self build waterproof basement membrane concrete formwork plan.jpg

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

Traditionally and still, professional formwork carpenters will use and re-use a steel threaded rod through a plastic tube to hold the two sides of formwork tightly together.

That is what the carpenters will use but all they do is put the formwork up.

Usually, professional formwork carpenters DO NOT
  1. Pour the concrete

  2. Strike the formwork (including pulling hundreds of nails out of timber formwork and damaging timber in the process)

  3. Fill the holes

  4. Fill voids in the concrete

  5. Repair leaks
  self build basement
And in my experience, the typical tradesman doesn't care about the extra work he creates for those following on.

So, if you are going to build the formwork yourself, I would like you to think about the other jobs as well, and make the whole process as easy as you can.

There is no need to leave anything to repair. No need to leave holes, voids or leaks.

self build basement formwork   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.

self build basement formwork

  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.

self build basement formwork   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, drill small holes through the timber to let the air out when the concrete is trying to get in.

If you think about it you will do better than if you didn't think about it.

You still have a few other things to think about.

First, one nut or two, second, thin nuts yes or no, third, 3 or 4 rows of rods.

These choices and the thickness of your wall determine the length of the rod. We have 600mm, 800mm, 1000mm, 1330mm and 2000mm. 1000mm is the most common and is supplied in 2m lengths you cut in half.

The way to spend least money with me is to have 3 rows of rods, leave the first half of formwork in place (with pairs of 6x2s tall enough for the whole wall), one nut on each end of each rod, no thin nuts.

You don't strike any formwork until the whole wall has hardened. You slice off the nuts and throw what's left away.

In this photo you see shorter lengths of 6x2, so the nuts need to be undone and the 6x2s slid up to the top for the top half of formwork.

self build basement formwork

The nuts grip the rods, so you need to use two nuts if you want to undo them. However, if you undo the nuts you can use that row of rods again.

And you can see it all here just before the second wall pour.

self build basement formwork

Except there are errors to avoid in this last photo.

  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.

    For the second pour he had extra bracing on the outside and over the top to keep the wall width correct, but it wasn't really necessary.

  2. 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.

I would:

Use thin nuts to control the width inside.

I would cut one thin nut, 5mm to 6mm, off one big nut as I need them. (Using a diamond blade and being careful no one breathes the dust).

Use 3 rows of rods, so the first pour has one almost-complete nut on the bottom and the middle has one almost-complete nut and a whole nut.

I undo the middle and for the second pour I have one almost-complete nut on the middle and I cut off another thin nut for the top so I have one almost-complete nut on the top row.

In that way, I avoid any obstruction across the opening along the top and my concreting is faster, easier and less messy - which is important because if there is anything I have forgotten I have the time to deal with it.

This is shortly before the first wall pour to about two-thirds height.

There are no obstructions along the top. He can swing his concrete pipe between the faces of steel through the top without having to lift it right up which would make a mess.

self build basement formwork


You should also plan good scaffold so the men can move around and poker safely. Make use of the triangular braces.

self build basement formwork

The slab concrete beneath the wall was scabbled and cleaned properly. The wall concrete will bond to it completely and the starter bars through the joint will prevent any cracks or leaks (Joints to BS 8007).

self build basement formwork

The blue lines are roof batten, Shorts lengths of batten with 18mm gaps between provide the holes for the threaded rods without drilling holes in the valuable timber that got sold on later.

When he sold smaller quantities to people who compared his price to that the timber merchants were quoting them, he got back what he paid despite a few screw holes.

The formwork begins with screwing two pieces of 4x2 together to make an L shape. Then, with two other lengths of 4x2, triangles are made and screwed down so that the plumb face of the L is a board thickness off what needs to be face of concrete.

I always use Screwfix M6 Turbo Coach Screws, which are put in and taken out just as easily with a cordless drill or impact driver, without damaging timber.

self build basement formwork

The formwork is held together with threaded rods and nuts.

I cut the nuts down to make thin nuts that go inside controlling the concrete width.

self build basement formwork   self build basement formwork   self build basement formwork

The first row of rods needs to be close to the bottom of the wall.

self build basement formwork

You cut through rods and nuts to remove the formwork.

If you don't cut much nut off, you will be able to use them again.

Or you can double up nuts increasing the chance of screwing them off. But unfortunately the frp is made never to come loose.

Elsewhere you will see how to use rods to get a slab level. If you do another basement you can save the offcuts of rod to re-use this way.

If you let the boards outside one wall fly by, you can use a piece of 2x2 to screw the two forms together to make corners strong.

self build basement formwork

You might avoid cutting some standard lengths of timber. You might want to make corners even stronger.

self build basement formwork

In some way, whether with ply or timber, you need to fix across joints in boards so the joints don't spread under the weight of wet concrete.

self build basement formwork

If you want more explanation than you found on this page and other pages you can use the "answer all your emails" service, by paying for it, or pay for a bespoke formwork design.

If you get to the end of this page and you are still tempted to hire in a formwork system be careful to understand the full cost of doing so. There are many reasons it will cost you more, for instance, hire longer than planned, damage charges, skills needed to get precise dimensions, they tell you to fill 3m high in sections = loads of leaks.

Or you might still fancy ICF. This is the page about why using ICF will cause you misery.

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