Shops for Woodframe House Prefabrication

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SerNA7110N21h8

no. 24c. 2 National Research Council

CanadaDivision of Building Research

H O U S I N G N O T E N O . 2 4

SMORS FOR WOOD-FRAME

BYR. E. Platts

Reprinted from Canadian Builder,Vol. XIV, No. 12, December 1964, p. 27-29

Ottawa, December 1964

This Note may be reprinted without amendment provided acknowledgment is given to Canadian Builder.


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f / G UW 1 A PREL/M/nA4Y LAYOUT: 1gOOD FRAME KOUgE PLANTa p p ~ o x / m ~ eAPACITY: 3 toy UOUSES/DAY, SINGLE SMFT

Shops for wood-framehouse prefabriicafionionThe availability and low cost of

equipment for prefab shops (com-pared to factories producing anyother "major appliance") may encour-age builders and dealers to jump intoprefabrication with little hard thinkingon the actual role of the shop. Thedominant aspect of wood-frame houseconstruction, with its many trades andhundreds of varying pieces, is materi-als handling. This is the aspect that asimple, well-planned shop can im-prove most radically, both for shopand subsequent field operations, aswas emphasized in the first note inthis series. Setting up a small shed ortwo and a bit of yard as a prefab"plant" does not readily allow ration-alized handling and logistics, and theoperation may gain little or nothing.An industrial engineering study ofwood-frame prefabrication is badly

needed, but apparently none has yetbeen attempted. The publication "Pre-fabrication in Canadian Housing",(NRC 7856) attempts to draw con-clusions from the many impressionsgained on diverse shop practice in thesurvey across Canada. It also pre-sents some discussion on productionaspects of stressed skin, structuralsandwich, and certain other innova-tions in house building. This note con-siders these impressions only as theyaffect builders' needs at present, deal-ing with wood - frame constructionalone.LAYOUT

The term "factory" always suggestsa picture of a smooth "flow-line" or"assembly - line" layout, and indeedmuch of our economy is based uponthe productivity attained by flow-lineproduction of repetitive units. With

wood - frame house production, thenumber and diversity of parts and thefluctuation in sizes, styles, and pro-duction volume make it very difficultto maintain a workable balance if atrue flow-line layout is used. Oneproduct will "pile up" on the next;one process will fall behind the others.It should be best to use "layout byprocess" for most measuring, cutting,and marshalling, and to lead this intoflow-line layout for handling and as-sembly. Figure 1 shows a hypotheti-cal plant layout incorporating theseand the following suggested points.

Materials handling is all-important,but wood-frame production does notlend itself to complex, fixed handlingequipment. It involves intermittentmoves, many paths and cross-paths,and bulky materials with some feasi-bility of unit loads. Simple handtrucks fit these requirements best ofall. Hand trucks do require largequarters and storage areas, but theprefab shop must be spacious and welllaid out to allow high productivity.

Lift trucks can best serve the out-


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side storage areas and sheds, but theyneed not work within the assemblyplant itself, except to serve th e cuttingand marshalling area near the receiv-ing end. Most of the han d trucksshould be fitted with ap propriate racksso that they can be loaded with theprepared an d cut mater ials within themarshalling area, and then can bemoved to the assembly area where thehand trucks themselves become thestorage racks at the lines. Transfersar e sharply minimized in this way.

Figure 1 suggests that a transverserun of a light overhead crane (lessthan 10-ton capacity, and of about40-ft. span) can allow its use for allloading of large parts and for trans-ferring heavy items such as windowsfrom sub-assembly to wall assemblylines.ASSEMBLY JIGSThe assembly jig is the coreof wood-frame prefabrication. Themarks of a good jig and its "feeder"racks are simple: the man at the tableshould rarely have to look for piecesor parts, alter them, look at draw-ings, use a tape or hold pieces inplace. All parts should have a placeat hand in suitable racks (the rackscan best be on the hand trucks, avoid-ing transfers). Th e racks should bemarked with the marking keyed tothe foreman's working plans and tothe "cut-off" sheets. Th e markin gshould include the number of par tsper house to facilitate the orderly re-fill of racks. These simple points areimportant par ts of optimum produc-tion in wood frame-as impo rtant asthe part that can be played by highcost mechanization.Although some use quite complexjig tables with hydraulic "popout"equipment, it appears that the verysimple open jig tables work as wellas any. These use guides or railsalong the edges only, the guides setslightly wider apart than the width ofthe wall o r partition section, with theflat table between them. Wood enwedges or other stops are readilydriven into the gap to secure theplates against the studs for nailing,sheathing, etc. "Jig sticks" can be laidalong the edge guides to allow thefast location of framing and openingsfor any model of house. These arelong wooden sticks or tapes which are

