outside with more crushed rock
Possible and necessary
Now that the multiple layers of polystyrene sheeting has been added to line the footing trench, the trench is able to withstand a lot more lateral force. The lumber bracing also helps a lot. It is now ok to add crushed rock nearly all the way up the outside of the outer wall of the trench. It is important to add this crushed rock because it can withstand the lateral force from the wet concrete when it is poured.
Required height
It makes sense to fill with crushed rock up to about 6 inches from the top of the outer upper Form-a-drain, ie to just under the lumber bracing that's attached to the outside of the Form-a-drain. The reason for not going all the way to the top is that it's good to have some space for storing the edges of the various polyethylene sheets.
Install official water barrier
Best to use a continuous sheet
It is very time consuming trying to do good joins between polyethylene sheets and the joins may decide to leak. It is far better to buy a custom made polyethylene sheet that is big enough to do the whole job without any joins. The largest commonly available sheets are 20 feet x 100 feet, however, it is possible to obtain polyethylene sheet with a larger width. Wide widths are used to make ice rinks. See here for purchasing details.
Try to get water out
We want to try to avoid trapping water under the final polyethylene sheet. Use a wet vac to suck out all the water from the trenches and try to pick dry weather for the installing of the final polyethylene sheet.
Mark centers
On the polystyrene over the slab, mark where the center of the slab is in both the x and y directions.
Lay starting strip of double sided tape (if joining sheets)
A strip of double sided tape on the polystyrene will stop the polyethylene sheet sliding around.
Make the join using double sided tape, single sided 2 inch tape. and single sided 4 inch tape.
The taping of the seam should be done both on the outside and the inside of the sheets. Note that doing the underside typically involves crawling under the sheet and working up-side-down (make sure you don't suffocate).
Put polyethylene sheet in footing and over slab area
This in the official impervious water/air/vapor membrane that lines the entire underground area. It starts with enough spare to go from 4 feet above grade, then down 2 feet to get to the Form-a-drain, then down the inside of the outer Form-a-drain, then along the bottom of the footing trench, then up the inner edge of the inner Form-a-drain, then across the slab area, then the same on the other side up to 4 feet above grade. It is held in place using the double sided tape .
Don't forget to provide movement slack. Put 1" extra flap in any place that the you think might experience movement. At minimum there must be an inch of slack on the vertical outside of the footings.
The 4 feet above grade (plus the 2 feet to the Form-a-drain) spare round the edges can be laid over the sides of the upper outer Form-a-drain. It will be rolled up (and temporarily taped) with the previous membrane sheet to prevent water getting between the sheets.
Cut undersized openings for pipes
You have pipes through the slab (eg radon vent) then you need to cut openings to allow the pipes to come through. Cut a cross in the polyethylene sheet at the carefully figured out place, but keep the cutting to an absolute minimum as we are aiming for a sealed membrane. The flaps of the cut cross should face upwards around the pipe so they can be taped. Leave a bit of extra slack in the polyethylene sheet between pipes if there are adjacent pipes.
Seal around pipes with tape
It is fundamental to have this sheet fully sealed so that no water gets through. Tape it well all around the perforations due to the pipes that come up through the slab. Use a couple of rings of double sided tape round the pipe and then single sided 2 inch tape on top, then single sided 4 inch tape on top of that.
Ensure no holes in polyethylene sheet
It is important that the polyethylene sheet has not been damaged. Thoroughly inspect it for holes. If any holes are found then repair them with single sided 2 inch tape on both sides of the polyethylene sheet and then single sided 4 inch tape on top of that on both sides.
Add two strips of double sided tape at upper Form-a-drains locations
This double run of double sided tape will be used to hold the membrane sheet (official 6 mil polyethylene water/air/vapor membrane) in place. It is stuck onto the polystyrene sheet in the same locations used before. Do this at both the inner and outer upper Form-a-drain locations on the trench facing sides (but in the inner case only one run of double sided tape is needed. The fixing with double sided tape doesn't need to be perfect because the wooden jigs added later will hold the sheet in place.
Roll up spare polyethylene sheet
There are now two pieces of 6 mil polyethylene sheet coming up round the outside of the footing trenches. To avoid letting water get between the sheets, make sure the official polyethylene membrane completely covers the more tightly rolled up first polyethylene radon/termite membrane.
Line footing trench with Fab-Form
Function
The aim of using this layer of Fab-Form in the footing trench is to protect the polyethylene official waterproof membrane from being damaged by the concrete or the concrete vibrator during the pour process. It is not practical to use 1" EPS in the footing for this protection as it's too hard to keep in place.
Implementation
Cut strips of the 62" Fab-Form to a length of 11'6". The Fab-Form is laid across (and into) the footing trench.
Use double sided tape and then single sided 2" tape to secure one end about 18" into the slab area.
For the other end put double sided tape on the polyethylene sheet in a position that corresponds to the inner top of the upper outer Form-a-drain. There should be about 12" of left over Fab-Form over the top edge of the foundation forming. It folds flat over the top of the polystyrene and the actual Form-a-drain, leaving about 3" beyond the actual Form-a-drain.
Hold everything together at the top
Wooden jigs
There is the possibility that the various sheets of EPS have bowed out away from the Form-a-drain. It's good to make jigs to hold everything together. They fit over the outer upper Form-a-drain. They also hold the membranes securely in place. They will stay in place until the concrete pour has reached up to about 6" from the top of the Form-a-drain.
Cover slab area with 1" polystyrene
Function and game plan
EPS (40psi) of 1" thickness will be used to cover the polyethylene sheet membrane over the slab area. Like with the Fab-Form used to line the footing trench, this protection stops it being punctured during the concrete pour or by any sharp edges on the set concrete.
Cover slab area with polystyrene
Cut 1" polystyrene sheeting such that it completely covers the slab area up to the place where it starts to angle down at 45 degrees. Chamfer the edge of the 1" EPS to continue the 45 degree slope. Use foam adhesive on the edges of the sheets to join them together to stop them moving about.
Put collars round pipes
Building code requirement
It is a building code requirement that pipes coming through foundation walls must be fitted with a collar. This also applies to pipes through the slab. The best way to implement a collar is to wrap a few thicknesses of sill plate foam around the pipe to a form a collar of about 1/2" thickness all the way round the pipe. This will mean you have a 4" pipe coming through a 5" hole in the concrete.
Build a walkway system
This walkway system will stay in place even while doing the concrete pour for the foundations. It is designed to not get in the way of the rebar that will be over the slab. It uses blocks that are 10.5" x 10.5" area to fit within the 12" on center grid of rebar. The blocks are 12.5" high to clear the screeding of the 8" concrete slab. It is best to build them using 2x12 lumber to form a box with air in the middle (rather than the 2x4 design shown in the photo). Don't put a bottom on the box. Only use screws in the top lid because it needs to be easy to break apart after the concrete has set.
Cover the outside of the block with polyethylene sheet as this will make it easier to extract the blocks from the concrete after the pour.
With a marker pen starting at the center of the footing trench draw lines on 12" centers across the slab area in both directions to form 12" squares. The lines show where the rebar will be. The blocks go near the corners and wherever else is needed. They position in the center of the appropriate drawn squares (in order to avoid the rebar). 2" thick lumber planks strong enough to walk on go between them. Screw the planks to the blocks so they don't slip off.
