Frequently asked questions answered by the Porotherm Team here at Wienerberger. Scroll down for more subjects.


Q: Do I have to be / use a specialist to lay Porotherm?
A: No. Anyone can lay Porotherm. We will provide full training and support. Visit the ‘Helpful Stuff’ section (above) for helpful step-by-step guides.

Q: How fast is the system compared with traditional blockwork?
A: Speed is only one of the advantages of the Porotherm system but a good guide is 30m² per man per day – which compares with 10-12m² per man per day for traditional. The full speed comes from utilising the system as a whole. We are currently achieving a watertight shell for a normal size pair of semi-detached houses/large detached house in 10 working days, removing the brickwork from the critical path and enabling follow-on trades to be inside working in the dry while the brickwork is brought up afterwards.

Q: Can the blocks be cut easily?
A: Yes – with an alligator-type saw, wet-cut block saw or they can even be cut using a hammer and bolster or bricking hammer by scoring down each side before laying on its side an “snapped”. This is because it is only thin layers that are being cut and not a solid lump of clay. Visit the ‘Helpful Stuff’ section (above) for more information.

Q: How high can I build with Porotherm in one lift?
A: Unlike traditional build, Porotherm is not limited to 1200mm maximum build height. It is quite normal for a storey height to be built in one day. However, consideration should be given to levels of wind exposure, lengths and heights of walls containing openings etc in determining whether temporary or permanent support is required.  In most cases, the external environment and safety must be considered. Propping in adverse weather conditions is recommended, as per standard practise. With a compressive strength of 10N/mm² it can be used for 4 storey buildings with concrete floors, which is the maximum height for structural block solutions due to progressive collapse. Individual buildings would always need to be checked and confirmed by the project engineer.

Q: Can Porotherm be used below dpc?
A: No. Porotherm is for use above ground only.

Q: How do I construct my party walls using Porotherm?
A: Porotherm has an approved robust detail (EWM-25) for the construction of party walls. This provides approx. 58db’s, way over the minimum requirement of 45!

Q: When building with Porotherm how level does my levelling (kicker) course need to be?
A: The plumb and level of the base course built off general purpose mortar needs to be very accurate. A straight edge run across the bed face of the finished levelling course should not show any unevenness between blocks. Time and care spent on this detail will pay dividends throughout the rest of the build as any error is not easily rectified using 1mm bed joints.

Q: Are Porotherm blocks laid using half bonding?
A: Porotherm blocks’ bonding requirement is no different to other masonry of comparable bed height. Design standards require a minimum bond of 90mm but at corners or junctions, the overlap of the units should not be less than the thickness of the blocks.

Q: How does Porotherm course in with the face brickwork?
A: Porotherm is a 224mm block with a 1mm bed joint, therefore it courses in at 225mm, the same as using a 215mm block with a 10mm traditional mortar bed.

Q: Ok what is the catch, this seems too good to be true?
A: Honestly as long as the site is organised and the brickwork contractors are competent it is very easy to achieve the full benefits of the system.

FAQ - jointsJOINTS

Q: At what centres do movement joints have to be provided in Porotherm walls?
A: Movement joints are required in Porotherm walls at not less than 20m centres. This means that majority of properties will not require any vertical movement joints when constructing with Porotherm.

Q: How do I incorporate a cavity tray membrane into a Porotherm thin bed joint?
A: The membrane should be placed onto the mortared bed joint and the blocks above mortared away from the wall and then placed so the membrane is sandwiched between the mortar joints.

Q: As the joints are dry surely it does not perform well with air tightness testing?
A: Once constructed, the Porotherm walls are coated internally with a product called Ecoparge. This is just like a white emulsion and applied very simply using a soft broom or roller, ensuring that all joints are fully covered. Once applied the wall achieves an air tightness of 1.2m³/m²/h.

FAQ - mortarMORTAR

Q: When mixing Porotherm Zero Plus mortar, do I need to clean out of the mixing tub remnants of old mortar.
A: Yes. It is good policy to do so as any hardened mortar particles, if introduced into the new mix, will disrupt the thickness of bed joint when placed on the block and provide ‘kick-up’.

Q: Porotherm has tongue and groove perpends. Do they need pointing up with mortar?
A: The tongue and groove system is an aid for alignment not bond. Only perp ends with visible daylight through will need to be pointed up with general purpose mortar. Porothem walls are sealed for air tightness using a thin coat of Eco parge after construction.

Q: When applying the Porotherm Zero plus mortar – will most of it fall into the voids thereby increasing wastage?
A: No. The roller applicator is designed to place mortar on the bed face surface only.

Q: Due to the thin mortar joints in Porotherm construction, do I need special wall ties, and at what spacings?
A: Wall ties for use with Porotherm construction are readily available from all of our distributors. Spacings are no different to other masonry constructions, unless the tie manufacturer state otherwise.

