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How to Save Money on Floor Framing and Sub-Floors
Ser
NA7110
N21h8
no.
6
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BLDG
National Research Council
Canada
Division of Building Research
HOUSING N O T E N O . 6
HOW TO SAVE MONEY ON
A.
T.
Hansen
, . -: . , i7 ,\Reprinted from Canadian Builder,
Vol.
XII,
No.
9,
September
1962, p.
65-67
'SEE HOUSING STANDARDS FOR MINIMUM WALL THICKNESS
FIGURE I
FLOOR SUPPORT ON EDGE LAID SlLL WlTH WOOD FRAME WALLS
ANCHOR BOLTS INSTALLED, NAILED IN PLACE IN THE
How to save money on
There are many factors forcing the cost of housing higher, year by year. For this reason alone it becomes increasingly important to re-examine present con- struction practices critically in order to determine the possibilities that exist for reducing construction costs. While it may be acknowledged that there is little pos- sibility of making drastic reductions through changes in construction practice, some real savings are possible. This paper discusses some of the changes that can lower construction costs of floor systems without impairing their quality.
Floor Joists
The allowable span of a floor joist is usually limited by either the stiffness or strength of the member. In many cases, however, joists have considerably more strength than is necessary, but are limited in span by stiffness, which is not signifi- cantly affected by the lumber grade (al- though the grade does directly a u e n c e
\SEE HOUSING STANDARDS FOR MINIMUM WALL THICKNESS
FIGURE 2
FLOOR SUPPORT ON EDGE LAlD SlLL WlTH BRICK
VENEER WALLS
strength). It is therefore possible in many cases to use a lower grade of lumber without reducing the allowable joist span. The span tables in the new *Housing Standards have been prepared for a number of different grades, so that it is now possible to select from these tables the most economical grade and size of a particular species to span a required distance without adversely affecting the serviceability of the floor system.
The design load, according to the new Housing Standards, is less for bedroom floors than for other areas of the house. This offers another opportunity for sav- ings in floor framing. In a bungalow, for example, where one would normally have the same joist size throughout, the joist spacing could be increased in the
*Housing Standards, Canada, 1962. Sup- plement No. 5 to the National Building Code, National Research Councd Assoc~ate Comm~ttee on the National ~uilhin* Code, Ottawa. NRC 6487.
By A. T. HANSEN, Housing Section, Division of Building Research, National Research Council, Ottawa.
bedroom areas, thus reducing the total number of joists normally installed. One could use 20-inch spacing for bedroom joists and 16-inch spacing for the rest of the house, provided that the joist spans in the bedrooms were not greater than those for the remainder of the house. As bedroom areas represent approximateIy 40% of the total floor area in a typical house, it may be seen that it should be possible to use 10% fewer joists by in- creasing the spacings from 16 to 20 inches in the bedroom areas. This repre- sents a saving of from 6 t o 7 joists in a typical bungalow, plus the labor to in- stall them.
At the same time, one should keep in mind that the minimum thickness for subfloors on 20-inch joist spacing is 94- inch boards or %-inch plywood. Since many builders use subfloors of this thick- ness in any event, this does not neces- sarily mean an increase in subflooring cost.
FIGURE 3
SUPPORT FOR FLOOR ON SOLID CONCRETE FOUNDATION TO ELIMINATE NEED FOR EXTERIOR STEP
a
TO OBTAIN LOW PROFILE,' -d.
.
SUBFLOOR SHOULD BE PLYWOOD OR DIAGONALLY LAID BOARDS 8 NAILED TO
'
*
THE SlLL TO PROVIDESUBFLOOR SHOULD BE PLYWOOD OR DIAGONALLY LAID BOARDS.
8 NAILED TO THE SlLL TO PROVIDE LATERAL SUPPORT FOR
f
T
-
'
THE TOP OF THE FOUNDATIONLATERAL SUPPORT FOR THE TOP OF THE FOUNDATION WALL UNLESS REDUCED SECTION IS REINFORCED
r
4 CAULKING COMPOUNO TO REDUCE AIR r.\A'
/ \ ; CAULKING COMPOUNO TO REDUCE AIR INFILTRATION IF SlLL IS NOT PLACE0 ON BED OF MORTAR-, FIGURE 4FLOOR SUPPORT ON BLOCK FOUNDATION TO ELIMINATE NECESSITY FOR EXTERIOR STEP B TO PROVIDE A LOW PROFILE
,*# J -
.
INFILTRATION IF SlLL IS NOT PLACED ON BED OF MORTAR.
WALL UNLESS THE REDUCED
- Sl
:i
;ol;;71 REINTO ' '.
.
_
---
.-
v -2'a 4' SILL -- 7r--'
//
+-
I
Q ' MIH WlTH W"3 OR PIIPIIROARS TYPE SICIIVS BridgingTwo reasons are usually cited for in- stalling cross-bridging: to distribute con- centrated loads on joists to adjacent
1---, ' EVEN 8 LEVEL
8 . ~ l k WITH WOOD
-4
' , ..
OR H1RDROARO TYPE'{ SIDING
'SEE HOUSING STANDARDS FOR MINIMUM WALL THICUMSS
,
.
