self build waterproof basement formwork membrane concrete rods

Threaded FRP rods and nuts - leave no holes through waterproof concrete.

frp threaded rod

frp threaded nut
  frp threaded rod

The rods are sliced off flush when they are finished with. In the photo above, the team used roofing batten to create holes for the rod without cutting the timber.

Whereas in the photo below a different team chose to notch their timber instead.

bare concrete wall regularised timber

Traditionally and still, formwork carpenters use a steel threaded rod through plastic tubes to hold formwork together.

They can wind them up very tight and undo them easily.

When the formwork is struck they re-use the steel rods again and again.

But they leave holes formed by plastic tubes.

Holes in otherwise waterproof concrete is fairly ridiculous.
  filling up dwvidag holes in concrete   dywidag leak   This is a threaded bar hole repair in Caltite waterproof concrete that did not work. Other similar leaks in this Cheshire footballer's new mansion with underground football pitch were behind plasterboard.

The most expensive projects are plagued with problems.
  1. It costs the sub contractor a lot of money to fill in hundreds of holes.

  2. It causes the main contractor a lot of worry - were any of the holes missed?

  3. It costs the client a huge headache in costs and delays when the building is ready but there is water coming in from behind the plasterboard. But where is the leak? How much plasterboard do you have to take down to find it?

Much simpler and cheaper, therefore, to use a single-use rod and nut that will be waterproof for all time, every time.

I regularly get calls from people about leaks in new basements. Last month (January 2018) an architect was telling me that his last basement project cost £1million and they were chasing leaks for a whole year.

Much easier and better to use FRP threaded rods costing £2.50 per m and nuts £1.50 each.

There are many variations how to use them.


half height formwork

3 rows of rods.

The first formwork should only cover the middle row of rods by no more than one width of timber. This is the best compromise between only pouring half and making it as easy as possible to clean the top of the concrete to form a waterproof joint.

Fill the wall to just over half height before continuing.

You can continue the formwork to the top as soon as you have washed off or scabbled the top of the first concrete that will form the joint.

Do not disturb the first formwork until the whole wall is filled and given at least 48 hours to get some strength.
self build formwork basement


4 rows of rods.

Here, the first pour had 2 rows and the second pour will have two additional rows.

This particular part of this basement project is under the double garage, so there is to be a cast insitu RC slab over the basement the footprint of the garage.


All these rods have two nuts on each end. This was only the second time we used these rods. The first time we discovered we could not undo a single nut without twisting and splitting the rod. So we were trying two nuts to see if they undid.

Two nuts will undo but only with 2 spanners and care.

We have found it cheaper, considering the cost of labour, to use one nut once, cut it off and throw it away.
4 rows of rods


Pour the concrete to half height or a little bit higher.

Wait for the concrete to gain some strength, minimum 48 hours.

Strip the timber, build it again to the top.

It might save timber but you might have trouble unwinding the nuts.

The reason we would not strip the formwork and use it again higher up any more is that it seems impossible to stop the second formwork pushing apart slightly so that grout from the second pour trickles down the first.
waterproof concrete basement resin rod formwork stripped

That would look a bit messy.

A more serious concern is that your top concrete is supposed to be waterproof. But it might not be if it lost too much grout.

I do not want you to make the mistakes I did learning how best to use the FRP threaded rods and timber formwork.

The only way to do that is to show you what went wrong to lead us to how we do it today.

The first option, above, is usually the best. You can call me and discuss your choice if you like.
concrete snots

The most popular way to use nuts is just one nut on each end of the rod and to cut that off with a mini angle grinder and thin blade when the formwork comes down.

Rods. £2.50 per m.

My stock is mostly 1m long. I have a few hundred 1.33m long.

Nuts. £1.50 each.

Thin nuts. £0.30 or £0.60 each. See the thin nut page (next) for full details.

07773 377087 or

But don't cut the rod and nut off too quickly.

Plan what you do next.

If you have a corbel to form, a concrete roof to pour, or anything else that would benefit if you already have a threaded rod cast securely in the wall below, use 2 nuts so that you can get them both off and use the threaded rod again.

2 frp nuts   reuse frp rods

These rods are stronger than steel in tension but they twist easily and the nuts are designed to bind on. That is why we use two nuts if we want to unscrew them or, more usually, chop them off.

Here you can see that we formed a chimney breast base in solid concrete. A terrific waste of concrete but the customer's structural engineer insisted it was the only way.

It is not the only way. I would usually expect the base slab to be bigger under the chimney breast, the retaining wall the same width alongside it and this mass of concrete formed with cheap concrete blocks.

However, you can see how rods can be used in two directions which is often useful forming odd details.

frp threaded rod stronger than steel


stock frp rods stock frp nuts

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