Screeds,Damp Proof Membranes,Costs and Problems

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In general terms the screeds used in Vinyl Floor laying are classed as smoothing underlayments,as opposed to other types of screeds used in the building industry such as sand and cement etc which are more commonly thicker and sometimes laid to falls, which is quite a different skill.The smoothing underlayment screeds are usually no more than 3 mm thick.They are most commonly either Water based or Latex based but the costs can vary quite considerably from manufacturer to manufacturer and the old adage you only get what you pay for generally applies.
Water Based Screeds
As the name would suggest these consist of a bag of powder,usually about 20-23kg which is then mixed with a set quantity of water in a mixing bucket with an electric mixing paddle.Water by it's very nature is self levelling and so this type of screed is generally used where there are large open areas of flooring,in big department stores etc on new subfloors where there are no old adhesive or prior floor residues but where any variations in floor height would be more readily noticeable.Also when high quality Amtico or Karndean flooring is specified.However because it is water based,the concrete subfloor must be primed prior to the application of the screed.Although not in itself a massive expensive it does add to the costs.
Latex Based Screeds
This type of screed is supplied in two parts.A bag of powder as before but instead of water,a bottle of Latex is supplied and added.The quantities are pre set so with a thorough mix there should be no inconsistencies in batches.This type of underlayment ordinarily doesn't need a primer and generally is a much tougher form of floor screed which can be laid over old floor adhesive residues etc.with more confidence.
 Under normal circumstances on a new concrete floor a unit of screed (approx 23kg) whether it is water based or latex based,can usually cover about 5m2 of floor at a depth of 3mm.Where the cost variations occur is that a unit of screed, can vary from £7.50 to £21.50 for technically the same thing from different manufacturers.So that for instance a good 40m2 kitchen diner which would need 8 units of screed can vary in the cost of the material only, from £60 - £172 which is a colossal difference when we are pricing to obtain work.In addition if the subfloor is not new and in poor condition,i.e not flat then instead of the 5m2 per unit of screed coverage,this can fall to 3 or 4m2 per unit,thus increasing the quantity required and hence the cost.Also some floor layers can just apply a "wash " of screed.i'e thinner than the advised 3mm and so possibly only 3 or 4 units of screed are required.Also most floor layers add extra water to the latex mixes to minimise trowel drag. So on the same piece of floor using a cheap screed with only a quick" wash" the costs would be say 4 units of screed at £7.50 i.e £30. The same floor using a top quality screed with a full 3 mm thickness would cost 8 units at £21.50 i.e £172.So a variance of £30 to £172 and these are costs without profit margins added on,and they can make a significant difference in whether an estimate is competitive or not.
So what do you get for the difference?Not being a chemist I don't know exactly what the different manufacturers use to produce their products and thus their costs.But we have used most of them at some time or another.From my point of view,as an owner of a flooring company I am responsible for the floor staying down and useable.Any problems and it becomes a cost.When a floor fails the costs can be enormous.Imagine a Marks and Spencers floor screed failing and a large floor area suddenly becoming unusable.Imagine a Hospital Operating theatre becoming out of use for any length of time.It could be ruinous.However all jobs don't carry that risk but it does tend to make flooring companies wary of using anything but the best.In our Industry Ardex is generally regarded as one of the foremost manufacturers of screeds etc.Whether its water based eg K15 or a latex based screed eg NA they are my preferred choice.Old floor layers will tell you that when you mix a good screed then your mixing bucket needs to be washed out and cleaned almost immediately otherwise if left until the next day the bucket is more or less useless.However when using a cheap screed a good bash with a hammer will clean the bucket of hardened screed days later.i.e the good stuff stays put,the cheap stuff breaks away.
If you add in a few complications in the subfloor,eg dusty concrete,damp concrete,floor paint,grease,damp proof membranes etc,etc it sometimes just isn't worth trying to save money on a cheap screed.
Damp Proof Membranes 
In new buildings where the conrete is green or freshly laid  the moisture in the concrete mix dries out roughly at a rate of 1 inch per month.Even though concrete sets within a day or so there is still moisture in the hardened subfloor.Because all new buildings must comply with building regulations and have a damp proof membrane ,the moisture in the mix can only evaporate upwards.So a 6 inch concrete floor in good weather conditions will take approximately 6 months to dry out.Conversely in some old buildings there are no damp proof membranes at all and moisture will be continuously rising up through the concrete.
Having established that a damp proof membrane is required there are quite a few to choose from.Nowadays most damp proof membranes come in the form of a two part epoxy mix.The quantities are pre set and the chemical reaction is instant and consistant.However the application can vary from floor layer to  floor layer.There are two coat membranes and single coat membranes.Each coat requires 24 hours to dry and time is money.Each return visit needs to be charged for.Consequently single coat membranes are preferred.The DPM is liquid and is spread on the floor with a notched trowel to give an even quantity across the floor.Once trowelled on, it is then rolled to give an even covering.The DPM itself is very expensive to buy,usually about £180 per 10kg unit.This will cover about 20m2 at the correct thickness.However if you need 30m2 you've still got to buy another unit and you can't mix half a batch,it's the whole lot or nothing and once mixed it cannot be stored or used at a later date.So you either pull out the first mix,i.e you roll it thinner or you do it correctly and apply a second mix.The cost difference is obvious.£180 immediate extra cost before labour and profit.The result is you appear uncompetitive.However the floor will be water proof and ready to be screeded and there should be no future damp problems and even if there are,a correct application thickness will ensure that it's not the floor layers problem,but possibly the manufacturer.
Sometimes because of a poor subfloor and the high cost of the damp proof membrane it becomes more cost efficient to sandwich the membrane between 2 screeds.i.e make the floor flat to use less DPM.On just such occasions it's doubly important to use high  quality screeds that can accept this procedure quite easily.Because some just can't and failure of a screed or membrane can be an expensive failure,because usually everything needs to be replaced.Better just to get it right first time and use good quality screeds and membranes.
Here's a quick video diary of a DPM we've laid.

Specialist Installers of Karndean,Amtico,Vinyl Flooring,Safety Flooring and Carpet Tiles throughout Newcastle Gateshead,Durham and Sunderland.