self build waterproof basement formwork membrane concrete rods

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

Everyone else leaves holes through otherwise waterproof concrete.
  self build basement

What does everyone else specify and build?

Seasoned professionals will all use steel threaded rods in a plastic sleeve. They leave the plastic sleeve as a hole through otherwise waterproof concrete; or else they fill the holes but some leak.

I was sent this first image by a prospective client looking for a different basement builder, so I don't know where he found it. He is planning a second basement and he says his first basement leaks - so perhaps this is it.

typical basement leaks

basement repair

Many hole repairs leak
  basement sub contractor left 400 holes

caltite leak

Everyone else leaves holes through otherwise waterproof concrete.

What should you do instead?

Threaded FRP rods and nuts. They 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 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 buy 1m FRP threaded rods costing £3.50 and nuts £2.00 each.

There are many variations how to use them.

Rods into piles.

This timber is largely scaffold board seconds and 4x2 and 6x3.

Here, the far wall has 4 rows of rods to pour the first 1850mm high because the rods were resin anchored into piles, and the pile concrete is the weakest part of the arrangement and we did not want the rods to pull off large chunks of pile concrete. This means the piles were the formwork one side and the timber the formwork the other side.

This represented a huge saving compared to hiring in single sided formwork.

The near wall was cast when the soil was still excavated and there were three rows of rods for the same 1850mm high concrete pour.

The top row of rods is being used a second time as the bottom row for the top pour. None of the timber was disturbed. It meant using a lot of timber once but it was to be used in the house anyway.
4 rows of rods

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.

But most customers since have used only one nut and undid them without too much trouble using a pipe wrench to hold the rod and an adjustable to undo the nut.

If your basement is narrow enough you can prop one side to the other with temporary joists and stand on those instead of staging. half height formwork

This next customer used three rows of rods using the middle row twice. He used nuts in pairs. His first pour was 7 scaffold boards not 8 so 1625mm high.

It turned out to be a mistake having the first row of rods two scaffold boards off the bottom. The bottom row should be above one timber plank only.

Fill the wall to just over half height before continuing.

The middle row of rods should be just below the top timber plank of the first pour. You can see that when the formwork was built to the top for the second pour that the timber strongbacks were lifted up and used a second time.
self build formwork basement

In due course customers complained about buying enough timber for all the walls and dismantled the timber after the first pour then rebuilt it for the second pour.

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.

Stripping the formwork and using it again higher up caused the second formwork to push apart slightly so that grout from the second pour trickled down the first. This is not good, it means that the concrete has lost grout and the stones somewhere have less cement bonding them.

I have found that if the top board of the first pour has screws through it, it can remain fixed to the concrete when the bottom formwork is struck. Then, the top formwork can be built off the fixed board and when it spreads slightly it doesn't leak like it did in this picture.

I would add that this issue is common to all formwork and 'industry specialists' often lose grout at the bottom of their walls against their kicker.
waterproof concrete basement resin rod formwork stripped

That looks 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.

You can call me and discuss your choice if you like.
concrete snots

All design is a compromise. If you are going to cover your wall later you might come to a different conclusion than if you are going to see your bare concrete.

This basement went well except we had to clean the snots off. That took only a day or two.
  1. If you form and pour a wall to full height in a single concrete pour, say 2.7m high, the nuts on the lower rods will bind on too tightly and need cutting off and discarding.

    The benefit would be you avoid the messy appearance in the photo above. But basement walls are usually covered and the 'snots' as they are aptly known knock and scrape off fairly easily.

    But the disadvantage would be voids through the concrete toward the bottom of the wall. Leaks and no guarantee because you poured too much in one go to get the concrete to the bottom in good condition and too far to get the poker down repeatedly all the way round because it is too heavy to do everywhere properly..

  2. To prevent leaks, repairs and internal drainage, walls need to be poured in two pours. Bottom no more than 2m high. The rest to the top.

  3. If you pour the wall in two goes: bottom half then top half, you are likely to get single nuts off and no leaks through the concrete.

    We experimented recently with fixing timbers to the top of the first pour of concrete before continuing up with concrete so that when the top formwork expanded slightly the fixed timber beneath it would reduce the snotting.

Rods. £3.50 per m. All 1m long.

Nuts. £2.00 each.

Thin nuts. The cheapest option is to cut 6mm off a 70mm nut yourself. I can do that for you for £0.30 a cut or I can cut a big nut into 4 thin nuts for £3.20 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 and the nuts 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 use the rod a second time, because two nuts each bind on a lot less than a single nut.

You can only use the rod and the nuts again if you used nuts in pairs.

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. Join rods with a whole 70mm nut.

frp threaded rod stronger than steel


I buy rods 5,000 at a time and nuts 10,000 at the same time. I have never run out of stock.

From the beginning of 2021, I VAT registered Basement Expert Ltd (Co. No. 09745193 Registered in England and Wales).

The limited company will supply all products and add VAT at 20%

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