Joists

Share

Introduction and Regulations

I.S. EN 1995-1 2005: Design of timber structures – Part 1 or simply Eurocode 5 outlines guidelines that should be followed when designing timber structures and elements. A series of harmonised structural design codes for building and civil engineering exists across the European Union, I.S. EN 1995 being just one. In Ireland, Eurocodes are published and made available by the National Standards Authority of Ireland (NSAI).

With the introduction of I.S. EN 1995, all conflicting national standards have been withdrawn. The NSAI are currently revising Irish Standard I.S. 444: The use of structural timber in buildings. The span table for floor joist spans referred to in this section and other guidance are reproduced from findings of the NSAI Timber Standards Consultative Committee and reflect the requirements set out in Eurocode 5 and the Irish National Annex. NSAI have available a full set of span tables.

Species of Timber

In Ireland, softwood timber is used as structural timber in domestic builds. The typical species, which are sourced either domestically or from overseas, include spruce, fir, pine, redwood, whitewood, and larch. In order to aid users in identifying timber, a system was created in which the species and source code as well as other information about the timber is stamped onto the timber.

Examples of abbreviations include:

  • WE/SG1 = Western European (including Britain) Sitka/Norway spruce, pine and Douglas fir – Species group 1. Source – Ireland

  • WE/SG2 = Larch, Scotch Pine – Species group 2. Source – Ireland

  • EW = Whitewood – Source North and North Eastern Europe (includes Scandinavian countries and Russia).

  • ER = Redwood – Source North and North Eastern Europe (includes Scandinavian countries and Russia).

  • EW/ER = Whitewood and redwood combined – Source North and North Eastern Europe (includes Scandinavian countries and Russia).

Timber Strength Classes/Grades

When dividing timber into strength classes, an abbreviated strength class divided in ascending order based on both the species and the grade of the timber is used, i.e. C14, C16, C18, C22, C24, C27. Two examples of strength grades are GS and SS, where GS stands for general structural grade and SS stands for special structural grade. Both are visually graded.

Identification markings that indicate the strength class and strength grade of timber should be stamped onto the timber in order to comply with the requirements of I.S. EN 1995.

Identification Marking Stamps

The following material is required by I.S. EN 1995:

  • Strength class

  • Species/Species combination of timber

  • Strength grade

  • Reference to the timber standard to which the timber was graded

Examples of Grading Stamps

Examples of markings that are compliant with I.S. 14801-1 are:

  • A waterproof inkpad should be used for stamp.

  • The minimum stamp size is 25 mm x 140 mm.

  • All stamps to have heavy duty facings and handles are to be heavy duty.

  • The reg. no, field is the company’s NSAI registration/grader’s number.

Moisture Content

The moisture content of timber should not exceed 20% at the time of construction and while in service. Individual moisture contents are allowed to be up to 24% at the time of construction but the average should be less than 20%. If the area of the timber is being used has high moisture content, this should be accounted for in the design.

Joist Span

Table C13 - Joist span table - Domestic

Guidance

  • The size of timber at 20% i.e. the height that all deviations are to be related to is known as the target size.

  • Where a span is greater than 2.7m, bridging is required at 1.35m centres. The depth of the bridge should be at least ¾ the depth of the joist. In the case of Herringbone strutting, 36mm should be the minimum target dimension, never use herringbone strutting in cases where the spacing of joists is greater than 3 times the joist depth.

  • Lateral stability of joists should be supplied at the ends and all along the length of the span. 3 ways of providing lateral support are:

    • Build joists into masonry
    • Bridge ends of joists with cuts of joists.
    • Using joist hangers which provide lateral support.
  • In cases where the joist hangers used do not provide adequate lateral support, use end bridging to support joists laterally. Seek hanger manufacturer’s guidance if this is the case.

  • If building joists into masonry, ensure ends are treated with 2 coats or colour-tinted preservative.

  • At each point of support, provide a minimum bearing of 50mm.

  • Partition point loads, point loads from dormer check studs etc. are not accounted for in the table above.

  • Alternatively, an engineer may design joists and possibly calculate smaller section sizes which are more economical.

External walls supporting floor joists provides further detail with respect to bearing and joist hanger type

EXAMPLE

Using the table provided above it can be determined which is the most appropriate and cost effect section to be used. For example assume the floor in question has a span of 4.5m. From the table above we can see that a joist 47mm x 225m of strength class C18 fixed at 400mm centres is permitted to have a span of 4.55m and is therefore suitable for use. A C18 joist of 44mm x 225mm fixed at 350mm centres ie permitted to span 4.62m and would also be suitable.

It should be noted that all spans in excess of 2.7m require bridging to be provided at 1.35m centres. using the table above the most cost effective section can be selected based on the required span.

