How to build a basement.

Unless you remember them clearly, re-read the pages about Joints Kickers BS8102 has changed, for the better. You must repair the leaks through your structure. Follow these pages and you won't have any leaks to repair. This is the page about choosing and buying in your basement wall formwork. The formwork build manual is the next page.

1. Traditional plywood and 4x2 formwork or hired-in system formwork.
   
   It's amazing, but Professional formwork carpenters will always think how to build a basement is with a system that leaves hundreds of holes through the structure. They use and re-use a steel threaded rod through a plastic sleeve to hold the two sides of formwork tightly together.
   
   That is what the carpenters always want to use but all they do is put the formwork up.
   
   Usually, professional formwork carpenters
   
   

   DO NOT pour the concrete DO NOT strike the formwork (including pulling hundreds of nails out of timber formwork and damaging timber in the process) DO NOT fill the holes DO NOT fill voids in the concrete DO NOT repair leaks

   
   
   So they don't even know what difficulties they leave for others to put right. They only care about themselves. They do not know how to build a basement not to leak, even if basements are all they do.
   
   Traditional formwork carpenters, who think they know exactly how to build a basement, will usually commit two other sins that suit them rather than their client.
   
   
   • They will form a kicker. A dwarf wall to clamp their formwork against. But the kicker will be very likely to leak and incredibly difficult to repair. A kicker is for your carpenter's benefit. Not yours. I have a page about kickers, already linked to at the top of the page here.
   
   
   • They will fill the wall with concrete to full height but in many sections, meaning many joints. Because narrow sections of wall and many concrete pours means that they buy or hire less formwork and use the same stuff repeatedly. It is for their benefit, not yours. Using the formwork many times is proof they do not know, or care, how to build a basement that will not leak.
     
     This means so many issues for the client.
     
     
     • Lots of vertical joints, lots of expensive hydrophylic strip that won't work everywhere and lots of leaks.
     
     
     • Lots of voids in the concrete at the bottom of the wall where they
       
       
       • Didn't get the concrete to the bottom of the formwork in good condition because it crashed past too much reinforcing steel, which got coated in cement, sand and water paste, and the stones got to the bottom surrounded with voids instead of sand and cement.
       
       
       • Didn't vibrate and compact the concrete at the bottom of the wall properly because they found it too hard lifting up a heavy vibrating poker (some weigh 25kgs) over 3m every 20cm along the wall. Plus pokers tend to get tangled up in the steel before they get 3m down inside formwork, so the bottom concrete gets missed and air remains inside it.

2. Blockwork walls, whether two faces of blockwork or hollow core blocks.
   
   Either will leak.
   
   Hollow core blocks will leak because water will get through the part of the block that is continuous front to back and water will get in through the mortar or open joints between blocks.
   
   If you have two faces of blockwork and concrete in between you might think you could use waterproof concrete, but you can't because:
   
   
   • The blockwork will fall over when you put concrete inside unless you use plenty of wall ties and only fill about 3 courses each time.
   
   
   • The bricklayer will drop mortar inside as he lays the blocks. You may well have a hydrophylic strip in the bottom but it will be covered in porous mortar droppings. Your waterproof concrete could not get right down to the slab beneath because it cannot displace the set-hard porous mortar droppings.
   
   
   Blockwork formwork is hopeless for basement retaining walls. It is a wasted opportunity to stop water getting in for no extra cost.
   
   It is mentioned in the new BS8102 as a bad choice.

3. ICF Insulated Concrete Form.
   
   Another very bad way to build basement walls. Also mentioned in the new BS8102 as a bad choice.
   
   I have a page explaining why ICF will cause you misery here.

Planning to use my resin rods in your formwork so you leave no holes.

The formwork starts with a string line along what will be face of concrete wall. Next the floor plates are screwed down to the concrete the correct offset for the uprights and the wall form timber. No kickers because kickers leak. After one course of boards, I would use 980mm lengths of 18mm roof batten and leave 18mm gaps. These give you 18mm square gaps for rods without having to cut valuable timber you will re-use.

Carry on. Usually, you want to form at least half height and no more than 2m tall for the first pour. This is so that you can get the concrete to the bottom inside in good condition and vibrate it to the bottom properly. A clean scabbled joint, no kickers and no more than 2m high removes most of the reasons concrete leaks. If your wall is a constant thickness bottom to top, divide the pours about half way.

If the wall is a bit complicated, you might have a corbel outside and an upstand inside, then make the more complicated top pour less than the first. You might want to deliberately stop the first pour where it makes continuing easier, for instance at the top of the corbel. If one pour finishes top of corbel, you don't need to cap your corbel, you can just trowel it off. On the other hand, this team backfilled the first pour before they carried on. If you cap a corbel or cap a ledge, drill small holes through the timber to let the air out when the concrete is trying to get in. I would say 4mm holes on 100mm centres at the farthest point your concrete has to reach. If you think about it all, you will do better than if you didn't think about it all.

Casting concrete against piles definitely requires more than 3 rows of rods because the weakest link is the rods pulling out of the piles because the piles themselves are made with relatively weak concrete. That entire far side is one side of timber formwork tied to concrete piles that are the second side of the formwork. I think we used 4 rows for the first pour, then the top two of those rows again with one more row near the top. This was massively cheaper than hiring in a system of single sided formwork.

