Product casement sashes

How to Measure for Window Replacement

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Mea­sure twice, buy once. Real­iz­ing you’ve improp­er­ly mea­sured the dimen­sions for a win­dow replace­ment can be a huge prob­lem in a ren­o­va­tion project. 

If the dimen­sions you’ve giv­en are too small, you’ll need shims and extra insu­la­tion to keep your win­dow in place; plus, you risk cre­at­ing air leaks that will pump up your ener­gy bills. 

And if you’ve ordered a win­dow that’s too big, you’ll either delay work while wait­ing for one in the cor­rect size, or you’ll need to sig­nif­i­cant­ly rebuild your wall to cre­ate an open­ing that fits, which can also involve remov­ing por­tions of or patch­ing your exte­ri­or siding.

Mea­sur­ing for win­dow replace­ment can seem over­whelm­ing. For some­thing that can feel as sim­ple as a sin­gle pane of glass, a win­dow actu­al­ly has a lot of dif­fer­ent parts. Should you mea­sure the trim? The frame? What about the jamb or the sash? Add in wood­en win­dows from man­u­fac­tur­ers that don’t exist any­more, and the process can get tricky.

We’re going to take some of the mys­tery out of how to mea­sure for win­dow replace­ment. While every home and win­dow is dif­fer­ent, if you use these win­dow mea­sur­ing tips, you’ll be able to select the win­dows for your next project with confidence.

Product casement sashes

What Part of the Window Are You Replacing?

Not all win­dow replace­ments are cre­at­ed equal. In some cas­es, you’ll only need to replace some or all of the sash. In oth­ers, you’ll need to replace every­thing includ­ing the frame. Inspect both the sash and frame close­ly for water dam­age. Some­times you can remove the dam­aged part or patch it. Oth­er times the dam­age is too exten­sive and a com­plete replace­ment is required.

If you’re replac­ing the frame, you’ll need to mea­sure the wall width. This is most eas­i­ly done once the frame has been removed. Oth­er­wise, you can mea­sure through an open win­dow from the back inte­ri­or edge of the frame to where it meets your exte­ri­or sid­ing. Dou­ble-check your mea­sure­ments once you’ve removed the win­dow to make sure it doesn’t include any ply­wood pan­el­ing inside the wall against the façade. 

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Measuring to Replace a Window Sash

If your frame is still in good con­di­tion, then you may only need to replace your win­dow sash. The sash is the wood­en cas­ing around the pane of glass. While wood­en win­dows have a rep­u­ta­tion for being more expen­sive than their vinyl and alu­minum coun­ter­parts, a replace­ment wood win­dow sash is actu­al­ly a cost-effec­tive solu­tion.

To mea­sure the case­ment sash, you’ll want to remove it from the frame so you know your mea­sure­ments are accu­rate. Do not only mea­sure the dimen­sions of exposed glass.

For a sim­ple case­ment win­dow, mea­sure the width and height of the sash, as shown above, both on the exte­ri­or side. Wood can warp when exposed to the ele­ments, and you’ll want to pro­vide the small­er dimen­sions to your new win­dow sash suppliers. 

If you’re replac­ing a hung-sash win­dow or dou­ble-hung win­dow, you’ll also want to mea­sure the dimen­sions inside your frame, to make sure your new sash will fit. 

If you’re replac­ing sash­es in a dou­ble-hung win­dow, you also want to make sure you remove the jambs and stops so you’re mea­sur­ing the full width and height inside the pock­ets. For dou­ble-hung win­dows, be sure to mea­sure both the upper and low­er sash­es and pro­vide the dimen­sions separately.

When you’re mea­sur­ing the pock­et grooves, take mea­sure­ments in at least three places, usu­al­ly the top, mid­dle and bot­tom for mea­sur­ing the width, and at the left, cen­ter and right for mea­sur­ing the height. Write all of the mea­sure­ments down, and choose the small­est dimen­sions to pro­vide to your con­trac­tor or sup­pli­er, so that you know the ordered mod­el will fit all sides.

Con­trac­tors and sup­pli­ers will also often rec­om­mend that you remove a small frac­tion from your mea­sured dimen­sions. This frac­tion is typ­i­cal­ly ⅛” or ¼”. For exam­ple, if your sash is 24” across, you would order a replace­ment sash that is 23 ⅞”. How­ev­er, some sup­pli­ers will remove this frac­tion as part of their process. Make sure you know if this will hap­pen, as you don’t want to order some­thing that is too small by remov­ing the frac­tion twice. 

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More Tips for Home Window Replacement

If you’re replac­ing more than one win­dow, make sure you care­ful­ly doc­u­ment which win­dow you’re mea­sur­ing. Num­ber them with painter’s tape, or label them with some­thing easy to remem­ber like liv­ing room, south wall.” There may be small vari­a­tions from one win­dow frame to the next, and you want to know which replace­ment win­dow sash fits in which frame when they arrive.

If you’re real­ly not sure you’ve mea­sured the right parts of the assem­bly — whether it’s the frame, the sash or the pock­et — it also nev­er hurts to draw a pic­ture and mark the dimen­sions direct­ly on it. That way, a con­trac­tor can eas­i­ly pull the win­dow mea­sure­ments they need, with no confusion.

If you’re very unsure, have the con­trac­tor come to your house and mea­sure them­selves. Although they may charge a small fee for this, the cost far out­weighs the penal­ty of need­ing to re-order win­dows when the first ones don’t fit, not to men­tion the inconvenience.

For more hints and tips on how to prop­er­ly mea­sure for your replace­ment win­dow sash, con­tact us today.

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