Joining Floor Joists to External Walls

Walls and floors are tied to aid stability to external walls. Recommendations for strapping to be used are outlined in Technical Guidance Document A of the building regulations. Galvanised or austenitic stainless steel straps 30 mm x 5 mm in cross section should be used when strapping floor joists to external walls.

Strapping should be carried over at least 2 joists, with bridging directly underneath, secured with at least 4 fixings with at least one of the 4 in the second joist. Build the strapping at 2 m centres into the external wall as the courses are built; include packing between joists and wall.

External Walls Supporting Floor Joists

If floor joists are supported by external walls, straps are not required in the longitudinal direction of joists in houses containing not more than two storeys where the joists are at centres of not more than 800mm and;

  • Have at least 90mm bearing on the supported walls or a 75mm bearing on timber wall plate at each end.

or

  • Joists are carried by joist hangers as outlined in I.S. EN 845-1:2013 + A1: 2016 Specification for ancillary components for masonry – Part 1: Ties, tension straps, hangers and brackets.

Joists should be notched for straps to ensure a level finish for flooring.

Diagram C123 - Typical strapping detail of floor joists to cavity wall

Diagram C124 - Typical strapping detail of floor joists to hollow blocks

Diagram C125 - Alternative strapping detail of floor joists to hollow blocks

Strapping and Packing

Straps are not required at party walls when:

  • Packing is placed between the last joist and the wall at no more than 2m centres.

  • The floors are approximately the same level.

  • Packing is as close as possible to in line across the wall.

  • Where the strapping/packing is interrupted by an opening such as a stairwell, ensure that the opening is less than 3 m parallel to the wall and also that extra strapping and packing is provided to compensate for the strapping and packing that will not be provided due to the opening i.e distance between packing pieces is reduced.

Diagram C126 - Typical packing detail to provide stability at a party wall

Diagram C127 - Typical packing detail around opes

Trimming Joists around Openings

Stair openings

Specialist design may be required for trimmer and trimming joists. Ensure the engineer employed is qualified by examination, in private practice, and possesses professional indemnity insurance.

Diagram C128 -Typical trimming detail around stair opening

Supporting Floor Joists with a Steel Beam

An issue with using steel members to support floor joists in the case of dormer or storey-and-a-half roofs is that, when joists are notched into the web, there may be a loss of continuity of position. This can be overcome by installing straps across the bottom of the beam. The minimum cross-section allowed for the straps is 30 mm x 2.5 mm and they must be nailed to the joists securely.

Diagram C129 - Typical fixing detail of floor joists to steel beams - from underside

Beam and fixing detail are to be the designer’s specification. The engineer appointed must be qualified by examination, in private practice, and possess professional indemnity insurance.

Ensuring Floors are Rigid

Adequate bridging ensures that joists do not twist and that the floor is stiff. Bridging should comprise of solid timber with a depth at least ¾ the depth of the floor joists. I.S. EN 1995 states that bridging is required in all floor joists where the span is greater than 2.7 m at centres less than 1350 mm.

Diagram C130 - Typical detail to ensure rigid floors

When building joists into walls, ensure that bearing of 90 mm is provided and that the void left between the joist and blockwork is packed tightly with mortar. Alternatives to building the joist into the wall comprise end bridging between joints or using appropriate hangers built into the masonry.

Diagram C130 (a) - Minimum bearing detail for floor joists built into walls

Diagram C131 - Typical packing and bridging to ensure rigid floors

Diagram C132 - Typical herring bone strutting and packing detail

Notching and Drilling

I.S. EN 1995 outlines notching and drilling requirements. Take extra care when notching and drilling joists. Notching and drilling should only be located in the positions and to the appropriate depth.

Only notch the top of joists, unless the joist being notched has been specially designed by an engineer who is qualified by examination, in private practice, and possesses professional indemnity insurance.

Ensure that the depth of saw cut is not greater than the depth of the notch itself. Inappropriate notching that weakens joists should be avoided and plumbers and electricians should be informed and made aware. Excessive notching is not permitted.

The horizontal distance between hole and notch should always be greater than the depth of the joist.

Diagram C133 - Simply supported joists - notching and drilling

Diagram C134 -Typical notching limitations

Diagram C135 - Typical drilling limitations

Proprietary Floor Joists

The following section provides guidance for construction as well as general information regarding the use of proprietary engineered timber joists such as I-Joists and open steel web joists in masonry construction. Any additional requirements or instructions outlined by the manufacturer should be followed also.

A timber I-Joist is a structural member engineered from wood and is primarily designed to be used in floor construction. Usually, a timber I-Joist consists of a solid timber or Laminated Veneered Lumber (LVL) flanges separated by a vertical web that connects the top and bottom flanges. The web is usually formed from OSB or plywood and glued into grooves in the top and bottom flanges.

An open steel web joist typically comprises of timber flanges that are similar to those used in the timber I-Joists but are generally formed from solid timber. The flanges are connected using Open Steel “V” shaped metal webs that are fixed using integral nail plate connectors.