made in sets for a particular housemodel.Co n~p licated and expensive "flop-over" rigs are sometimes installed toturn the wall or partition sections over,al lowing them to be completed onboth sides while o n the flat. A se-quence evolved by some mobile homemanufacturers and house prefabrica-tors avoids such a rig and is at leastequally effective. Th e fram e is as-sembled, insulation pushed in place,and sheathing applied on top, in thefirst few stations on the flat. Thi srequires the use of friction-fit insula-tion batts and a separate vapour bar-rier applied later, which are recom-mended for Canadian conditions inany case. The n the panels are hoistedto a vertical position and moved alongto subsequent stations where wiring,wallboard, windows, doors, and trimare set in, and siding may be applied(Figure 2).Overhead monorail conveyors areoften considered for this purpose butthey involve a considerable invest-ment in multiple rails, sidings, andswitch gear, since it is almost impos-sible to achieve balance in house pre-fabrication on a single line. Muchlower costs and greater flexibility inscheduling and in the use of spacecan be gained by using small dolliesto handle the panels on edge. Twomen can readily handle the panels inthis way, and vertical stations can thenbe set side by side. La g screws setinto the top plates of the panels al-low loading on transport trailers byoverhead cranes within the shop, andallow optimum unloading and hand-ling on the site by truck-mountedboom cranes.EQUIPMENTIt is difficult to envisage completemechanization of wood-frame houseproduction. Th e variability of thelumber, the number of pieces in-volved, and the low labour content ofany one operation all work againsthigh mechanization and automation.The gains achievable through properorganization of materials handling canscarcely be equalled by the adoptionof high cost mechanical tools. A fewof the larger machinery manufactur-ers are now concentrat ing on thewood-frame production field, with in-triguing results, but the marginal na-

ture of the mechanization questionconfirms the wisdom of many housemanufacturers who lease all majorequipment, rather than buying it atthe outset.Precise precutting is the most im-portant process in wood-frame pre-fabrication. Most Canadian plants usean in-line arrangement of radial armcutoff saws for this function. A re-cent t ime and motion study confirmsthis observer's general impression thatthe process is quite inefficient, due tothe end-to-end handling that is in-volved. Th e radial saws are invalu-able for job-cutting, but using themfor standard, high-volume pieces suchas common studs can create the worst

and most wasteful bottle-neck in theshop. In some areas it is now ad-vantageous to buy precut f raming forcommon studs and other repetitivepieces.Volumes of four houses and overper day for much of the year mayjustify the trial leasing of a double-end saw. The saw may cost f rom$8,000 to $20,000 but i t can precutall members for a house in a fewhours. Outside sales of precu t com -mon studs are desirable to use to the

maximum the high production capac-ity of these saws. Setting up may tak econsiderable time, so long productionruns of any one piece are desirable.For this reason the saw should havegood storage and marshalling areasa r o ~ ~ n dt.Som e pieces-of equipm ent are basicto any shop, large or small. A panelsaw is well justified for all sheet cut-ting. A selfguiding router, or at leasta sabre saw mounted in a cradle, willbe necessary for quickly cutting outwindow and door openings af ter thesheathing has been placed over thef rame. And n o one doubts the ad-vantages of power staplers for secur-ing all sheathing. Ma ny are interestedin the fast-improving pressure-sensi-tive adhesives for applying wallboard.but any adhesive appears difficult toapply rapidly on narrow frames withuniform results. Few yet use such ad-hesives, but many have adopted aproprietary power-screw method toapply drywall to reduce popping.

Roof trusses have greatly profitedfrom advances in engineering criteria,fastening devices, and mechanization.


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A small shop showing horizontal and vertical assembly sta tion s (Rothwell-Perrin, Portland, Ont.)

Their costs have been sharply re-duced and they are commonly usedby prefabricators and others. Th ecommon method of fixing plates fromboth sides at two separate stations andthen sliding the whole assemblythrough rollers suffers from the timeconsumed and from the space re-quired. Oth er methods use a massive"beam press", o r hydra ulic "C"clamps to fix all the joints at one sta-tion, requiring much less time andspace.Nailing machines are still contro-versial items, especially the self-feed-ing types. Thos e that use commonframing nails are subject to jamming,while those using special nails haveless jamming trouble but the nails arecostly. Pneumatic hammers are fa-voured by many. They are slow and

must be "fed" one nail at a time, butthey relieve fatigue.

The decision to incorporate a cab-inet shop in the prefab operation willlargely depend on the possibility ofoutside sales. Considerable spac e isrequired (Figure 1) and the types ofwood, production methods, an d equip-ment are somewhat different than inhouse shell prefabrication. On theother hand, production of cabinetryallows profit to be taken on some ofthe highest cost items in the housepackage. This, of course, appliesequally well to windows, doors, "built-ins", and all mill work.INNOVATIONSIn contrast to wood-frame con-struction, the promising innovationsin housing technology have in com-mon a n amenability t o highly mechan-ized processes of production. Theleading new systems are characterizedby fewer pieces to be handled andassembled, fewer materials and proc-

esses, more controllable materials andprocesses, and a higher continuity ofprocess, all favouring optimum ma-chine production. Com posite materi-als of structural sandwich form showgreat potential over the long range.Developments in bonded wood fibrematerials may lead to complete floor,wall, partition, and roof componentsthat satisfy all requirements at sharplycompetitive costs. P olyure thane coresmay allow continuous belt productionof sandwich enclosure componentsthat can be thermally and structurallysound with a thickness of only oneinch. But wood-frame constructioncontinues to provide technical ade-quacy at low costs, and still remainsa remarkably difficult yardstickagainst which proposed innovationsmust be measured. It will be verydifficult to replace for the bulk ofhouse structures.

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Shops for Woodframe House Prefabrication