Q: In certain detailing there are short lengths of Porotherm blockwork that I am unable to apply mortar onto with the roller, can I use my trowel to apply the bed joint.
A: No. Accuracy of mortar bed depth will be lost and this will introduce level error into the build. Blocks for use at these details should be mortared prior to placing by turning them over, putting them onto a flat area of ground and mortaring with the roller. The blocks should then be turned over and placed into position within the detail.

Q: Does the mortar drip down in to the cells of the block?
A: No, when applied by the roller correctly it just sticks to the surfaces that it touches leaving an even 1mm mortar layer over the whole bed joint.

Q: How do I bed the wall plate?
A: Exactly the same as you’ve always done, on traditional mortar. This also applies for lintels.

FAQ - weather2WEATHER

Q: When working in winter what is the minimum temperature requirement for building with Porotherm?
A: The mortar can be used at Zero degrees C on a rising scale. However good site practice should be employed to ensure Porotherm blocks are kept protected from sub-zero temperatures as the thin layer of mortar will soon become the same temperature of the block once bedded.

Q: Can Porotherm be used in wet weather?
A: Like other forms of masonry construction, building should not take place in heavy rain. Construction with Porotherm can be undertaken in light rain but care should be taken not to leave extended runs of bed mortar exposed. The mortar tub should be covered to protect it from excess water which will affect its workability and setting properties.

Q: Can I add retarders to Porotherm Zero Plus mortar when working in hot weather?
A: No. Porotherm Zero Plus is a mortar specially designed to provide both workability and early strength in Porotherm construction. Any additives will alter the mix properties and design strength. In hot weather the mortar tub should be placed out of direct sunlight and excessive runs of uncovered bed mortar be avoided.


Q: How do the ties work –  as the bed joint is only 1mm thick?
A: Specific ties are required for the system. Manufactured by Ancon they are a 2 or 3 part tie with a flat plate that is built in to the bed joint and the outer or middle section is simply hooked on to a small loop which is left protruding from the block-work. All ties are placed at the same number per m² as traditional block-work.

Q: How easy is it to fix in to the Porotherm blocks?
A: Very simple, you just use a masonry bit but with a percussion drill action which will give you a perfect whole to the diameter of the drill bit. Then fixings for hollow blocks should be used rather than the usual solid block wall plugs which are readily available and manufacture by the likes of Spit, Rawl or Fischer.

Q: When building a Porotherm inner leaf without following up with outer brickwork, how do I support cavity lintels?
A: Our stockists can supply a range of 2 part lintels designed for this use. The outer face of the lintel only being connected to the inner face at the time of brickwork construction. Alternatively, box lintels can be used on the inner leaf of Porotherm with an angle plate lintel on the external leaf.

Q: What sort of lintel do I need?
A: Porotherm has a two part lintel system that all major lintel manufacturers produce. Just tell your supplier that you building using Porotherm.

Q: Is it easy to fix to Porotherm?
A: Extremely easy. Porotherm drills very accurately, with the use of a masonry bit, but without the use of any percussion. A compressible fixing, as opposed to an expandable fixing is then used to secure any component.

Q: How do I fit joist hangers into the Porotherm thin mortar joint?
A: Joist hangers cannot fit within a 1mm bed joint, so the block above will need to be recessed to allow the thickness of hanger. Excess recess will need to be mortared using general purpose mortar. Always ensure that the joist hanger sits on a whole block.


Q: Can I render directly onto Porotherm?
A: Yes. Please refer to our Technical department for advice on recommended renders for given areas of wind driven rain exposure.

Q: Do all Porotherm internal walls that are to be dry lined have to receive a coat of Eco parge?
A: Yes. Although there is no requirement for air tightness it provides additional sound insulation benefits.


Q: Can I complete the carpentry work to the roof?
A: Yes. The only consideration being – how will the brick layer complete the brickwork if soffit & facia is in place? A cloaking strip can be fitted to the underside of the soffit in this case.

Q: Can I fully load out / and or tile the roof?
A: Yes the only consideration being the completion of the roof carpentry works. It may be beneficial to only fly batten the roof until the external brickwork has been completed and soffit & fascia installed.


Q: How does the system perform with the Code For Sustainable Homes?
A: All code levels have been achieved using Porotherm and there are particular advantages with our modelled Psi values and thermal linear bridging which may enable cost savings to be made in other areas of the building.

Q: How do the blocks perform thermally?
A: With a Lamda Value of 0.29 for 100mm and 0.26 for 140mm & 190mm it performs better than dense concrete but not quite as well as aircrete blocks.

Q: How does Porotherm perform acoustically?
A: Very well due to its cellular structure and considering its low density. 100mm achieves the 40 dBRw partition wall requirement and our separating wall EWM-25 achieves an average of 56-58 DnT,W + Ctr dB.