Choosing and Bedding a Lintel Over a 2.1 m Sliding Door Opening
You are on a bungalow, building a 215 mm cavity wall: 102.5 mm facing brick outer leaf, 100 mm cavity, 100 mm block inner leaf. A sliding patio door is scheduled with a 2.1 m clear structural opening. The two prestressed concrete lintels on site are 2.4 m long and are in the spec as suitable for the leaf and the load, which is the outer leaf plus the roof bearing on the inner leaf. The blockwork is up to lintel level. The actual formed opening measures 2.13 m because the right-hand reveal is 30 mm out. The site engineer has not been on site. There are two extra wall ties in the back of the van and the foreman says to 'just bed them in, there's plenty of bearing'.
1. Show how you check whether the available lintels are adequate as they stand, and what you would do about the 2.13 m opening.
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The lintel check has three parts: length/bearing, load capacity, and bearing stress.
Bearing: each end of the lintel must have a proper bearing on the masonry, normally at least 150 mm, and that bearing must be on full, solid masonry - not on the cavity, not on a cold bridge, and not on a half-block. A 2.4 m lintel over a 2.1 m clear opening gives (2400 − 2100)/2 = 150 mm at each end. That is the minimum, with zero tolerance. Over the as-built 2.13 m opening the bearing becomes (2400 − 2130)/2 = 135 mm at each end, which fails the minimum. The 'plenty of bearing' claim is arithmetically false.
Load: the lintel must be checked against the manufacturer's load table for the actual leaf, the actual span and the load above it - here the outer leaf plus roof load. An S2 mortar and the specified block strength tell me nothing about the lintel capacity, so I read the manufacturer's data, not the spec phrase.
Bearing stress: a prestressed lintel bearing 150 mm on 100 mm blockwork can exceed the block's local bearing strength if the load is heavy or the block is hollow. If the tables require it, a padstone or a solid concrete block is needed at each bearing.
So the answer is: do not lay. Regain the 30 mm on the right reveal so the opening is 2.1 m, or obtain 2.7 m lintels (300 mm bearing each end) from the engineer. Either way, get the engineer on site first.
2. Assuming the opening is corrected, describe how you bed the lintels and detail the brickwork and ties around the opening - and explain why each item matters.
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Bedding: lay a full mortar bed on the blockwork at each bearing, no dry packing, and check the lintel is level along its length and across the wall. If it is not level, the courses above will taper, and the load goes into one end. Check the cavity width is maintained at each end - the lintel should not squeeze the cavity closed, or it becomes a bridge for heat and water. Where the manufacturer's table requires it, set a padstone or solid unit under each bearing.
Level and alignment: I check the toe of the lintel so the door head has a consistent margin, and I check that the two lintels (if separate leaves) are levelled to the same line, because the door frame fixes to both.
Brickwork above: build the wall above to the same gauge, keep the cavity clean of mortar droppings, and do not bridge the cavity. If the specification requires a cavity tray over the opening, it must be fitted with the upstand correct and weep holes clear at the outer leaf, otherwise water that collects at the lintel tracks inward.
Ties: add extra wall ties within about 225 mm of the opening at the jambs, and at closer centres around it, because the reveals are where the cavity is at its most vulnerable and where wind load is concentrated. Ties must engage both leaves and be bedded in a full mortar joint, not dropped into the cavity - a tie in the cavity is doing nothing.
Significance: every one of those items stops a small detail turning into a cracked quoin, a leaking head or a dropped frame.