,.
-.
rvL/
1,-a SEE HOUSING STANDARDS FOR
MINIMUM WILL THtCHNESS
11
-
joists; and to prevent twisting of joists. Tests conducted at the U. S. Forest Prod- ucts Laboratory and more recently at the NAHB Research Institute Laboratory in- dicate that the effectiveness of cross- bridging in transferring loads to ad- jacent joists is slight.
f
-
r/
,,-7 L-
-
-
2 . ~ 4 " SILL
-SILL SHOULD BE
1
PLACED ON BED OF MORTAR IF TOP OF WALL IS NOT EVEN 8 LEVEL TOE NAIL JOISTSTO PLATE -
0
! C
2 I
L
Resistance to joist twisting can be pro- vided as an acceptable alternative by nailing continuous 1 by 4 strips across the bottoms of the joists in lines where cross-bridging would normally be in- stalled. The use of continuous strapping eliminates the often time-consuming op-
*'
I -SILL SHOULD BE PLACED
4
ON BED OF MORTAR IFTOP OF WALL IS NOT
--I-'-'
eration of cutting and fitting the many pieces of wood that constitute cross- bridging. (According to some authorities this operation requires from 3 to 4 man- hours in an average house.)
Sill Plate
The setting of sill plates on foundation walls in the conventional manner can b- time consuming. Anchor bolt holes have to be drilled, the plate shimmed and leveled and placed on a mortar bed to provide even bearing. The new Housing Standards no longer require a mortar bed if the top of the foundation wall is even and level and the junction between the plate and wall is calked to prevent air infiltration.
One method of simplifying the instal- lation of sill plates in solid concrete walls is shown in Figs. 1 and 2.
The 2 by 4 sill plate is placed on ed,ge in the concrete forms and nailed in place at the proper elevation, with the anchor bolts in place, before the concrete is placed. The top of the sill plate is used as a screed to level off the top of the concrete, thus ensuring an even, level wall surface on which to place the joists. It is recommended, however, that the junction of the header plate and foun- dation walls be calked to guard against air infiltration a t this point.
Lower Floor Support
A current trend in house design is to provide a low silhouette by setting the house close to the ground. As well as providing a pleasing appearance, it makes it possible in many instances to eliminate the necessity for steps at exterior en- trances.
There is an additional advantage in the reduction of heat loss by reducing the amount of exposed foundation wall. Fig-
ures 3 and 4 show details of how the floor can be constructed close to grade to accomplish this.
These details also offer advantages where 8-foot sheet material is used for exterior siding. The necessity of pro- viding an additional p i d e of siding to bridge the sill plate and subfloor can be eliminated. There is one drawback, how- ever: there is a possibility of differential
wood shrinkage between the outside wall sill and the top of the joists. Unless reas- onably seasoned joists are used, it may be necessary to shim in order to bring the tops of the joists to the same ele- vation as the top of the exterior sill plate if the joists shrink from drying.
Elimination of Subflooring
Subflooring serves two major func- tions:
-
to support the finish floor and any loads that may occur;- to provide a working platform for the convenient performance of the vari- ous building operations.
If sufficient strength is provided by the finish floor itself to support the floor loads, the subfloor may be omitted as a structural necessity. Such would be the case when %-inch matched wood strip flooring is installed at right angles to joists spaced 16 inches on centre.
To provide a convenient work plat- form, a temporary plywood deck tacked in place may be used and removed as the finish flooring is placed. Alternative- ly, a relatively inexpensive plywood sheathing may be left in place beneath the finish flooring. If prefabricated wall panels and roof trusses are used, even the necessity for a working platform could be eliminated, although there would be some initial inconvenience.
Board Subfloors
not be laid parallel to board subfloors, subfloors are usually laid diagonally to permit the finish floor to be installed either parallel or at right angles to the joists as may be desired in any particular area.
The necessity for changing direction of the finish flooring in different rooms is dictated presumably by the tradition that requires the finish floor to run parallel to the long dimension of the room. In many house plans, however, it would be quite acceptable to lay the finish floor parallel to the joists in all rooms so finished. In these cases the subfloor could be satisfactorily laid at right angles. The same would be true if
a resilient floor were used over a sepa- rate underlay.
There are several advantages in laying subfloors at right angles. It has been
estimated that they require about 15%
less labor than do those laid diagonally, and there is substantially less waste from end cuttings. Some authorities estimate
that they take from 5% to 10 less
lumber. In addition, they are considerably stronger and stiffer than those laid diagonally.
Many builders use tongue and groove or shiplap lumber for subfloors. While there is nothing wrong with this practice, there seems to be more advantage in
using square-edge lumber, particularly in view of the fact that more coverage is obtained per board foot. When tongue and groove material is used in the winter, snow and ice can clog the grooves and make the fitting of the boards difficult.
Conclusion
The construction techniques mentioned here as possible methods for reducing costs are not new. Some are now being used by Canadian builders; some have been introduced in the new Housing Standards; while others were permitted
in the 1958 Housing Standards, although
they have not been generally adopted by the building industry.