Diagram C136 - Typical proprietary timber I-joist and steel web joist

Third party certification, which outlines compliance with the Building Regulations namely, Irish Agrement Board (IAB) certification, British Board of Agrement (BBA) certification and, occasionally, European Technical Approval (ETA) certification, must be provided where proprietary timber I-Joists and open steel web joists are used. The company or some 3rd party must show how the product complies with the Irish Building Regulations where a product has ETA certification. Usually, open steel web joists are manufactured under license and under the supervision of the accrediting body. The technical literature provided by the manufacturer will include important information about the design and use of the joists. This literature should be read in conjunction with this section.

Please Note: It is important to ensure the correct information for the correct product is being used and that the recommendations in the information and this section are adhered to due to the large number of I-Joist and open steel web joist systems and manufacturers to choose from.

Construction Guidance

With all the following topics, the manufacturer will provide in-depth specific information. The following merely lists general guidance for the topics.

Design of Joists

Although the relevant approval certification contains some design information, most engineered joist systems provide a design service. The design service and the manufacturer’s technical literature contains any requirements such as joist sizes and layouts, bearing, drilling, and lateral restraint.

Cantilevered Joists

In the case of cantilevered joists, due to the special design and construction requirements, care should be taken. Ensure preservative treatment is added where they extend over an external wall. Be sure to seek advice from both the joist and preservative manufacturers as there may be additional requirements such as blocking and vapour control layers.

Moisture uptake is a problem to which this kind of cantilevered joist is quite susceptible. To avoid this problem, ensure good construction details are designed and that on-site workmanship is of a good standard. During construction, joists must be sheltered from excessive moisture and weather. I-Joists and steel web joists are not appropriate for use where the joist is exposed to external conditions.

Moisture Content

The approval certification outlines advice regarding moisture content of joists. It is important to protect joists from exposure to moisture and protect them from weather on site during construction.

Drilling and Notching

Ensure that the webs and flanges in both I-Joists and steel web joists are never drilled, notched, or altered at all without the manufacturer giving approval in writing. Knockout holes are pre-punched holes provided by the manufacturer in order to accommodate services like plumbing, heating pipes, and electrical cables. In the event that extra holes are required, seek the manufacturer’s guidance regarding location so as not to affect the shear capacity. Services may be passed through the open web of joists in steel web joists.

Bearing

Minimum bearing requirements will be specified by manufacturers. When using hangers, the instructions of both the joist and hanger manufacturer should be followed when fixing the joist to the hanger. Typical end bearing details have been provided below.

Diagram C137 (a) - Various end bearing typical details

Diagram C137 (b) - Various end bearing typical details

Handling and Storage

Joists should be stored vertically and evenly stacked with bearers in line off the ground, and protected from weather when being stored. In the case of timber I-Joists and open steel web joists, the manufacturer’s instructions should be followed during handling and installation as they are lightweight and have unique characteristics. Take care to make sure that the I-Joists are braced horizontally during construction and avoid damage to the steel webs throughout construction.

Where heavy loads such as blocks must be supported by proprietary floor joists, ensure the manufacturer’s recommendations regarding propping are followed.

Restraining walls laterally

Technical Guidance Document A of the Building regulations states that external walls must be restrained by the floor; the same as with conventional floor joists. Some typical restraint strapping have been shown below. Also, the manufacturers will provide supplementary guidance if restraint is required in the longitudinal direction of joists.

Diagram C138 -Typical lateral restraint strapping detail to external walls

Web and Joist Stiffeners

To avoid joists buckling, it may be necessary to include web stiffeners at critical load points, such as at heavy point loads, cantilever supports, or where load bearing partitions are located. For open steel web joists, stiffen using solid timber braced laterally; for I-Joists, include plywood or OSB fixed to each side of the joist. Illustrated below are details of both types of web stiffener. Manufacturers will provide information regarding location and fixing of stiffeners.

In addition to the situations outlined, it may be necessary to include web stiffeners at the ends of joists being supported. Generally blocking pieces are sufficient for this purpose.

Diagram C139 -Typical details for web and joist stiffeners

Joist Hangers

Ensure joist hangers used are appropriate for the specific joist being used. Manufacturers will generally outline the appropriate hanger along with guidelines for the particular joist as the hanger will most likely be unique to the joist.

In order to fix an I-Joist to the hanger, blocking pieces or possibly web stiffeners may need to be fixed to either side of the web. Where I-Joists are supported by other I-Joists, backing timber needs to be included to fix the hanger to.

Diagram C140 - Typical joist hanger details

Bridging/Strutting

Generally, the manufacturers will provide site drawings that provide specific guidance on bridging /strutting requirements.

Timber Preservation

When treatment of the timber is necessary, the manufacturer’s advice should be sought regarding which treatment is compatible with the particular type of joist.

Shop Screws