This customer wanted to be quite certain nothing moved. He got 25mm good quality plywood cheap and used 4x2 uprights not the 6x2s I would have used. He used 4 rows of rods where 3 would do (had he used 6x2s). However, he absolutely succeeded. Nothing moved. No one knows how strong formwork needs to be. But if it wasn't strong enough it can move or burst and you have a lot of work chiselling concrete out.

I use this same picture again a little lower down, before you think you are going mad.

This customer decided that he would try to only pay for 2½ rows of rods. He used the lower two rows twice and he wanted a 50% refund on a third, top row that he used only once and was able to return because he kept them clean in plastic sleeves. He also thought about how to buy the least timber. The first two rows of rods supported the first wall pour to about 1800mm high. When he poured to the top he only added the top row of rods in plastic sleeves. This is how he used two rows twice. Nothing wrong with using two rows twice in this way, but he would have had an easier time and neater finish if the top row of rods was one board down from the top, and if his formwork was a few inches higher than the top of the wall. It is impossible to fill formwork to the top, then stick a great big vibrating poker inside, without a lot of spillage, waste concrete and formwork to clean of concrete. Plus you have fill the wall to the top with concrete from a bucket. This is hard work and it takes time. You might not mind the hard work, but if the concrete is setting before you get all this extra work complete you will not get a good finish. Some days after the first, lower, pour had cooled, he loosened off the two rows of nuts to retrieve most of the timber, tightened the nuts back up, used the retrieved timber to build his formwork to the top and he put his top row of rods just above the formwork in plastic sleeves so that he could get some money back returning me 38 rods.

1. He used two boards before the first row of nuts and the concrete pushed those two boards out very slightly because he did not fill sufficiently higher than the first rods. Had he used thin nuts in his middle row that might not have happened. But the proper solution is to have the first row of rods above the first board instead.
   
   For the second pour he had extra bracing on the outside and over the top to keep the wall width correct. It wasn't really necessary, but it did make steering the concrete pump a lot harder, it cause a lot of unnecessary mess and wasted some concrete and it made floating the top off neatly a lot more difficult and slower.


2. He cut 6x2s in half.
   
   He needed to buy a lot fewer 6x2s but they were relatively useless in short lengths to re-use, neither could he sell them on as profitably as long lengths.


3. In other ways he went over the top.
   
   The problem with the horizontal timbers over the top, maintaining the wall width, are that the concreting pipe and the vibrating poker have to be pulled right out before they can be moved along. This means extra work, extra time and unnecessary mess as concrete dribbles everywhere.
   
   Whereas if he had cut 6mm of each of the top nuts to use inside as thin nuts he wouldn't have needed to lift the concreting pipe up and clear of the cross timbers as he went round and round concreting, and he would have made less mess.

1. The pump comes round and the form is filled with concrete to the top. This concrete might have overflowed slightly but it contains air. A bit like the narrow ledge that hasn't been scraped level yet.


2. The poker is inserted to remove the air and the poker displaces about 2 litres.


3. 2 litres of concrete spills out every time the poker is inserted, every 200mm to 300mm. The level of the top of concrete is now 20mm to 40mm low and it needs topping up. This means finding some concrete, putting it in a bucket, carrying it round and trowelling it in before tamping the air out.
   
   All this before the concrete you have in a pile nearby starts to set.


4. Sometimes the concrete can be partially topped up with the concrete pump pipe being swung back where it has been, but it is just as likely to overfill 40mm deep and more concrete spills out.

Screws at top of concrete level.

Underside of batten shows us top of concrete.

The formwork is built up in a single width. Void formers are fixed inside. Air holes beneath the void formers are drilled through the formwork so the concrete can get into the corners.

You use your timber to create the two widths and the two top levels. The disadvantage, clearly, is that to get the lower concrete level correct you will spill some concrete as you vibrate the higher level and you will manually have to clean the lower level off with a trowel. The void formers create a better finish, especially if the upstand formwork is a bit high with the top of concrete level clearly marked.

1. A timber that creates the face of concrete.
   
   If this is 18mm shuttering ply it needs 4x2 ribs on 400mm centres as well or else the curvature under load is too great.
   
   I have found scaffold boards and 6x2 to be excellent. Scaffold boards need the steel straps removing. If the 6x2 is regularised then the chamfers create proud lines of concrete which might be a nuisance later.


2. You need uprights in pairs: called soldiers or strongbacks.
   
   You can hire steel versions or use 6x2s.


3. You need frames to hold the formwork upright.
   
   This is because I avoid kickers and these rods and nuts cannot be done up as tightly as the steel alternatives. The formwork is strong enough but not strong enough to self support upright.
   
   Triangular frames. I find 10.8m of 4x2 in 3.6m lengths is plenty to form the L shape with 2 screwed together, the floor plate and the diagonal. All on a maximum of 2m centres. The maximum is also dependent on the boards you will walk on on your scaffold.


4. You need a scaffold to stand on.
   
   If you have already made the triangular frames then adding a horizontal support with a leg and 3 scaffold boards wide to stand on is simplicity itself.
   
   I have known customers make me walk along cheap B&Q trestles not tall enough or strong enough.
   
   I have known customers pay a scaffold company hundreds of pounds for loads of scaffold only used for a few hours.
   
   My triangles are a hit when the scaffold is added very cheaply. See the note about screws and taking them out below.
   
   6x2s can be used as the boards you will walk on as well.

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