End-grain timber block flooring: specification, installation and movement

At a glance

Sources: a UK distributor (Nov 2022), NWFA's magazine, our range pages. None is a standard.

There is no standard: what we read

There is no industry standard for installing end-grain block flooring. Trade press has said so (from our research notes: we could not read that article, because the site blocked automated access). We found no British Standard written for it either, though we did not read the parquet product standards or BS 8201:2011, the UK code of practice for wood flooring. What does exist is thin: a distributor's two-page installation sheet, and articles in NWFA's trade-association magazine. A distributor is not an independent trade body, and a magazine article is not a standard.

So the primary source for any one floor is the maker's own instruction. End-grain blocks are a product class that the partner workshops make themselves, so the workshop should sign the specification. Treat everything below as the questions to ask and the numbers to compare. Where the two sources we could read disagree, the page says so.

Why it is different: movement in two directions

Timber moves most across the grain and hardly at all along it. In a plank the grain runs along the length, so the floor moves across the width. In an end-grain block the cut end is the face, so the block moves in both directions across the face, along a radius and along a ring, and hardly at all in its depth. NWFA puts it plainly: it shrinks and swells on all four sides, unlike a plain-sawn plank, and the blocks equilibrate quickly. The US Forest Products Laboratory adds that water enters most rapidly through exposed end grain.

For a 3 point change in moisture content, say from 8% to 11% between a heated winter and a humid summer, our arithmetic from the Wood Handbook's white oak values (radial 5.6%, tangential 10.5% from green to dry) is:

Our arithmetic: shrinkage % = tabulated % x change in moisture content / 30, as the handbook allows; white oak, not European oak; a guide to scale, not a prediction.

A plank floor moves one way, across its width. A field of end-grain blocks moves both ways, so a large field has to be given room in both directions, and this is why a perimeter gap and a stable climate matter more here than on a plank floor.

Three constructions, one name

Three constructions are sold under the name, and the specification differs for each. The drawing shows A and C in section: the blocks in A are glued one by one; in C the thin top layer sits on a ply carrier that is itself bonded to the subfloor.

Which one you are specifying changes the answers below: how much movement the joints take, whether a filler is sensible, how many times the surface can be sanded, and what the heating allows. The studio's end-grain ranges are made by partner workshops: see Forêt, Élément, Cube and Opus.

Site, subfloor, fixing and heating

The distributor's guidelines ask for the building to be weathertight and the wet trades dry, the room held at 45 to 65% relative humidity and 15 to 25 °C from delivery and for the life of the floor, and the blocks stored in the room for at least 72 hours, off the walls and radiators and on battens. The subfloor is to be clean, dry and flat to SR1, and the blocks fully bonded to a cementitious screed, a glued and fixed plywood subfloor, or an adequate fully bonded underlay, with a flexible adhesive and the notched trowel the adhesive maker names. Blocks are to be taken from three separate packs, and the most critical time is the installation and the 48 hours after.

NWFA's article differs in one important way: if solid end-grain flooring is delivered at the moisture content its maker requires for the in-use conditions, and those conditions are kept, no on-site conditioning may be needed. It asks for records at delivery, a wood subfloor no more than 2 percentage points wetter than the flooring, and a Class I vapour retarder over concrete on or below grade. If conditioning is needed, it suggests separating the blocks into small lots and restacking them on edge in the building's in-use conditions, because blocks equilibrate quickly. Our sheets on acclimatisation and site conditions and subfloor requirements set the numbers side by side: write moisture readings into the specification, not a number of days.

Underfloor heating. The distributor says end grain is compatible with some systems. The system is to be commissioned, run for a full cycle to a maximum floor temperature of 27 °C and left running for 2 weeks before fitting, with floor probes connected to room thermostats; after fitting the temperature goes up by no more than 3 °C a day to 27 °C; the heating is never on while the floor is protected. Our own range pages are not uniform: one says compatible, one suitable subject to substrate and acclimatisation, and two that it is confirmed with the maker for each project, which is the safest wording. See underfloor heating under wood and stone.

Gaps, filling, sanding and finish

The sources split on gaps. The distributor accepts a gap round each block, which opens and closes, plus a perimeter gap of 1.5 mm per metre with a minimum of 10 mm, subject to site conditions; our wood flooring installation guide asks for at least 15 mm at the perimeter and every vertical obstruction. NWFA's article says that where a building cannot hold a steady climate, end grain often needs 3/4 in (19 mm) at the perimeter and at vertical obstructions and also inside the floor, commonly 1/8 to 1/4 in (3 to 6 mm), and that a slight gap between blocks is not unusual when the floor is fitted in the heating season. Our range pages describe joints under 1 mm on a ply carrier. These are not the same floor: ask the workshop what the carrier restrains, where the movement goes at the panel edges, and what to expect in a dry winter.

Filling. The distributor sands the floor flat, fills with a resin mixed with fine sawdust, fine sands and finishes. Our range pages offer a brown or black filler in the joints. A rigid filler in a joint that opens and closes can crack or be squeezed out: ask the maker which filler, when it goes in and whether it suits a floor that moves.

Sanding and finish. The distributor leaves the floor for 10 days after fitting before sanding, then sands through the grits, fills, fine sands and finishes to the finish maker's instructions. Two practitioners quoted in NWFA's magazine say the dense end grain is hard to get flat and smooth, so the fitting should be as flat as possible to limit the sanding, and that it is a ‘thirsty’ product. Ask how many times a thin top layer can be sanded. Care: our wood flooring guide says an oiled end-grain surface can be refreshed in worn areas without sanding the whole floor, and our range pages say a varnished surface is swept or vacuumed and wiped with a barely damp mop. No re-oiling interval is in any source we read: ask the maker for a schedule.

End-grain [oak] floor, construction [loose blocks / blocks in strips / pre-assembled tiles on a [ ] mm ply carrier with a [ ] mm top layer], block depth [ ] mm, face [ ] x [ ] mm, laid in [half bond / pattern ref] to the workshop drawing [ref]. Finish [oil / varnish, ref]. Maker's installation instructions [ref] form part of this specification and prevail.

Subfloor [cementitious screed / plywood] flat to [SR1: no gap over 3 mm under a 2 m straight edge] and dry to the maker's limit of [ ], tested and recorded before delivery. Building weathertight and wet trades dry. Room held at [15 to 25] degrees C and [45 to 65] % relative humidity from before delivery for the life of the floor. Block moisture content recorded on delivery; conditioning [as the maker instructs: ref].

Fully bonded with [flexible adhesive named by the maker], full coverage, notched trowel [ ] mm. Blocks drawn from at least [three] packs. Perimeter gap [ ] mm, covered by [skirting / shadow gap]; movement provision inside the field [none / as drawing ref]; gaps between blocks [ ] mm at fitting. Joints [left unfilled / filled with [ ] after [ ] days]. Doorway gaps covered by [T-section].

Underfloor heating [none / system ref], commissioned and run to a maximum floor temperature of [27] degrees C for [2] weeks before fitting, probes [ref], raised after fitting by no more than [3] degrees C a day. Floor left for [10] days before sanding. Sanding sequence [ref] agreed on a sample. Care and re-finishing schedule supplied by the maker at handover.

Standards and sources

• No industry standard for end-grain block installation was found: BS 8201:2011 (code of practice for wood flooring) named; not read. Parquet product standards not read.
• A UK distributor's end grain installation guidelines (Nov 2022): read in full; not a trade body.
• NWFA, Hardwood Floors magazine (2017, 2019): read.
• USDA Forest Products Laboratory, Wood Handbook ch. 4 (FPL-GTR-282, 2021): read: Table 4-3.

Questions we are asked

Is there a standard for end-grain block flooring?

None that we found. Trade press says there is no industry guideline, and the only technical sheet we could read is a UK distributor's two-page installation guide. We did not read BS 8201:2011, the UK code of practice for wood flooring, or the parquet product standards. Treat the maker's instructions as the primary source and have the partner workshop sign the specification.

Can end-grain flooring go over underfloor heating?

The distributor says it is compatible with some systems: commissioned, run to no more than 27 degrees C at the floor for 2 weeks before fitting, then raised by no more than 3 degrees C a day. Our own range pages are not uniform on this (compatible, suitable subject to substrate, or confirmed with the maker project by project), so ask before you specify.

Why do gaps open between end-grain blocks?

A block moves along a radius and along a ring, so it moves in both directions across its face. On our arithmetic from Wood Handbook values, a 100 mm oak block changes by about 0.6 to 1.1 mm across the face for a 3 point change in moisture content. The distributor calls gaps round each block acceptable and says they open and close with the seasons.

How long should the floor wait before sanding?

The distributor says 10 days after fitting, then sanding through the grits, filling, fine sanding and finishing to the finish maker's instructions. A thin top layer limits how often a floor can be sanded: ask the maker how many times.

Does end-grain flooring need acclimatising?

The distributor asks for at least 72 hours in the room. NWFA says that if the blocks arrive at the moisture content the maker requires for the conditions in use, and those conditions are held, no on-site conditioning may be needed, and that blocks equilibrate quickly. Record moisture readings; do not rely on a number of days.

How thick should the blocks be?

No standard says. Examples: oak 19 mm thick in a reported strip project, and a 6 or 8 mm top layer on a 10 or 12 mm ply carrier in the studio's range pages (the makers' figures). State the depth, or the top layer and the carrier, in the specification.

Can the joints be filled?

The distributor sands, fills with a resin mixed with fine sawdust and fine sands the floor, and the studio's range pages offer a brown or black joint filler. A rigid filler in a joint that opens and closes can crack, so ask the maker which filler suits the construction and when it goes in.